Credibility alone cannot and should not carry the full burden of convincing readers. Indeed, many are inclined to think that the logic of the argument—the reasoning behind it—is as important as its ethos.

Examples, precedents, and narratives

Just as a picture can sometimes be worth a thousand words, so can a well-conceived example be extremely valuable in arguing a point. Examples are used most often to support generalizations or to bring abstractions to life. In making the general statement that popular media send the message that a woman must be thin to be attractive; you might include these examples:

At the supermarket checkout, a tabloid publishes unflattering photographs of a young singer and comments on her apparent weight gain in shocked captions that ask “What happened?!?” Another praises a star for quickly shedding “ugly pounds” after the recent birth of a child. The cover of Cosmopolitan features a glamorously made-up and airbrushed actress in an outfit that reveals her remarkably tiny waist and flat stomach. Every woman in every advertisement in the magazine is thin—and the context makes it clear that we’re supposed to think that she is beautiful.

Precedents are examples taken from the past. If, as part of a proposal for increasing lighting in the library garage, you point out that the university has increased lighting in four other garages in the past year, you are arguing on the basis of precedent.

The following questions can help you check any use of example or precedent:

Because storytelling is universal, narratives can be very persuasive in helping readers understand and accept the logic of an argument. Narratives that use video and audio to capture the faces and voices of the people involved are often particularly compelling. In As We Sow, a documentary arguing against corporate pork production methods, the farmers shown here tell stories of their struggle to continue raising animals as their families have for generations.

Stories drawn from your own experience can appeal particularly to readers, for they not only help make your point in true-to-life, human terms but also help readers know you better and therefore identify with you more closely.

When you include stories in an argument, ask yourself the following questions:

In research writing, you must identify your sources for any examples, precedents, or narratives not based on your own knowledge.

Authority and testimony

Another way to support an argument logically is to cite an authority. The use of authority has figured prominently in the controversy over smoking. Since the U.S. surgeon general’s 1964 announcement that smoking is hazardous to health, many Americans have quit smoking, largely persuaded by the authority of the scientists offering the evidence.

Ask the following questions to be sure you are using authorities effectively:

Testimony—the evidence that an authority presents in support of a claim—is a feature of much contemporary argument. If testimony is timely, accurate, representative, and provided by a respected authority, then it, like authority itself, can add powerful support.

In research writing (see Chapters 15–19), you should cite your sources for authority and for testimony not based on your own knowledge.

Causes and effects

Showing that one event is the cause or the effect of another can help support an argument. Suppose you are trying to explain, in a petition to change your grade in a course, why you were unable to take the final examination. You would probably trace the causes of your failure to appear—your illness or the theft of your car, perhaps—so that the committee reading the petition would reconsider the effect—your not taking the examination.

Tracing causes often lays the groundwork for an argument, particularly if the effect of the causes is one we would like to change. In an environmental science class, for example, a student may argue that a national law regulating smokestack emissions from utility plants is needed because (1) acid rain on the East Coast originates from emissions at utility plants in the Midwest, (2) acid rain kills trees and other vegetation, (3) utility lobbyists have prevented midwestern states from passing strict laws controlling emissions from such plants, and (4) if such laws are not passed, acid rain will soon destroy most eastern forests. In this case, the fourth point ties all of the previous points together to provide an overall argument from effect: if X, then Y.

Inductive and deductive reasoning

Traditionally, logical arguments are classified as using either inductive or deductive reasoning; in practice, the two almost always work together. Inductive reasoning is the process of making a generalization based on a number of specific instances. If you find you are ill on ten occasions after eating seafood, for example, you will likely draw the inductive generalization that seafood makes you ill. It may not be an absolute certainty that seafood is to blame, but the probability lies in that direction.

Deductive reasoning, on the other hand, reaches a conclusion by assuming a general principle (known as a major premise) and then applying that principle to a specific case (the minor premise). In practice, this general principle is usually derived from induction. The inductive generalization Seafood makes me ill, for instance, could serve as the major premise for the deductive argument Since all seafood makes me ill, the shrimp on this buffet is certain to make me ill.

Deductive arguments have traditionally been analyzed as syllogisms—reasoning that contains a major premise, a minor premise, and a conclusion.

Syllogisms, however, are too rigid and absolute to serve in arguments about questions that have no absolute answers, and they often lack any appeal to an audience. Aristotle’s simpler alternative, the enthymeme, asks the audience to supply the implied major premise. Consider the following example:

Since violent video games can be addictive and cause psychological harm, players and their parents must carefully evaluate such games and monitor their use.

You can analyze this enthymeme by restating it in the form of two premises and a conclusion.

Note that the major premise is one the writer can count on an audience agreeing with or supplying: safety and common sense demand that potentially harmful games be used with great care. By implicitly asking an audience to supply this premise to an argument, a writer engages the audience’s participation.

Toulmin’s system (13d) looks for claims, reasons, and assumptions instead of major and minor premises.

Whether it is expressed as a syllogism, an enthymeme, or a claim, a deductive conclusion is only as strong as the premise or reasons on which it is based.

Visuals that make logical appeals

Visuals that make logical appeals can be especially useful in arguments, since they present factual information that can be taken in at a glance. Mother Jones used the following simple chart to carry a big message about income distribution in the United States. Consider how long it would take to explain all the information in this chart with words alone.