While the character a writer presents in writing always exerts a strong appeal (or lack of appeal) in an argument, 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. Logical appeals, known to the ancient Greeks as logos, can thus be very effective; particularly useful types of logical appeals include examples, precedents, and narratives; authority and testimony; causes and effects; and inductive and deductive reasoning. In addition, visuals can help you enhance your logical appeals.

Using 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 an argument about American mass media and body image, for instance, you might make the general statement that popular media send the message that a woman must be thin to be attractive; you might then illustrate your generalization with these examples:

The generalization would mean far less without the examples.

Examples can also help us understand abstractions. Poverty, for instance, may be difficult for us to think about in the abstract, but a description of several residents of a poverty-stricken community, vying for low-paying jobs, visiting local food pantries and soup kitchens, or facing homelessness, speaks directly to our understanding.

Precedents are particular kinds of examples taken from the past. The most common use of precedent occurs in law, where an attorney may ask for a certain ruling based on a similar earlier case. Precedent appears in everyday arguments as well. 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.

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

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.

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. In arguing for a stronger government campaign against smoking, for example, former President Clinton often drew on personal stories of his own family’s experience with lung cancer.

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

In general, do not rely solely on the power of stories to carry your argument, since readers usually expect writers to state and argue their reasons more directly and abstractly as well. An additional danger if you use only your own experiences is that you can seem focused too much on yourself (and perhaps not enough on your readers).

As you develop your own arguments, keep in mind that while narratives can provide effective logical support, they may be used equally effectively for ethical or emotional appeals as well.

Citing authority and testimony

Another way to support an argument logically is to cite an authority. For nearly fifty years, 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, millions of Americans have quit smoking, largely persuaded by the authority of the scientists offering the evidence.

But as with other strategies for building support for an argumentative claim, citing authorities demands careful consideration. Ask yourself the following questions to be sure you are using authorities effectively:

Authorities are commonly cited in research writing, which often relies on the findings of other people. In addition, you may cite authorities in an assignment that asks you to review the literature of any field.

Testimony—the evidence 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 to an argument. In an essay for a literature class, for example, you might argue that a new edition of a literary work will open up many new areas of interpretation. You could strengthen this argument by adding a quotation from the author’s biographer, noting that the new edition carries out the author’s intentions much more closely than the previous edition did.

In research writing you should cite your sources for authority and testimony not based on your own knowledge.

Showing causes and effects

Showing that one event is the cause—or the effect—of another can sometimes 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. In such a case, you would probably try to trace the causes of your failure to appear (the death of your grandmother followed by 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) in the absence of such laws, acid rain will destroy a high percentage of eastern forests. In this case, the first point is that the emissions cause acid rain; the second, that acid rain causes destruction in eastern forests; and the third, that states have not acted to break the cause-effect relationship established by the first two points. The fourth point ties all of the previous points together to provide an overall argument from effect: unless a national law is passed, most eastern forests are doomed.

In fact, a cause-effect relationship is often extremely difficult to establish. Scientists and politicians continue to disagree, for example, over the extent to which acid rain is responsible for the so-called dieback of many eastern forests. If you can show strong evidence that a cause produces an effect, though, you will have a powerful argument at your disposal.

Using inductive and deductive reasoning

Traditionally, logical arguments are classified as using either inductive or deductive reasoning, but in practice, the two types of reasoning usually appear 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 shellfish, for example, you will likely draw the inductive generalization that shellfish makes you ill. It may not be an absolute certainty that shellfish was the culprit, 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 Shellfish makes me ill, for instance, could serve as the major premise for the deductive argument Since all shellfish makes me ill, the shrimp on this buffet is certain to make me ill.

Deductive arguments have traditionally been analyzed as syllogisms—three-part statements that contain 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, calls on the audience to supply the implied major premise. Consider the following example:

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 schools should discipline children who harm other students. As such, this premise is assumed rather than stated in the enthymeme. By implicitly asking the audience to supply this premise to the argument, the writer engages the audience’s participation.

Note that a deductive conclusion is only as strong as the premises on which it is based. The citizen who argues that Ed is a crook and shouldn’t be elected to public office is arguing deductively, based on an implied major premise: No crook should be elected to public office. Most people would agree with this major premise. So the issue in this argument rests on the minor premise that Ed is a crook. Satisfactory proof of that premise will make us likely to accept the deductive conclusion that Ed shouldn’t be elected.

At other times, the unstated premise may be more problematic. The person who says Don’t bother to ask for Ramon’s help with physics—he’s a jock is arguing deductively on the basis of an implied major premise: Jocks don’t know anything about physics. In this case, careful listeners would demand proof of the unstated premise. Because bigoted or prejudiced statements often rest on this kind of reasoning—a type of fallacy—writers should be particularly alert to it.

A helpful variation on the syllogism and the enthymeme is the Toulmin system, which looks for claims, reasons, and assumptions rather than major and minor premises.

Note that in this system the assumption—which may be unstated—serves the same function as the assumed major premise in an enthymeme.

Making logical appeals with visuals

Charts, graphs, tables, maps, photographs, and so on can be especially useful in arguments because they present factual information that can be taken in at a glance.