Adapted from Writing and Reading Across the Curriculum, 6th Edition
By Laurence Behrens and Leonard J. Rosen
New York: HarperCollins, 1997. Pp. 32-58.
WRITING A THESIS
A thesis statement is a one-sentence summary of a paper's content. It is similar, actually, to a paper's conclusion but lacks the conclusion's concern for broad implications and significance. For a writer in the drafting stages, the thesis establishes a focus, a basis on which to include or exclude information. For the reader of a finished product, the thesis anticipates the author's discussion. A thesis statement, therefore, is an essential tool for both writers and readers of academic material.
This last sentence is our thesis for this section. Based on this thesis, we, as the authors, have limited the content of the section; and you, as the reader, will be able to form certain expectations about the discussion that follows. You can expect a definition of a thesis statement; an enumeration of the uses of a thesis statement; and a discussion focused on academic material. As writers, we will have met our obligations to you only if in subsequent paragraphs we satisfy these expectations.
The Components of a Thesis
Like any other sentence, a thesis includes a subject and a predicate, which consists of an assertion about the subject. In the sentence "Lee and Grant were different kinds of generals," "Lee and Grant" is the subject and "were different kinds of generals" is the predicate. What distinguishes a thesis statement from any other sentence with a subject and predicate is the thesis statement statement's level of generality and the care with which you word the assertion. The subject of a thesis must present the right balance between the general and the specific to allow for a thorough discussion within the allotted length of the paper. The discussion might include definitions, details, comparisons contrasts - whatever is needed to illuminate a subject and carry on an intelligent conversation. (If the sentence about Lee and Grant were a thesis, the reader would assume that the rest of the essay contained comparisons and contrasts between the two generals.)
Bear in mind when writing thesis statements that the more general your subject and the more complex your assertion, the longer your paper will be. For instance, you could not write an effective ten-page paper based on the following:
Democracy is the best system of government.
Consider the subject of this sentence, "democracy," and the assertion of its predicate, "is the best system of government." The subject is enormous in scope; it is a general category composed of hundreds of more specific sub-categories, each of which would be appropriate for a paper ten pages in length. The predicate of our example is also a problem, for the claim that democracy is the best system of government would be simplistic unless accompanied by a thorough, systematic, critical evaluation of every form of government yet devised. A ten-page paper governed by such a thesis simply could not achieve the level of detail and sophistication expected of college students.
Limiting the Scope of the Thesis
Before you can write an effective thesis and thus a controlled, effective paper, you need to limit your intended discussions by limiting your subject and your claims about it. Two strategies for achieving a thesis statement of manageable proportions are (1) to begin with a working thesis (this strategy assumes that you are familiar with your topic) and (2) to begin with a broad area of interest and narrow it (this strategy assumes that you are unfamiliar with your topic).
Begin with a Working Thesis
Professionals thoroughly familiar with a topic often begin writing with a clear thesis in mind - a happy state of affairs unfamiliar to most college students who are assigned term papers. But professionals usually have an important advantage over students: experience. Because professionals know their material, are familiar with the ways of approaching it, are aware of the questions important to practitioners, and have devoted considerable time to study of the topic, they are naturally in a strong position to begin writing a paper. Not only do professionals have experience in their fields, but they also have a clear purpose in writing; they know their audience and are comfortable with the format of their papers.
Experience counts - there's no way around it. As a student, you are not yet an expert and therefore don't generally have the luxury of beginning your writing tasks with a definite thesis in mind. Once you choose and devote time to a major field of study, however, you will gain experience. In the meantime, you'll have to do more work than the professional to prepare yourself for writing a paper.
But let's assume that you do have an area of expertise, that you are in your own right a professional (albeit not in academic matters). We'll assume that you understand your nonacademic subject - say, backpacking - and have been given a clear purpose for writing: to discuss the relative merits of backpack designs. Your job is to write a recommendation for the owner of a sporting-goods chain, suggesting which line of backpacks the chain should carry. The owner lives in another city, so your remarks have to be written. Since you already know a good deal about backpacks, you may already have some well-developed ideas on the topic before you start doing additional research.
Yet even as an expert in your field, you will find that beginning the writing task is a challenge, for at this point it is unlikely that you will be able to conceive a thesis perfectly suited to the contents of your paper. After all, a thesis statement is a summary, and it is difficult to summarize a presentation yet to be written - especially if you plan to discover what you want to say during the process of writing. Even if you know your material well, the best you can do at the early stages is to formulate a working thesis - a hypothesis of sorts, a well-informed hunch about your topic and the claim to be made about it. Once you have completed a draft, you can evaluate the degree to which your working thesis accurately summarizes the content of your paper. 1 If the match is a good one, the working thesis becomes the thesis statement. If, however, sections of the paper drift from the focus set out in the working thesis, you'll need to revise the thesis and the paper itself to ensure that the presentation is unified. (You'll know that the match between the content and thesis is a good one when every paragraph directly refers to and develops some element of the thesis.)
Begin with a Subject and Narrow It
Let's assume that you have moved from making recommendations about backpacks (your territory) to writing a paper for your government class (your professor's territory). Whereas you were once the professional who knew enough about your subject to begin writing with a working thesis, you are now the student, inexperienced and in need of a great deal of information before you can begin begin to think of thesis statements. It may be a comfort to know that your government professor would likely be in the same predicament if asked to recommend backpack designs. He would need to spend several weeks, at least, backpacking to become as experienced as you; and it is fair to say that you will need to spend several hours in the library before you are in a position to choose a topic suitable for an undergraduate paper.
Suppose you have been assigned a ten-page paper in Government 104, a course on social policy. Not only do you not have a thesis - you don't have a subject! Where will you begin? First, you need to select a broad area of interest and make yourself knowledgeable about its general features. What if no broad area of interest occurs to you? Don't despair - there's usually a way to make use of discussions you've read in a text or heard in a lecture. The trick is to find a topic that can become personally important, for whatever reason. (For a paper in your biology class, you might write on the digestive system because a relative has stomach troubles. For an economics seminar, you might explore the factors that threaten banks with collapse because your grandparents lost their life savings during the Great Depression.) Whatever the academic discipline, try to discover a topic that you'll enjoy exploring; that way, you'll be writing for yourself as much as for your professor. Some specific strategies to try if no topics occur to you: Review material covered during the semester, class by class if need be; review the semester's readings, actually skimming each assignment. Choose any subject that has held your interest, if even for a moment, and use that as your point of departure.
Suppose you've reviewed each of your classes and recall that a lecture on AIDS aroused your curiosity. Your broad subject of interest, then, will be AIDS. At this point, the goal of your research is to limit this subject to a manageable scope. Although your initial, broad subject will often be more specific than our example, "AIDS," we'll assume for the purposes of discussion the most general case (the subject in greatest need of limiting).
A subject can be limited in at least two ways. First, a general article like an encyclopedia entry may do the work for you by presenting the subject in the form of an outline, with each item in the outline representing a separate topic (which, for your purposes, may need further limiting). Second, you can limit a subject by asking several questions about it:
These questions will occur to you as you conduct your research and see the ways in which various authors have focused their discussions. Having read several sources and having decided that you'd like to use them, you might limit the subject "AIDS" by asking who - AIDS patients; and which aspect - civil rights of AIDS patients.
Certainly, "the civil rights of AIDS patients" offers a more specific focus than does "AIDS"; still, the revised focus is too broad for a ten-page paper in that a comprehensive discussion would obligate you to review numerous particular rights. So again you must try to limit your subject by posing a question. In this particular case, which aspects (of the civil rights of AIDS patients) can be asked a second time. Six aspects may come to mind:
- Rights in the workplace
- Rights to hospital care
- Rights to insurance benefits
- Rights to privacy
- Rights to fair housing
- Rights to education
Any one of these aspects could provide the focus of a ten-page paper, and you do yourself an important service by choosing one, perhaps two, of the aspects; to choose more would obligate you to too broad a discussion and you would frustrate yourself: Either the paper would have to be longer than ten pages or, assuming you kept to the page limit, the paper would be superficial in its treatment. In both instances, the paper would fail, given the constraints of the assignment. So it is far better that you limit your subject ahead of time, before you attempt to write about it. Let's assume that you settle on the following as an appropriately defined subject for a ten-page paper:
the rights of AIDS patients in the workplace
The process of narrowing an initial subject depends heavily upon the reading you do. The more you read, the deeper your understanding of a topic. The deeper your understanding, the likelier it will be that you can divide a broad and complex topic into manageable - that is, researchable - categories. Identify these categories that compose the larger topic and pursue one of them. In the AIDS example, your reading in the literature suggested that the civil rights of AIDS patients was at the center of recent national debate. So reading allowed you to narrow the subject "AIDS" by answering the initial questions - the who and which aspects. Once you narrowed your focus to "the civil rights of AIDS patients," you read further and quickly realized that civil rights in itself was a broad concern that also should be limited. In this way, reading provided an important stimulus as you worked to identify an appropriate subject for your paper.
Make an Assertion
Once you have identified the subject, you can now develop it into a thesis by making an assertion about it. If you have spent enough time reading and gathering information, you will be knowledgeable enough to have something to say about the subject, based on a combination of your own thinking and the thinking of your sources. If you have trouble making an assertion, try writing your topic at the top of a page and then listing everything you know and feel about it. Often from such a list you will discover an assertion that you then can use to fashion a working thesis. A good way to gauge the reasonableness of your claim is to see what other authors have asserted about the same topic. In fact, keep good notes on the views of others; the notes will prove a useful counterpoint to your own views as you write, and you may want to use them in your paper.
Next, make three assertions about your topic, in order of increasing complexity.
- During the past few years, the rights of AIDS patients in the workplace have been debated by national columnists.
- Several columnists have offered convincing reasons for protecting the rights of AIDS patients in the workplace.
- The most sensible plan for protecting the rights of AIDS patients in the workplace has been offered by columnist Anthony Jones.
Keep in mind that these are working thesis statements. Because you haven't written a paper based on any of them, they remain hypotheses to be tested. After completing a first draft, you would compare the contents of the paper to the thesis and make adjustments as necessary for unity. The working thesis is an excellent tool for planning broad sections of the paper, but - again - don't let it prevent you from pursuing related discussions as they occur to you.
Notice how these three statements differ from one another in the forcefulness of their assertions. The third thesis is strongly argumentative. "Most sensible" implies that the writer will explain several plans for protecting the rights of AIDS patients in the workplace. Following the explanation would come a comparison of plans and then a judgment in favor of Anthony Jones. Like any working thesis, this one helps the writer plan the paper. Assuming the paper follows the three-part structure we've inferred, the working thesis would become the final thesis, on the basis of which a reader could anticipate sections of the essay to come.
The first of the three thesis statements, by contrast, is explanatory:
During the past few years, the rights of AIDS patients in the workplace have been debated by national columnists.
In developing a paper based on this thesis, the writer would assert only the existence of a debate, obligating himself merely to a summary of the various positions taken. Readers, then, would use this thesis as a tool for anticipating the contours of the paper to follow. Based on this particular thesis, a reader would not expect to find the author strongly endorsing the views of one or another columnist. The thesis does not require the author to defend a personal opinion.
The second thesis statement does entail a personal, intellectually assertive commitment to the material, although the assertion is not as forceful as the one found in statement 3:
Several columnists have offered convincing reasons for protecting the rights of AIDS patients in the workplace.
Here we have an explanatory, mildly argumentative thesis that enables the writer to express an opinion. We infer from the use of the word convincing that the writer will judge the various reasons for protecting the rights of AIDS patients; and, we can reasonably assume, the writer himself believes in protecting these rights. Note the contrast between this second thesis and the first one, where the writer committed himself to no involvement in the debate whatsoever. Still, the present thesis is not as ambitious as the third one, whose writer implicitly accepted the general argument for safeguarding rights (an acceptance he would need to justify) and then took the additional step of evaluating the merits of those arguments in relation to each other. (Recall that Anthony Jones's plan was the "most sensible.")
As you can see, for any subject you might care to explore in a paper, you can make any number of assertions - some relatively simple, some complex. It is on the basis of these assertions that you set yourself an agenda in writing a paper - and readers set for themselves expectations for reading. The more ambitious the thesis, the more complex will be the paper and the greater will be the readers' expectations.
Using the Thesis
Different writing tasks require different thesis statements. The explanatory thesis is often developed in response to short-answer exam questions that call for information, not analysis (e.g., "List and explain proposed modifications to contemporary American democracy"). The explanatory but mildly argumentative thesis is appropriate for organizing reports (even lengthy ones), as well as essay questions that call for some analysis (e.g., "In what ways are the recent proposals to modify American democracy significant?"). The strongly argumentative thesis is used to organize papers and exam questions that call for information, analysis, and the writer's forcefully stated point of view (e.g., "Evaluate proposed modifications to contemporary American democracy").
The strongly argumentative thesis, of course, is the riskiest of the three, since you must unequivocally state your position and make it appear reasonable - which requires that you offer evidence and defend against logical objections. But such intellectual risks pay dividends, and if you become involved enough in your work to make challenging assertions, you will provoke challenging responses that enliven classroom discussions. One of the important objectives of a college education is to extend learning by stretching, or challenging, conventional beliefs. You breathe new life into this broad objective, and you enliven your own learning as well, every time you adopt a thesis that sets a challenging agenda both for you (as writer) and for your readers. Of course, once you set the challenge, you must be equal to the task. As a writer, you will need to discuss all the elements implied by your thesis.
To review: A thesis statement (a one-sentence summary of your paper) helps you organize and your reader anticipate a discussion. Thesis statements are distinguished by their carefully worded subjects and predicates, which should be just broad enough and complex enough to be developed within the length limitations of the assignment. Both novices and experts in a field typically begin the initial draft of a paper with a working thesis - a statement that provides writers with structure enough to get started but with latitude enough to discover what they want to say as they write. Once you have completed a first draft, you should test the "fit" of your thesis with the paper that follows. Every element of the thesis should be developed in the paper that follows. Discussions that drift from your thesis should be deleted, or the thesis changed to accommodate the new discussions.
A quotation records the exact language used by someone in speech or in writing. A summary, in contrast, is a brief restatement in your own words of what someone else has said or written. And a paraphrase is also a restatement, although one that is often as long as the original source. Any paper in which you draw upon sources will rely heavily on quotation, summary, and paraphrase. How do you choose among the three?
Remember that the papers you write should be your own - for the most part, your own language and certainly your own thesis, your own inferences, and your own conclusions. It follows that references to your source materials should be written primarily as summaries and paraphrases, both of which are built on restatement, not quotation. You will use summaries when you need a brief restatement, and paraphrases, which provide more explicit detail than summaries, when you need to follow the development of a source closely. When you quote too much, you risk losing ownership of your work: more easily than you might think, your voice can be drowned out by the voices of those you've quoted. So use quotations sparingly, as you would a pungent spice.
Nevertheless, quoting just the right source at the right time can significantly improve your papers. The trick is to know when and how to use quotations.
- Use quotations when another writer's language is particularly memorable and will add interest and liveliness to your paper.
- Use quotations when another writer's language is so clear and economical that to make the same point in your own words would, by comparison, be ineffective.
- Use quotations when you want the solid reputation of a source to lend authority and credibility to your own writing.
Quoting Memorable Language
Assume you're writing a paper on Napoleon Bonaparte's relationship with the celebrated Josephine. Through research you learn that two days after their marriage Napoleon, given command of an army, left his bride for what was to be a brilliant military campaign in Italy. How did the young general respond to leaving his wife so soon after their wedding? You come across the following, written from the field of battle by Napoleon on April 3, 1796:
I have received all your letters, but none has had such an impact on me as the last. Do you have any idea, darling, what you are doing, writing to me in those terms? Do you not think my situation cruel enough without intensifying my longing for you, overwhelming my soul? What a style! What emotions you evoke! Written in fire, they burn my poor heart!2
A summary of this passage might read as follows:
On April 3, 1796, Napoleon wrote to Josephine, expressing how sorely he missed her and how passionately he responded to her letters.
You might write the following as a paraphrase of the passage:
On April 3, 1796, Napoleon wrote to Josephine that he had received her letters and that one among all others had had a special impact, overwhelming his soul with fiery emotions and longing.
How feeble this summary and paraphrase are when compared with the original! Use the vivid language that your sources give you. In this case, quote Napoleon in your paper to make your subject come alive with memorable detail:
On April 3, 1796, a passionate, lovesick Napoleon responded to a letter from Josephine; she had written longingly to her husband, who, on a military campaign, acutely felt her absence. "Do you have any idea, darling, what you are doing, writing to me in those terms? . . . What emotions you evoke!" he said of her letters. "Written in fire, they burn.my poor heart!"
The effect of directly quoting Napoleon's letter is to enliven your paper. A direct quotation is one in which you record precisely the language of another, as we did with the sentences from Napoleon's letter. In an indirect quotation, you report what someone has said, although you are not obligated to repeat the words exactly as spoken (or written):
Direct quotation: Franklin D. Roosevelt said: "The only thing we have to fear is fear itself."
Indirect quotation: Franklin D. Roosevelt said that we have nothing to fear but fear itself.
The language in a direct quotation, which is indicated by a pair of quotation marks (" "), must be faithful to the language of the original passage. When using an indirect quotation, you have the liberty of changing words (although not changing meaning). For both direct and indirect quotations, you must credit your sources, naming them either in (or close to) the sentence that includes the quotation [or, in some disciplines, in a footnote].
Quoting Clear and Concise Language
You should quote a source when its language is particularly clear and economical - when your language, by contrast, would be wordy. Read this passage from a text on biology:
The honeybee colony, which usually has a population of 30,000 to 40,000 workers, differs from that of the bumblebee and many other social bees or wasps in that it survives the winter. This means that the bees must stay warm despite the cold. Like other bees, the isolated honeybee cannot fly if the temperature falls below 10°C (50°F) and cannot walk if the temperature is below 7°C (45°F). Within the wintering hive, bees maintain their temperature by clustering together in a dense ball; the lower the temperature, the denser the cluster. The clustered bees produce heat by constant muscular movements of their wings, legs, and abdomens. In very cold weather, the bees on the outside of the cluster keep moving toward the center, while those in the core of the cluster move to the colder outside periphery. The entire cluster moves slowly about on the combs, eating the stored honey from the combs as it moves.3
A summary of this paragraph might read as follows:
Honeybees, unlike many other varieties of bee, are able to live through the winter by "clustering together in a dense ball" for body warmth.
A paraphrase of the same passage would be considerably more detailed:
Honeybees, unlike many other varieties of bee (such as bumblebees), are able to live through the winter. The 30,000 to 40,000 bees within a honeybee hive could not, individually, move about in cold winter temperatures. But when "clustering together in a dense ball," the bees generate heat by constantly moving their body parts. The cluster also moves slowly about the hive, eating honey stored in the combs. This nutrition, in addition to the heat generated by the cluster, enables the honeybee to survive the cold winter months.
In both the summary and the paraphrase we've quoted Curtis's "clustering together in a dense ball," a phrase that lies at the heart of her description of wintering honeybees. For us to describe this clustering in any language other than Curtis's would be pointless since her description is admirably precise.
Quoting Authoritative Language
You will also want to use quotations that lend authority to your work. When quoting an expert or some prominent political, artistic, or historical figure, you elevate your own work by placing it in esteemed company. Quote respected figures to establish background information in a paper, and your readers will tend to perceive that information as reliable. Quote the opinions of respected figures to endorse some statement that you've made, and your statement becomes more credible to your readers. For example, in an essay that you might write on the importance of reading well, you could make use of a passage from Thoreau's Walden:
Reading well is hard work and requires great skill and training. It "is a noble exercise," writes Henry David Thoreau in Walden, "and one that will task the reader more than any exercise which the customs of the day esteem. It requires a training such as the athletes underwent.... Books must be read as deliberately and reservedly as they were written."
By quoting a famous philosopher and essayist on the subject of reading, you add legitimacy to your discussion. Not only do you regard reading to be a skill that is both difficult and important; so too does Henry David Thoreau, one of our most influential American thinkers. The quotation has elevated the level of your work.
You can also quote to advantage well-respected figures who've written or spoken about the subject of your paper. Here is a discussion of space flight. Author David Chandler refers to a physicist and an astronaut:
A few scientists - notably James Van Allen, discoverer of the Earth's radiation belts - have decried the expense of the manned space program and called for an almost exclusive concentration on unmanned scientific exploration instead, saying this would be far more cost-effective.
Other space scientists dispute that idea. Joseph Allen, physicist and former shuttle astronaut, says, "It seems to be argued that one takes away from the other. But before there was a manned space program, the funding on space science was zero. Now it's about $500 million a year."
Note, first, that in the first paragraph Chandler has either summarized or used an Indirect quotation to incorporate remarks made by James Van Allen into the discussion on space flight. In the second paragraph, Chandler directly quotes his next source, Joseph Allen. Both quotations, indirect and direct, lend authority and legitimacy to the article, for both James Van Allen and Joseph Allen are experts on the subject of space flight. Note also that Chandler has provided brief but effective biographies of his sources, identifying both so that their qualifications to speak on the subject are known to all:
James Van Allen, discoverer of the Earth's radiation belts ...
Joseph Allen, physicist and former shuttle astronaut ...
The phrases in italics are called appositives. Their function is to rename the nouns they follow by providing explicit, identifying detail. Any information about a person that can be expressed in the following sentence pattern can be made into an appositive phrase:
James Van Allen is the discoverer of the Earth's radiation belts.
James Van Allen has decried the expense of the manned space program
James Van Allen, discoverer of the Earth's radiation belts, has decried the expense of the manned space program.
Use appositives to identify authors whom you quote.
Incorporating Quotations into Your Sentences
Quoting Only the Part of a Sentence or Paragraph That You Need
As you've seen, a writer selects passages for quotation that are especially vivid and memorable, concise, or authoritative. Now we will put these principles into practice. Suppose that while conducting research on the topic of college sports you've come across the following, written by Robert Hutchins, former president of the University of Chicago:
If athleticism is bad for students, players, alumni and the public, it is even worse for the colleges and universities themselves. They want to be educational institutions, but they can't. The story of the famous halfback whose only regret, when he bade his coach farewell, was that he hadn't learned to read and write is probably exaggerated. But we must admit that pressure from trustees, graduates, "friends," presidents and even professors has tended to relax academic standards. These gentry often overlook the fact that a college should not be interested in a fullback who is a half-wit. Recruiting, subsidizing and the double educational standard cannot exist without the knowledge and the tacit approval, at least, of the colleges and universities themselves. Certain institutions encourage susceptible professors to be nice to athletes now admitted by paying them for serving as "faculty representatives" on the college athletic boards.4
Suppose that from this entire paragraph you find a gem, a quotable grouping of words that will enliven your discussion. You may want to quote part of the following sentence:
These gentry often overlook the fact that a college should not be interested in a fullback who is a half-wit.
Incorporating the Quotation into the Flow of Your Own Sentence
Once you've selected the passage you want to quote, work the material into your paper in as natural and fluid a manner as possible. Here's how we would quote Hutchins:
Robert Hutchins, a former president of the University of Chicago, asserts that "a college should not be interested in a fullback who is a half-wit."
Note that we've used an appositive to identify Hutchins. And we've used only the part of the paragraph - a single clause - that we thought memorable enough to quote directly.
Avoiding Freestanding Quotations
A quoted sentence should never stand by itself - as in the following example:
Various people associated with the university admit that the pressures of athleticism have caused a relaxation of standards. "These gentry often overlook the fact that a college should not be interested in a fullback who is a half-wit." But this kind of thinking is bad for the university and even worse for the athletes.
Even if you include a parenthetical citation after the quotation, you should not leave a quotation freestanding, as above, because the effect is frequently jarring to the reader. Introduce the quotation by attributing the source in some other part of the sentence - beginning, middle, or end. Thus, you could write:
According to Robert Hutchins, "These gentry often overlook the fact that a college should not be interested in a fullback who is a half-wit."
"These gentry," asserts Robert Hutchins, "often overlook the fact that a college should not be interested in a fullback who is a half-wit."
Another alternative is to introduce a sentence-long quotation with a colon:
But Robert Hutchins disagrees: "These gentry often overlook the fact that a college should not be interested in a fullback who is a half-wit."
Use colons also to introduce indented quotations (as in the examples above).
When attributing sources, try to vary the standard "states," "writes," "says," and so on. Other, stronger verbs you might consider: "asserts," "argues," "maintains," "insists," "asks," and even "wonders."
Using Ellipsis Marks
Using quotations is made somewhat complicated when you want to quote the beginning and end of a passage but not its middle - as was the case when we quoted Henry David Thoreau. Here's part of the paragraph in Walden from which we quoted a few sentences:
To read well, that is, to read true books in a true spirit, is a noble exercise, and one that will task the reader more than any exercise which the customs of the day esteem. It requires a training such as the athletes underwent, the steady intention almost of the whole life to this object. Books must be read as deliberately and reservedly as they were written.5
And here was how we used this material:
Reading well is hard work and requires great skill and training. It "is a noble exercise," writes Henry David Thoreau in Walden, "and one that will task the reader more than any exercise which the customs of the day esteem. It requires a training such as the athletes underwent.... Books must be read as deliberately and reservedly as they were written."
Whenever you quote a sentence but delete words from it, as we have done, indicate this deletion to the reader by placing an ellipsis mark, three spaced periods, in the sentence at the point of deletion. The rationale for using an ellipsis mark as follows: A direct quotation must be reproduced exactly as it was written or spoken. When writers delete or change any part of the quoted material, readers must be alerted so they don't think that the changes were part of the original. Ellipsis marks and brackets serve this purpose.
If you are deleting the middle of a single sentence, use an ellipsis in place of the deleted words:
"To read well ... is a noble exercise, and one that will task the reader more than any exercise which the customs of the day esteem."
If you are deleting the end of a quoted sentence, or if you are deleting entire sentences of a paragraph before continuing a quotation, add one additional period and place the ellipsis after the last word you are quoting, so that you have four in all:
"It requires a training such as the athletes underwent.... Books must be read as deliberately and reservedly as they were written."
If you begin your quotation of an author in the middle of a sentence, you need not indicate deleted words with an ellipsis. Be sure, however, that the syntax of the quotation fits smoothly with the syntax of your sentence:
Reading "is a noble exercise," writes Henry David Thoreau.
Use square brackets whenever you need to add or substitute words in a quoted sentence. The brackets indicate to the reader a word or phrase that does not appear in the original passage but that you have inserted to avoid confusion. For example, when a pronoun's antecedent would be unclear to readers, delete the pronoun from the sentence and substitute an identifying word or phrase in brackets. When you make such a substitution, no ellipsis marks are needed. Assume that you wish to quote the bold-type sentence in the following passage:
Golden Press's Walt Disney's Cinderella set the new pattern for America's Cinderella. This book's text is coy and condescending. (Sample: "And her best friends of all were - guess who - the mice!") The illustrations are poor cartoons. And Cinderella herself is a disaster. She cowers as her sisters rip her homemade ball gown to shreds. (Not even homemade by Cinderella, but by the mice and birds.) She answers her stepmother with whines and pleadings. She is a sorry excuse for a heroine, pitiable and useless. She cannot perform even a simple action to save herself, though she is warned by her friends, the mice. She does not hear them because she is "off in a world of dreams." Cinderella begs, she whimpers, and at last has to be rescued by - guess who - the mice!6
In quoting this sentence, you would need to identify whom the pronoun she refers to. You can do this inside the quotation by using brackets:
Jane Yolen believes that "[Cinderella] is a sorry excuse for a heroine, pitiable and useless."
If the pronoun begins the sentence to be quoted, as it does in this example, you can identify the pronoun outside of the quotation and simply begin quoting your source one word later:
Jane Yolen believes that Cinderella "is a sorry excuse for a heroine, pitiable and useless."
If the pronoun you want to identify occurs in the middle of the sentence to be quoted, then you'll need to use brackets. Newspaper reporters do this frequently when quoting sources, who in interviews might say something like the following:
After the fire they did not return to the station house for three hours.
If the reporter wants to use this sentence in an article, he or she needs to identify the pronoun:
An official from City Hall, speaking on the condition that he not be identified, said, "After the fire [the officers] did not return to the station house for three hours."
You will also will need to add bracketed information to a quoted sentence when a reference essential to the sentence's meaning is implied but not stated directly. Read the following paragraphs from Robert Jastrow's "Toward an Intelligence Beyond Man's":
These are amiable qualities for the computer; it imitates life like an electronic monkey. As computers get more complex, the imitation gets better. Finally, the line between the original and the copy becomes blurred. In another 15 years or so - two more generations of computer evolution, in the jargon of the technologists - we will see the computer as an emergent form of life.
The proposition seems ridiculous because, for one thing, computers lack the drives and emotions of living creatures. But when drives are useful, they can be programmed into the computer's brain, just as nature programmed them into our ancestors' brains as a part of the equipment for survival. For example, computers, like people, work better and learn faster when they are motivated. Arthur Samuel made this discovery when he taught two IBM computers how to play checkers. They polished their game by playing each other, but they learned slowly. Finally, Dr. Samuel programmed in the will to win by forcing the computers to try harder - and to think out more moves in advance - when they were losing. Then the computers learned very quickly. One of them beat Samuel and went on to defeat a champion player who had not lost a game to a human opponent in eight years.7
If you wanted to quote only the sentence in bold type, you would need to provide readers with a bracketed explanation; otherwise, the words "the proposition" would be unclear. Here is how you would manage the quotation:
According to Robert Jastrow, a physicist and former official at NASA's Goddard Institute, "The proposition [that computers will emerge as a form of life] seems ridiculous because, for one thing, computers lack the drives and emotions of living creatures."
Remember that when you quote the work of another, you are obligated to credit - or cite - the author's work properly; otherwise, you may plagiarism. [See your Allyn and Bacon Handbook for guidance on citing sources.]
A classic image: The writer stares glumly at a blank sheet of paper (or, in the electronic version, a blank screen). Usually, however, this is an image of a writer who hasn't yet begun to write. Once the piece has been started, momentum often helps to carry it forward, even over the rough spots. (These can always be fixed later.) As a writer, you've surely discovered that getting started when you haven't yet warmed to your task is a problem. What's the best way to approach your subject? With high seriousness, a light touch, an anecdote? How best to engage your reader?
Many writers avoid such agonizing choices by putting them off - productively. Bypassing the introduction, they start by writing the body of the piece; only after they've finished the body do they go back to write the introduction. There's a lot to be said for this approach. Because you have presumably spent more time thinking about the topic itself than about how you're going to introduce it, you are in a better position, at first, to begin directly with your presentation (once you've settled on a working thesis). And often, it's not until you've actually seen the piece on paper and read it over once or twice that a "natural" way of introducing it becomes apparent. Even if there is no natural way to begin, you are generally in better psychological shape to write the introduction after the major task of writing is behind you and you know exactly what you're leading up to.
Perhaps, however, you can't operate this way. After all, you have to start writing somewhere, and if you have evaded the problem by skipping the introduction, that blank page may loom just as large wherever you do choose to begin. If this is the case, then go ahead and write an introduction, knowing full well that it's probably going to be flat and awful. Set down any kind of pump- priming or throat-clearing verbiage that comes to mind, as long as you have a working thesis. Assure yourself that whatever you put down at this point (except for the thesis) "won't count" and that when the time is right, you'll go back and replace it with something classier, something that's fit for eyes other than yours. But in the meantime, you'll have gotten started.
The purpose of an introduction is to prepare the reader to enter the world of your essay. The introduction makes the connection between the more familiar world inhabited by the reader and the less familiar world of the writer's particular subject; it places a discussion in a context that the reader can understand.
There are many ways to provide such a context. We'll consider just a few of the most common.
In introduction to a paper on democracy:
"Two cheers for democracy" was E. M. Forster's not-quite-wholehearted judgment. Most Americans would not agree. To them, our democracy is one of the glories of civilization. To one American in particular, E. B. White, democracy is "the hole in the stuffed shirt through which the sawdust slowly trickles . . . the dent in the high hat . . . the recurrent suspicion that more than half of the people are right more than half of the time" (915). American democracy is based on the oldest continuously operating written constitution in the world - a most impressive fact and a testament to the farsightedness of the founding fathers. But just how farsighted can mere humans be? In Future Shock, Alvin Toffler quotes economist Kenneth Boulding on the incredible acceleration of social change in our time: "The world of today . . . is as different from the world in which I was born as that world was from Julius Caesar's" (13). As we move toward the twenty-first century, it seems legitimate to question the continued effectiveness of a governmental system that was devised in the eighteenth century; and it seems equally legitimate to consider alternatives.
The quotations by Forster and White help set the stage for the discussion of democracy by presenting the reader with some provocative and well-phrased remarks. Later in the paragraph, the quotation by Boulding more specifically prepares us for the theme of change that will be central to the essay as a whole.
In many cases, the reader will be unprepared to follow the issue you discuss unless you provide some historical background. Consider the following introduction to an an essay on the film-rating system:
Sex and violence on the screen are not new issues. In the Roaring Twenties there was increasing pressure from civic and religious groups to ban depictions of "immorality" from the screen. Faced with the threat of federal censorship, the film producers decided to clean their own house. In 1930, the Motion Picture Producers and Distributors of America established the Production Code. At first, adherence to the Code was voluntary; but in 1934 Joseph Breen, newly appointed head of the MPPDA, gave the Code teeth. Henceforth all newly produced films had to be submitted for approval to the Production Code Administration which had the power to award or withhold the Code seal. Without a Code seal, it was virtually impossible for a film to be shown anywhere in the United States, since exhibitors would not accept it. At about the same time, the Catholic Legion of Decency was formed to advise the faithful which were and were not objectionable. For several decades the Production Code Administration exercised powerful control over what was portrayed in American theatrical films. By the 1960s, however, changing standards of morality had considerably weakened the Code's grip. In 1968, the Production Code was replaced with a rating system designed to keep younger audiences away from films with high levels of sex or violence. Despite its imperfections, this rating system has proved more beneficial to American films than did the old censorship system.
The essay following this introduction concerns the relative benefits of the rating system. By providing some historical background on the rating system, the writer helps readers to understand his arguments. Notice the chronological development of details.
Review of a Controversy
A particular type of historical review is the review of a controversy or debate. Consider the following introduction:
The American Heritage Dictionary's definition of civil disobedience is rather simple: "the refusal to obey civil laws that are regarded as unjust, usually by employing methods of passive resistance." However, despite such famous (and beloved) examples of civil disobedience as the movements of Mahatma Gandhi in India and the Reverend Martin Luther King, Jr., in the United States, the question of whether or not civil disobedience should be considered an asset to society is hardly clear cut. For instance, Hannah Arendt, in her article "Civil Disobedience," holds that"to think of disobedient minorities as rebels and truants is against the letter and spirit of a constitution whose framers were especially sensitive to the dangers of unbridled majority rule." On the other hand, a noted lawyer, Lewis Van Dusen, Jr., in his article "Civil Disobedience: Destroyer of Democracy," states that "civil disobedience, whatever the ethical rationalization, is still an assault on our democratic society, an affront to our legal order and an attack on our constitutional government." These two views are clearly incompatible. I believe, though, that Van Dusen's is the more convincing. On balance, civil disobedience is dangerous to society.8
The negative aspects of civil disobedience, rather than Van Dusen's essay, the topic of this essay. But to introduce this topic, the writer has provided quotations that represent opposing sides of the controversy over civil disobedience, as well as brief references to two controversial practitioners. By focusing at the outset on the particular rather than the abstract aspects of the subject, the writer hoped to secure the attention of her readers and to involve them in the controversy that forms the subject of her essay.
From General to the Specific
Another way of providing a transition from the reader's world to the less familiar world of the essay is to work from a general subject to a specific one. The following introduction to a discussion of the 1968 massacre at My Lai, Vietnam, begins with general statements and leads to the particular subject at hand:
Though we prefer to think of man as basically good and reluctant to do evil, such is not the case. Many of the crimes inflicted on humankind can be dismissed as being committed by the degenerates of society at the prompting of the abnormal mind. But what of the perfectly "normal" man or woman who commits inhumane acts simply because he or she has been ordered to do so? It cannot be denied that such acts have occurred, either in everyday life or in war-time situations. Unfortunately, even normal, well-adjusted people can become cruel, inhumane, and destructive if placed in the hands of unscrupulous authority. Such was the case in the village of My Lai, Vietnam, on March 16, 1968, when a platoon of American soldiers commanded by Lt. William Calley massacred more than 100 civilians, including women and children.
Specific to the General: Anecdote, Illustration
Consider the following paragraph:
In late 1971 astronomer Carl Sagan and his colleagues were studying data transmitted from the planet Mars to the earth by the Mariner 9 spacecraft. Struck by the effects of the Martian dust storms on the temperature and on the amount of light reaching the surface, the scientists wondered about the effects on earth of the dust storms that would be created by nuclear explosions. Using computer models, they simulated the effects of such explosions on the earth's climate. The results astounded them. Apart from the known effects of nuclear blasts (fires and radiation), the earth, they discovered, would become enshrouded in a "nuclear winter." Following a nuclear exchange, plummeting temperatures and pervading darkness would destroy most of the Northern Hemisphere's crops and farm animals and would eventually render much of the planet's surface uninhabitable. The effects of nuclear war, apparently, would be more catastrophic than had previously been imagined. It has therefore become more urgent than ever for the nations of the world to take dramatic steps to reduce the threat of nuclear war.
The previous introduction went from the general (the question of whether or not man is basically good) to the specific (the massacre at My Lai); this one goes from the specific (scientists studying data) to the general (the urgency of reducing the nuclear threat). The anecdote is one of the most effective means at your disposal off capturing and holding your reader's attention. For decades, speakers have begun their general remarks with a funny, touching, or otherwise appropriate story; in fact, there are plenty of books that are nothing but collections of such stories, arranged by subject.
Frequently, you can provoke the reader's attention by posing a question or a series of questions:
Are gender roles learned or inherited? Scientific research has established the existence of biological differences between the sexes, but the effect of biology's influence on gender roles cannot be distinguished from society's influence. According to Michael Lewis of the Institute for the Study of Exceptional children, "As early as you can show me a sex difference, I can show you the culture at work." Social processes, as well as biological differences, are responsible for the separate roles of men and women.9
Opening your essay with a question can be provocative, since it places the reader in an active role: He or she begins by considering answers. Are gender roles learned? Are they inherited? In this active role, the reader is likely to continue reading with interest.
Statement of Thesis
Perhaps the most direct method of introduction is to begin immediately with the thesis:
Computers are a mixed blessing. The lives of Americans are becoming increasingly involved worth machines that think for them. "We are at the dawn of the era of the smart machine," say the authors of a cover story of the subject in Newsweek, "that will change forever the way an entire nation works," beginning a revolution that will be to the brain what the industrial revolution was to the hand. Tiny silicon chips already process enough information to direct air travel, to instruct machines how to cut fabric - even to play chess with (and defeat) the masters. One can argue that development of computers for the household, as well as industry, will change for the better the quality of our lives: computers help us save energy, reduce the amount of drudgery that most of us endure around tax season, make access to libraries easier. Yet there is a certain danger involved with this proliferation of technology.
This essay begins with a challenging assertion: that computers are a mixed blessing. It is one that many readers are perhaps unprepared to consider, since they may have taken it for granted that computers are an unmixed blessing. The advantage of beginning with a provocative (thesis) statement is that it forces the reader to sit up and take notice perhaps even to begin protesting. The paragraph goes on to concede some of the "blessings" of computerization but then concludes with the warning that there is "a certain danger" associated with the new technology - a danger, the curious or even indignant reader has a right to conclude, that will be more fully explained in the paragraphs to follow.
One final note about our model introductions: They may be longer than introductions you have been accustomed to writing. Many writers (and readers) prefer shorter, snappier introductions. This is largely a matter of personal or corporate style: there is no rule concerning the correct length of an introduction. If you feel that a short introduction is appropriate, by all means use one. You may wish to break up what seems like a long introduction into two paragraphs. (Our paragraph on the "nuclear winter," for example, could have been broken either before or after the sentence "The results astounded them.")
One way to view the conclusion of your paper is as an introduction worked in reverse, a bridge from the world of your essay back to the world of your reader. A conclusion is the part of your paper in which you restate and (if necessary) expand on your thesis. Essential to any conclusion is the summary, which is not merely a repetition of the thesis but a restatement that takes advantage of the material you've presented. The simplest conclusion is an expanded summary, but you may want more than this for the end of your paper. Depending on your needs, you might offer a summary and then build onto it a discussion of the paper's significance or its implications for future study, for choices that individuals might make, for policy, and so on. you might also want to urge the reader to change an attitude or to modify behavior. Certainly, you are under no obligation to discuss the broader significance of your work (and a summary, alone, will satisfy the formal requirement that your paper have an ending); but the conclusions of better papers often reveal authors who are "thinking large" and want to connect the particular concerns of their papers with the broader concerns of society.
Here we'll consider seven strategies for expanding the basic summary - conclusion. But two words of advice are in order. First, no matter how clever or beautifully executed, a conclusion cannot salvage a poorly written paper. Second, by virtue of its placement, the conclusion carries rhetorical weight. It is the last statement a reader will encounter before turning from your work. Realizing this, writers who expand on the basic summary-conclusion often wish to give their final words a dramatic flourish, a heightened level of diction. Soaring rhetoric and drama in a conclusion are fine as long as they do not unbalance the paper and call attention to themselves. Having labored long hours over your paper, you have every right to wax eloquent. But keep a sense of proportion and timing. Make your points quickly and end crisply.
Statement of the Subject's Significance
One of the more effective ways to conclude a paper is to discuss the larger significance of what you have written, providing readers with one more reason to regard your work as a serious effort. When using this strategy, you move from the specific concern of your paper to the broader concerns of the reader's world. Often, you will need to choose among a range of significances: A paper on the Wright brothers might end with a discussion of air travel as it affects economies, politics, or families; a paper on contraception might end with a discussion of its effect on sexual mores, population, or the church. But don't overwhelm your reader with the importance of your remarks. Keep your discussion well focused.
The following paragraphs conclude a paper on George H. Shull, a pioneer in the inbreeding and crossbreeding of corn:
. . . Thus, the hybrids developed and described by Shull 75 years ago have finally dominated U.S. corn production.
The adoption of hybrid corn was steady and dramatic in the Corn Belt. From 1930 through 1979 the average yields of corn in the U.S. increased from 21.9 to 95.1 bushels per acre, and the additional value to the farmer is now several billion dollars per year.
The success of hybrid corn has also stimulated the breeding of other crops, such as sorghum hybrids, a major feed grain crop in arid parts of the world. Sorghum yields have increased 300 percent since 1930. Approximately 20 percent of the land devoted to rice production in China is planted with hybrid seed, which is reported to yield 20 percent more than the best varieties. And many superior varieties of tomatoes, cucumbers, spinach, and other vegetables are hybrids. Today virtually all corn produced in the developed countries is from hybrid seed. From those blue bloods of the plant kingdom has come a model for feeding the world.10
The first sentence of this conclusion is a summary, and from it the reader can infer that the paper included a discussion of Shull's techniques for the hybrid breeding of corn. The summary is followed by a two-paragraph discussion on the significance of Shull's research for feeding the world.
Call for Further Research
In the scientific and social scientific communities, papers often end with a review of what has been presented (as, for instance, in an experiment) and the ways in which the subject under consideration needs to be further explored. If you raise questions that you call on others to answer, however, make sure you know that the research you are calling for hasn't already been conducted.
This next conclusion comes from a sociological report on the placement of elderly men and women in nursing homes.
Thus, our study shows a correlation between the placement of elderly citizens in nursing facilities and the significant decline of their motor and intellectual skills over the ten months following placement. What the research has not made clear is the extent to which this marked decline is due to physical as opposed to emotional causes. The elderly are referred to homes at that point in their lives when they grow less able to care for themselves - which suggests that the drop-off in skills may be due to physical causes. But the emotional stress of being placed in a home, away from family and in an environment that confirms the patient's view of himself as decrepit, may exacerbate - if not itself be a primary cause of - the patient's rapid loss of abilities. Further research is needed to clarify the relationship between depression and particular physical ailments as these affect the skills of the elderly in nursing facilities. There is little doubt that information yielded by such studies can enable health care professionals to deliver more effective services.
Notice how this call for further study locates the author in a large community of researchers on whom she depends for assistance in answering the questions that have come out of her own work. The author summarizes her findings (in the first sentence of the paragraph), states what her work has not shown, and then extends her invitation.
The purpose of your paper might be to review a problem or controversy and to discuss contributing factors. In such a case, it would be appropriate, after summarizing your discussion, to offer a solution based on the knowledge you've gained while conducting research. If your solution is to be taken seriously, your knowledge must be amply demonstrated in the body of the paper.
. . . The major problem in college sports today is not commercialism - it is the exploitation of athletes and the proliferation of illicit practices which dilute educational standards.
Many universities are currently deriving substantial benefits from sports programs that depend on the labor of athletes drawn from the poorest sections of America's population. It is the responsibility of educators, civil rights leaders, and concerned citizens to see that these young people get a fair return for their labor both in terms of direct remuneration and in terms of career preparation for a life outside sports.
Minimally, scholarships in revenue-producing sports should be designed to extend until graduation, rather than covering only four years of athletic eligibility, and should include guarantees of tutoring, counseling, and proper medical care. At institutions where the profits are particularly large (such as Texas A &M, which can afford to pay its football coach $280,000 a year), scholarships should also provide salaries that extend beyond room, board, and tuition. The important thing is that the athlete be remunerated fairly and have the opportunity to gain skills from a university environment without undue competition from a physically and psychologically demanding full-time job. This may well require that scholarships be extended over five or six years, including summers.
Such a proposal, I suspect, will not be easy to implement. The current amateur system, despite its moral and educational flaws, enables universities to hire their athletic labor at minimal cost. But solving the fiscal crisis of the universities on the backs of America's poor and minorities is not, in the long run, a tenable solution. With the support of concerned educators, parents, and civil rights leaders, and with the help from organized labor, the college athlete, truly a sleeping giant, will someday speak out and demand what is rightly his - and hers - a fair share of the revenue created by their hard work.11
In this conclusion, the author summarizes his article in one sentence: "The major problem in college sports today is not commercialism - it is the exploitation of athletes and the proliferation of illicit practices which dilute educational standards." In paragraph 2, he continues with an analysis of the problem just stated and follows with a general recommendation - that "educators, civil rights leaders, and concerned citizens" be responsible for the welfare of college athletes. In paragraph 3, he makes a specific proposal, and in the final paragraph, he anticipates resistance to the proposal. He concludes by discounting this resistance and returning to the general point, that college athletes should receive a fair deal.
An anecdote is a briefly told story or joke, the point of which in a conclusion is to shed light on your subject. The anecdote is more direct than an allusion. With an allusion, you merely refer to a story ("Too many people today live in Plato's cave . . ."); with the anecdote, you actually retell the story. The anecdote allows readers to discover for themselves the significance of a reference to another source - an effort most readers enjoy because they get to exercise their creativity.
The following anecdote concludes an article on homicide. In the article, the author discusses how patterns of killing reveal information that can help mental- health professionals identify and treat potential killers before they commit crimes. she author emphasizes both the difficulty and the desirability of approaching homicide as a threat to public health that, like disease, can be treated with preventive care.
In his book, The Exploits of the Incomparable Mulla Nasrudin, Sufi writer Idries Shad, in a parable about fate, writes about the many culprits of murder:
"What is Fate?" Nasrudin was asked by a scholar.
"An endless succession of intertwined events, each influencing the other."
"That is hardly a satisfactory answer. I believe in cause and effect."
"Very well," said the Mulla, "look at that." He pointed to a procession passing in the street.
"That man is being taken to be hanged. Is that because someone gave him a silver piece and enabled him to buy the knife with which he committed the murder; or because someone saw him do it; or because nobody stopped him?"12
The writer chose to conclude the article with this anecdote. She could have developed an interpretation, but this would have spoiled the dramatic value for the reader. The purpose of using an anecdote is to make your point with subtlety, so resist the temptation to interpret. Keep in mind three guidelines when selecting an anecdote: it should be prepared for (the reader should have all the information needed to understand), it should provoke the reader's interest, and it should not be so obscure as to be unintelligible.
A favorite concluding device is the quotation - the words of a famous person or an authority in the field on which you are writing The purpose of quoting another is to link your work to theirs, thereby gaining for your work authority and credibility. The first criterion for selecting a quotation is its suitability to your thesis. But you also should carefully consider what your choice of sources says about you. Suppose you are writing a paper on the American work ethic. If you could use a line by comedian David Letterman or one by the current secretary of labor to make the final point of your conclusion, which would you choose and why? One source may not be inherently more effective than the other, but the choice certainly sets a tone for the paper.
There is no doubt that machines will get smarter and smarter, even designing their own software and making new and better chips for new generations of computers.... More and more of their power will be devoted to making them easier to use - "friendly," in industry parlance - even for those not trained in computer science. And computer scientists expect that public ingenuity will come up with applications the most visionary researchers have not even considered. One day, a global network of smart machines will be exchanging rapid-fire bursts of information at unimaginable speeds. If they are used wisely, they could help mankind to educate its masses and crack new scientific frontiers. "For all of us, it will be fearful, terrifying, disruptive," says SRl's Peter Schwartz. In the end there will be those whose lives will be diminished. But for the vast majority, their lives will be greatly enhanced." In any event, there is no turning back: if the smart machines have not taken over, they are fast making themselves indispensable - and in the end, that may amount to very much the same thing.13
Notice how the quotation is used to position the writer to make one final remark.
Particularly effective quotations may themselves be used to end an essay, as in the following example. Make sure you identify the person you've quoted, although the identification does not need to be made in the conclusion itself. For example, earlier in the paper from which the following conclusion was taken, Maureen Henderson was identified as an epidemiologist exploring the ways in which a change in diet can prevent the onset of certain cancers.
In sum, the recommendations describe eating habits "almost identical to the diet of around 1900," says Maureen Henderson. "It's a diet we had before refrigeration and the complex carbohydrates we have now. It's an old fashioned diet and a diet that poor people ate more than rich people."
Physical science, the systematic study of the inorganic world, as distinct from the study of the organic world, which is the province of biological science. Physical science is ordinarily thought of as consisting of four broad areas: astronomy, physics, chemistry, and the Earth sciences. Each of these is in turn divided into fields and subfields. This article discusses the historical development—with due attention to the scope, principal concerns, and methods—of the first three of these areas. The Earth sciences are discussed in a separate article.
Physics, in its modern sense, was founded in the mid-19th century as a synthesis of several older sciences—namely, those of mechanics, optics, acoustics, electricity, magnetism, heat, and the physical properties of matter. The synthesis was based in large part on the recognition that the different forces of nature are related and are, in fact, interconvertible because they are forms of energy.
The boundary between physics and chemistry is somewhat arbitrary. As it developed in the 20th century, physics is concerned with the structure and behaviour of individual atoms and their components, while chemistry deals with the properties and reactions of molecules. These latter depend on energy, especially heat, as well as on atoms; hence, there is a strong link between physics and chemistry. Chemists tend to be more interested in the specific properties of different elements and compounds, whereas physicists are concerned with general properties shared by all matter. (Seechemistry: The history of chemistry.)
Astronomy is the science of the entire universe beyond Earth; it includes Earth’s gross physical properties, such as its mass and rotation, insofar as they interact with other bodies in the solar system. Until the 18th century, astronomers were concerned primarily with the Sun, Moon, planets, and comets. During the following centuries, however, the study of stars, galaxies, nebulas, and the interstellar medium became increasingly important. Celestial mechanics, the science of the motion of planets and other solid objects within the solar system, was the first testing ground for Newton’s laws of motion and thereby helped to establish the fundamental principles of classical (that is, pre-20th-century) physics. Astrophysics, the study of the physical properties of celestial bodies, arose during the 19th century and is closely connected with the determination of the chemical composition of those bodies. In the 20th century physics and astronomy became more intimately linked through cosmological theories, especially those based on the theory of relativity. (Seeastronomy: History of astronomy.)
Heritage of antiquity and the Middle Ages
The physical sciences ultimately derive from the rationalistic materialism that emerged in classical Greece, itself an outgrowth of magical and mythical views of the world. The Greek philosophers of the 6th and 5th centuries bce abandoned the animism of the poets and explained the world in terms of ordinarily observable natural processes. These early philosophers posed the broad questions that still underlie science: How did the world order emerge from chaos? What is the origin of multitude and variety in the world? How can motion and change be accounted for? What is the underlying relation between form and matter? Greek philosophy answered these questions in terms that provided the framework for science for approximately 2,000 years.
Ancient Middle Eastern and Greek astronomy
Western astronomy had its origins in Egypt and Mesopotamia. Egyptian astronomy, which was neither a very well-developed nor an influential study, was largely concerned with time reckoning. Its main lasting contribution was the civil calendar of 365 days, consisting of 12 months of 30 days each and five additional festival days at the end of each year. This calendar played an important role in the history of astronomy, allowing astronomers to calculate the number of days between any two sets of observations.
Babylonian astronomy, dating back to about 1800 bce, constitutes one of the earliest systematic, scientific treatments of the physical world. In contrast to the Egyptians, the Babylonians were interested in the accurate prediction of astronomical phenomena, especially the first appearance of the new Moon. Using the zodiac as a reference, by the 4th century bce, they developed a complex system of arithmetic progressions and methods of approximation by which they were able to predict first appearances. The mass of observations they collected and their mathematical methods were important contributions to the later flowering of astronomy among the Greeks.
The Pythagoreans (5th century bce) were responsible for one of the first Greek astronomical theories. Believing that the order of the cosmos is fundamentally mathematical, they held that it is possible to discover the harmonies of the universe by contemplating the regular motions of the heavens. Postulating a central fire about which all the heavenly bodies including Earth and the Sun revolve, they constructed the first physical model of the solar system. Subsequent Greek astronomy derived its character from a comment ascribed to Plato, in the 4th century bce, who is reported to have instructed the astronomers to “save the phenomena” in terms of uniform circular motion. That is to say, he urged them to develop predictively accurate theories using only combinations of uniform circular motion. As a result, Greek astronomers never regarded their geometric models as true or as being physical descriptions of the machinery of the heavens. They regarded them simply as tools for predicting planetary positions.
Eudoxus of Cnidus (4th century bce) was the first of the Greek astronomers to rise to Plato’s challenge. He developed a theory of homocentric spheres, a model that represented the universe by sets of nesting concentric spheres the motions of which combined to produce the planetary and other celestial motions. Using only uniform circular motions, Eudoxus was able to “save” the rather complex planetary motions with some success. His theory required four homocentric spheres for each planet and three each for the Sun and Moon. The system was modified by Callippus, a student of Eudoxus, who added spheres to improve the theory, especially for Mercury and Venus. Aristotle, in formulating his cosmology, adopted Eudoxus’s homocentric spheres as the actual machinery of the heavens. The Aristotelian cosmos was like an onion consisting of a series of some 55 spheres nested about Earth, which was fixed at the centre. In order to unify the system, Aristotle added spheres in order to “unroll” the motions of a given planet so that they would not be transmitted to the next inner planet.
The theory of homocentric spheres failed to account for two sets of observations: (1) brightness changes suggesting that planets are not always the same distance from Earth, and (2) bounded elongations (i.e., Venus is never observed to be more than about 48° and Mercury never more than about 24° from the Sun). Heracleides of Pontus (4th century bce) attempted to solve these problems by having Venus and Mercury revolve about the Sun, rather than Earth, and having the Sun and other planets revolve in turn about Earth, which he placed at the centre. In addition, to account for the daily motions of the heavens, he held that Earth rotates on its axis. Heracleides’ theory had little impact in antiquity except perhaps on Aristarchus of Samos (3rd century bce), who apparently put forth a heliocentric hypothesis similar to the one Copernicus was to propound in the 16th century.
Hipparchus (flourished 130 bce) made extensive contributions to both theoretical and observational astronomy. Basing his theories on an impressive mass of observations, he was able to work out theories of the Sun and Moon that were more successful than those of any of his predecessors. His primary conceptualtool was the eccentric circle, a circle in which Earth is at some point eccentric to the geometric centre. He used this device to account for various irregularities and inequalities observed in the motions of the Sun and Moon. He also proved that the eccentric circle is mathematically equivalent to a geometric figure called an epicycle-deferent system, a proof probably first made by Apollonius of Perga a century earlier.
Among Hipparchus’s observations, one of the most significant was that of the precession of the equinoxes—i.e., a gradual apparent increase in longitude between any fixed star and the equinoctial point (either of two points on the celestial sphere where the celestial equator crosses the ecliptic). Thus, the north celestial pole, the point on the celestial sphere defined as the apparent centre of rotation of the stars, moves relative to the stars in its vicinity. In the heliocentric theory, this effect is ascribed to a change in Earth’s rotational axis, which traces out a conical path around the axis of the orbital plane.
Ptolemy (flourished 140 ce) applied the theory of epicycles to compile a systematic account of Greek astronomy. He elaborated theories for each of the planets, as well as for the Sun and Moon. His theory generally fitted the data available to him with a good degree of accuracy, and his book, the Almagest, became the vehicle by which Greek astronomy was transmitted to astronomers of the Middle Ages and Renaissance. It essentially molded astronomy for the next millennium and a half.
Several kinds of physical theories emerged in ancient Greece, including both generalized hypotheses about the ultimate structure of nature and more specific theories that considered the problem of motion from both metaphysical and mathematical points of view. Attempting to reconcile the antithesis between the underlying unity and apparent multitude and diversity of nature, the Greek atomists Leucippus (mid-5th century bce), Democritus (late 5th century bce), and Epicurus (late 4th and early 3rd century bce) asserted that nature consists of immutable atoms moving in empty space. According to this theory, the various motions and configurations of atoms and clusters of atoms are the causes of all the phenomena of nature.
In contrast to the particulate universe of the atomists, the Stoics (principally Zeno of Citium, bridging 4th and 3rd centuries bce, Chrysippus [3rd century bce], and Poseidonius of Apamea [flourished c. 100 bce]) insisted on the continuity of nature, conceiving of both space and matter as continuous and as infused with an active, airlike spirit—pneuma—which serves to unify the frame of nature. The inspiration for the Stoic emphasis on pneumatic processes probably arose from earlier experiences with the “spring” (i.e., compressibility and pressure) of the air. Neither the atomic theory nor Stoic physics survived the criticism of Aristotle and his theory.
In his physics, Aristotle was primarily concerned with the philosophical question of the nature of motion as one variety of change. He assumed that a constant motion requires a constant cause; that is to say, as long as a body remains in motion, a force must be acting on that body. He considered the motion of a body through a resisting medium as proportional to the force producing the motion and inversely proportional to the resistance of the medium. Aristotle used this relationship to argue against the possibility of the existence of a void, for in a void resistance is zero, and the relationship loses meaning. He considered the cosmos to be divided into two qualitatively different realms, governed by two different kinds of laws. In the terrestrial realm, within the sphere of the Moon, rectilinear up-and-down motion is characteristic. Heavy bodies, by their nature, seek the centre and tend to move downward in a natural motion. It is unnatural for a heavy body to move up, and such unnatural or violent motion requires an external cause. Light bodies, in direct contrast, move naturally upward. In the celestial realm, uniform circular motion is natural, thus producing the motions of the heavenly bodies.
Archimedes (3rd century bce) fundamentally applied mathematics to the solution of physical problems and brilliantly employed physical assumptions and insights leading to mathematical demonstrations, particularly in problems of statics and hydrostatics. He was thus able to derive the law of the lever rigorously and to deal with problems of the equilibrium of floating bodies.
Islamic and medieval science
Greek science reached a zenith with the work of Ptolemy in the 2nd century ce. The lack of interest in theoretical questions in the Roman world reduced science in the Latin West to the level of predigested handbooks and encyclopaedias that had been distilled many times. Social pressures, political persecution, and the anti-intellectual bias of some of the early Church Fathers drove the few remaining Greek scientists and philosophers to the East. There they ultimately found a welcome when the rise of Islam in the 7th century stimulated interest in scientific and philosophical subjects. Most of the important Greek scientific texts were preserved in Arabic translations. Although the Muslims did not alter the foundations of Greek science, they made several important contributions within its general framework. When interest in Greek learning revived in western Europe during the 12th and 13th centuries, scholars turned to Islamic Spain for the scientific texts. A spate of translations resulted in the revival of Greek science in the West and coincided with the rise of the universities. Working within a predominantly Greek framework, scientists of the late Middle Ages reached high levels of sophistication and prepared the ground for the scientific revolution of the 16th and 17th centuries.
Mechanics was one of the most highly developed sciences pursued in the Middle Ages. Operating within a fundamentally Aristotelian framework, medieval physicists criticized and attempted to improve many aspects of Aristotle’s physics.
The problem of projectile motion was a crucial one for Aristotelian mechanics, and the analysis of this problem represents one of the most impressive medieval contributions to physics. Because of the assumption that continuation of motion requires the continued action of a motive force, the continued motion of a projectile after losing contact with the projector required explanation. Aristotle himself had proposed explanations of the continuation of projectile motion in terms of the action of the medium. The ad hoc character of these explanations rendered them unsatisfactory to most of the medieval commentators, who nevertheless retained the fundamental assumption that continued motion requires a continuing cause.
The most fruitful alternative to Aristotle’s attempts to explain projectile motion resulted from the concept of impressed force. According to this view, there is an incorporeal motive force that is imparted to the projectile, causing it to continue moving. Such views were espoused by John Philoponus of Alexandria (flourished 6th century), Avicenna, the Persian philosopher (died 1037), and the Arab Abū al Barakāt al-Baghdādi (died 1164). In the 14th century the French philosopher Jean Buridan developed a new version of the impressed-force theory, calling the quality impressed on the projectile “impetus.” Impetus, a permanent quality for Buridan, is measurable by the initial velocity of the projectile and by the quantity of matter contained in it. Buridan employed this concept to suggest an explanation of the everlasting motions of the heavens.
During the 1300s certain Oxford scholars pondered the philosophical problem of how to describe the change that occurs when qualities increase or decrease in intensity and came to consider the kinematic aspects of motion. Dealing with these problems in a purely hypothetical manner without any attempt to describe actual motions in nature or to test their formulas experimentally, they were able to derive the result that in a uniformly accelerated motion, distance increases as the square of the time.
Although medieval science was deeply influenced by Aristotle’s philosophy, adherence to his point of view was by no means dogmatic. During the 13th century, theologians at the University of Paris were disturbed by certain statements in Aristotle that seemed to imply limitations of God’s powers as well as other statements, such as the eternity of the world, which stood in apparent contradiction to scripture. In 1277 Pope John XXI condemned 219 propositions, many from Aristotle and St. Thomas Aquinas, which had clearly theological consequences. Many of these condemned propositions had scientific implications as well. For example, one of these propositions states, “That the first cause (i.e., God) could not make several worlds.” Although it is unlikely that anyone in the Middle Ages actually asserted the existence of many worlds, the condemnation led to the discussion of that possibility, as well as other important problems such as the possibility that Earth moved.
The scientific revolution
During the 15th, 16th, and 17th centuries, scientific thought underwent a revolution. A new view of nature emerged, replacing the Greek view that had dominated science for almost 2,000 years. Science became an autonomousdiscipline, distinct from both philosophy and technology, and it came to be regarded as having utilitarian goals. By the end of this period, it may not be too much to say that science had replaced Christianity as the focal point of European civilization. Out of the ferment of the Renaissance and Reformation there arose a new view of science, bringing about the following transformations: the reeducation of common sense in favour of abstract reasoning; the substitution of a quantitative for a qualitative view of nature; the view of nature as a machine rather than as an organism; the development of an experimental method that sought definite answers to certain limited questions couched in the framework of specific theories; the acceptance of new criteria for explanation, stressing the “how” rather than the “why” that had characterized the Aristotelian search for final causes.
The scientific revolution began in astronomy. Although there had been earlier discussions of the possibility of Earth’s motion, the Polish astronomer Nicolaus Copernicus was the first to propound a comprehensive heliocentric theory equal in scope and predictive capability to Ptolemy’s geocentric system. Motivated by the desire to satisfy Plato’s dictum, Copernicus was led to overthrow traditional astronomy because of its alleged violation of the principle of uniform circular motion and its lack of unity and harmony as a system of the world. Relying on virtually the same data as Ptolemy had possessed, Copernicus turned the world inside out, putting the Sun at the centre and setting Earth into motion around it. Copernicus’s theory, published in 1543, possessed a qualitative simplicity that Ptolemaic astronomy appeared to lack. To achieve comparable levels of quantitative precision, however, the new system became just as complex as the old. Perhaps the most revolutionary aspect of Copernican astronomy lay in Copernicus’s attitude toward the reality of his theory. In contrast to Platonic instrumentalism, Copernicus asserted that to be satisfactory astronomy must describe the real, physical system of the world.
The reception of Copernican astronomy amounted to victory by infiltration. By the time large-scale opposition to the theory had developed in the church and elsewhere, most of the best professional astronomers had found some aspect or other of the new system indispensable. Copernicus’s book De revolutionibus orbium coelestium libri VI (“Six Books Concerning the Revolutions of the Heavenly Orbs”), published in 1543, became a standard reference for advanced problems in astronomical research, particularly for its mathematical techniques. Thus, it was widely read by mathematical astronomers, in spite of its central cosmological hypothesis, which was widely ignored. In 1551 the German astronomer Erasmus Reinhold published the Tabulae prutenicae (“Prutenic Tables”), computed by Copernican methods. The tables were more accurate and more up-to-date than their 13th-century predecessor and became indispensable to both astronomers and astrologers.
During the 16th century the Danish astronomer Tycho Brahe, rejecting both the Ptolemaic and Copernican systems, was responsible for major changes in observation, unwittingly providing the data that ultimately decided the argument in favour of the new astronomy. Using larger, stabler, and better calibrated instruments, he observed regularly over extended periods, thereby obtaining a continuity of observations that were accurate for planets to within about one minute of arc—several times better than any previous observation. Several of Tycho’s observations contradicted Aristotle’s system: a nova that appeared in 1572 exhibited no parallax (meaning that it lay at a very great distance) and was thus not of the sublunary sphere and therefore contrary to the Aristotelian assertion of the immutability of the heavens; similarly, a succession of comets appeared to be moving freely through a region that was supposed to be filled with solid, crystalline spheres. Tycho devised his own world system—a modification of Heracleides’—to avoid various undesirable implications of the Ptolemaic and Copernican systems.
At the beginning of the 17th century, the German astronomer Johannes Kepler placed the Copernican hypothesis on firm astronomical footing. Converted to the new astronomy as a student and deeply motivated by a neo-Pythagorean desire for finding the mathematical principles of order and harmony according to which God had constructed the world, Kepler spent his life looking for simple mathematical relationships that described planetary motions. His painstaking search for the real order of the universe forced him finally to abandon the Platonic ideal of uniform circular motion in his search for a physical basis for the motions of the heavens.
In 1609 Kepler announced two new planetary laws derived from Tycho’s data: (1) the planets travel around the Sun in elliptical orbits, one focus of the ellipse being occupied by the Sun; and (2) a planet moves in its orbit in such a manner that a line drawn from the planet to the Sun always sweeps out equal areas in equal times. With these two laws, Kepler abandoned uniform circular motion of the planets on their spheres, thus raising the fundamental physical question of what holds the planets in their orbits. He attempted to provide a physical basis for the planetary motions by means of a force analogous to the magnetic force, the qualitative properties of which had been recently described in England by William Gilbert in his influential treatise, De Magnete, Magneticisque Corporibus et de Magno Magnete Tellure (1600; “On the Magnet, Magnetic Bodies, and the Great Magnet of the Earth”). The impending marriage of astronomy and physics had been announced. In 1618 Kepler stated his third law, which was one of many laws concerned with the harmonies of the planetary motions: (3) the square of the period in which a planet orbits the Sun is proportional to the cube of its mean distance from the Sun.
A powerful blow was dealt to traditional cosmology by Galileo Galilei, who early in the 17th century used the telescope, a recent invention of Dutch lens grinders, to look toward the heavens. In 1610 Galileo announced observations that contradicted many traditional cosmological assumptions. He observed that the Moon is not a smooth, polished surface, as Aristotle had claimed, but that it is jagged and mountainous. Earthshine on the Moon revealed that Earth, like the other planets, shines by reflected light. Like Earth, Jupiter was observed to have satellites; hence, Earth had been demoted from its unique position. The phases of Venus proved that that planet orbits the Sun, not Earth.
The battle for Copernicanism was fought in the realm of mechanics as well as astronomy. The Ptolemaic–Aristotelian system stood or fell as a monolith, and it rested on the idea of Earth’s fixity at the centre of the cosmos. Removing Earth from the centre destroyed the doctrine of natural motion and place, and circular motion of Earth was incompatible with Aristotelian physics.
Galileo’s contributions to the science of mechanics were related directly to his defense of Copernicanism. Although in his youth he adhered to the traditional impetus physics, his desire to mathematize in the manner of Archimedes led him to abandon the traditional approach and develop the foundations for a new physics that was both highly mathematizable and directly related to the problems facing the new cosmology. Interested in finding the natural acceleration of falling bodies, he was able to derive the law of free fall (the distance, s, varies as the square of the time, t2). Combining this result with his rudimentary form of the principle of inertia, he was able to derive the parabolic path of projectile motion. Furthermore, his principle of inertia enabled him to meet the traditional physical objections to Earth’s motion: since a body in motion tends to remain in motion, projectiles and other objects on the terrestrial surface will tend to share the motions of Earth, which will thus be imperceptible to someone standing on Earth.
The 17th-century contributions to mechanics of the French philosopher René Descartes, like his contributions to the scientific endeavour as a whole, were more concerned with problems in the foundations of science than with the solution of specific technical problems. He was principally concerned with the conceptions of matter and motion as part of his general program for science—namely, to explain all the phenomena of nature in terms of matter and motion. This program, known as the mechanical philosophy, came to be the dominant theme of 17th-century science.
Descartes rejected the idea that one piece of matter could act on another through empty space; instead, forces must be propagated by a material substance, the “ether,” that fills all space. Although matter tends to move in a straight line in accordance with the principle of inertia, it cannot occupy space already filled by other matter, so the only kind of motion that can actually occur is a vortex in which each particle in a ring moves simultaneously.
According to Descartes, all natural phenomena depend on the collisions of small particles, and so it is of great importance to discover the quantitative laws of impact. This was done by Descartes’s disciple, the Dutch physicist Christiaan Huygens, who formulated the laws of conservation of momentum and of kinetic energy (the latter being valid only for elastic collisions).
The work of Sir Isaac Newton represents the culmination of the scientific revolution at the end of the 17th century. His monumental Philosophiae Naturalis Principia Mathematica (1687; Mathematical Principles of Natural Philosophy) solved the major problems posed by the scientific revolution in mechanics and in cosmology. It provided a physical basis for Kepler’s laws, unified celestial and terrestrial physics under one set of laws, and established the problems and methods that dominated much of astronomy and physics for well over a century. By means of the concept of force, Newton was able to synthesize two important components of the scientific revolution, the mechanical philosophy and the mathematization of nature.
Newton was able to derive all these striking results from his three laws of motion:
1. Every body continues in its state of rest or of motion in a straight line unless it is compelled to change that state by force impressed on it;
2. The change of motion is proportional to the motive force impressed and is made in the direction of the straight line in which that force is impressed;
3. To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal.
The second law was put into its modern form F = ma (where a is acceleration) by the Swiss mathematician Leonhard Euler in 1750. In this form, it is clear that the rate of change of velocity is directly proportional to the force acting on a body and inversely proportional to its mass.
In order to apply his laws to astronomy, Newton had to extend the mechanical philosophy beyond the limits set by Descartes. He postulated a gravitational force acting between any two objects in the universe, even though he was unable to explain how this force could be propagated.
By means of his laws of motion and a gravitational force proportional to the inverse square of the distance between the centres of two bodies, Newton could deduce Kepler’s laws of planetary motion. Galileo’s law of free fall is also consistent with Newton’s laws. The same force that causes objects to fall near the surface of Earth also holds the Moon and planets in their orbits.
Newton’s physics led to the conclusion that the shape of Earth is not precisely spherical but should bulge at the Equator. The confirmation of this prediction by French expeditions in the mid-18th century helped persuade most European scientists to change from Cartesian to Newtonian physics. Newton also used the nonspherical shape of Earth to explain the precession of the equinoxes, using the differential action of the Moon and Sun on the equatorial bulge to show how the axis of rotation would change its direction.
The science of optics in the 17th century expressed the fundamental outlook of the scientific revolution by combining an experimental approach with a quantitative analysis of phenomena. Optics had its origins in Greece, especially in the works of Euclid (c. 300 bce), who stated many of the results in geometric optics that the Greeks had discovered, including the law of reflection: the angle of incidence is equal to the angle of reflection. In the 13th century, such men as Roger Bacon, Robert Grosseteste, and John Pecham, relying on the work of the Arab Ibn al-Haytham (died c. 1040), considered numerous optical problems, including the optics of the rainbow. It was Kepler, taking his lead from the writings of these 13th-century opticians, who set the tone for the science in the 17th century. Kepler introduced the point by point analysis of optical problems, tracing rays from each point on the object to a point on the image. Just as the mechanical philosophy was breaking the world into atomic parts, so Kepler approached optics by breaking organic reality into what he considered to be ultimately real units. He developed a geometric theory of lenses, providing the first mathematical account of Galileo’stelescope.
Descartes sought to incorporate the phenomena of light into mechanical philosophy by demonstrating that they can be explained entirely in terms of matter and motion. Using mechanical analogies, he was able to derive mathematically many of the known properties of light, including the law of reflection and the newly discovered law of refraction.
Many of the most important contributions to optics in the 17th century were the work of Newton, especially the theory of colours. Traditional theory considered colours to be the result of the modification of white light. Descartes, for example, thought that colours were the result of the spin of the particles that constitute light. Newton upset the traditional theory of colours by demonstrating in an impressive set of experiments that white light is a mixture out of which separate beams of coloured light can be separated. He associated different degrees of refrangibility with rays of different colours, and in this manner he was able to explain the way prisms produce spectra of colours from white light.
His experimental method was characterized by a quantitative approach, since he always sought measurable variables and a clear distinction between experimental findings and mechanical explanations of those findings. His second important contribution to optics dealt with the interference phenomena that came to be called “Newton’s rings.” Although the colours of thin films (e.g., oil on water) had been previously observed, no one had attempted to quantify the phenomena in any way. Newton observed quantitative relations between the thickness of the film and the diameters of the rings of colour, a regularity he attempted to explain by his theory of fits of easy transmission and fits of easy reflection. Notwithstanding the fact that he generally conceived of light as being particulate, Newton’s theory of fits involves periodicity and vibrations of ether, the hypothetical fluid substance permeating all space (see above).
Huygens was the second great optical thinker of the 17th century. Although he was critical of many of the details of Descartes’s system, he wrote in the Cartesian tradition, seeking purely mechanical explanations of phenomena. Huygens regarded light as something of a pulse phenomenon, but he explicitly denied the periodicity of light pulses. He developed the concept of wave front, by means of which he was able to derive the laws of reflection and refraction from his pulse theory and to explain the recently discovered phenomenon of double refraction.
Chemistry had manifold origins, coming from such diverse sources as philosophy, alchemy, metallurgy, and medicine. It emerged as a separate science only with the rise of mechanical philosophy in the 17th century. Aristotle had regarded the four elements earth, water, air, and fire as the ultimate constituents of all things. Transmutable each into the other, all four elements were believed to exist in every substance. Originating in Egypt and the Middle East, alchemy had a double aspect: on the one hand it was a practical endeavour aimed to make gold from baser substances, while on the other it was a cosmological theory based on the correspondence between man and the universe at large. Alchemy contributed to chemistry a long tradition of experience with a wide variety of substances. Paracelsus, a 16th-century Swiss natural philosopher, was a seminal figure in the history of chemistry, putting together in an almost impenetrable combination the Aristotelian theory of matter, alchemical correspondences, mystical forms of knowledge, and chemical therapy in medicine. His influence was widely felt in succeeding generations.
During the first half of the 17th century, there were few established doctrines that chemists generally accepted as a framework. As a result, there was little cumulative growth of chemical knowledge. Chemists tended to build detailed systems, “chemical philosophies,” attempting to explain the entire universe in chemical terms. Most chemists accepted the traditional four elements (air, earth, water, fire), or the Paracelsian principles (salt, sulfur, mercury), or both, as the bearers of real qualities in substances; they also exhibited a marked tendency toward the occult.
The interaction between chemistry and mechanical philosophy altered this situation by providing chemists with a shared language. The mechanical philosophy had been successfully employed in other areas; it seemed consistent with an experimental empiricism and seemed to provide a way to render chemistry respectable by translating it into the terms of the new science. Perhaps the best example of the influence of the mechanical philosophy is the work of Robert Boyle. The thrust of his work was to understand the chemical properties of matter, to provide experimental evidence for the mechanical philosophy, and to demonstrate that all chemical properties can be explained in mechanical terms. He was an excellent laboratory chemist and developed a number of important techniques, especially colour-identification tests.