When you evaluate an argument, look closely at the reasoning and evidence behind it. A number of unreasonable argumentative tactics are known as logical fallacies. Most of the fallacies—such as hasty generalizations and false analogies—are misguided or dishonest uses of legitimate argumentative strategies. The examples in this section suggest when such strategies are reasonable and when they are not.

Generalizing (inductive reasoning)

Writers and thinkers generalize all the time. We look at a sample of data and conclude that data we have not observed will most likely conform to what we have seen. From a spoonful of soup, we conclude just how salty the whole bowl will be. After numerous unpleasant experiences with an airline, we decide to book future flights with a competitor.

When we draw a conclusion from an array of facts, we are engaged in inductive reasoning. Such reasoning deals in probability, not certainty. For a conclusion to be highly probable, it must be based on evidence that is sufficient, representative, and relevant. (See the chart below.)

The fallacy known as hasty generalization is a conclusion based on insufficient or unrepresentative evidence.

Data from one state do not justify a conclusion about the whole United States.

A stereotype is a hasty generalization about a group. Here are a few examples.

Stereotyping is common because of our tendency to perceive selectively. We tend to see what we want to see; we notice evidence confirming our already formed opinions and fail to notice evidence to the contrary. For example, if you have concluded that politicians are corrupt, your stereotype will be confirmed by news reports of legislators being indicted—even though every day the media describe conscientious officials serving the public honestly and well.

Drawing analogies

An analogy points out a similarity between two things that are otherwise different. Analogies can be an effective means of arguing a point. It is not always easy to draw the line between a reasonable and an unreasonable analogy. At times, however, an analogy is clearly off base, in which case it is called a false analogy.

The writer has falsely assumed that because two things are alike in one respect, they must be alike in others. Exploring the outer reaches of the solar system and finding a cure for the common cold are both scientific challenges, but the problems confronting medical researchers are quite different from those solved by space scientists.

Tracing causes and effects

Demonstrating a connection between causes and effects is rarely simple. For example, to explain why a chemistry course has a high failure rate, you would begin by listing possible causes: inadequate preparation of students, poor teaching, lack of qualified tutors, and so on. Next you would investigate each possible cause. Only after investigating the possible causes would you be able to weigh the relative impact of each cause and suggest appropriate remedies.

Because cause-and-effect reasoning is so complex, it is not surprising that writers frequently oversimplify it. In particular, writers sometimes assume that because one event follows another, the first is the cause of the second. This common fallacy is known as post hoc, from the Latin post hoc, ergo propter hoc, meaning “after this, therefore because of this.”

Is the governor solely responsible for the decrease? Are there other reasons? The writer must show that Governor Cho’s policies are responsible for the decrease in unemployment; it is not enough to show that the decrease followed the governor’s taking office.

Weighing options

Especially when reasoning about problems and solutions, writers must weigh options. To be fair, a writer should mention the full range of options, showing why one is superior to the others or might work well in combination with others.

It is unfair to suggest that only two alternatives exist when in fact there are more. Writers who set up a false choice between their preferred option and one that is clearly unsatisfactory are guilty of the either . . . or fallacy.

Are these the only solutions—legalizing drugs and calling out the army? Other options, such as funding for drug abuse prevention programs, are possible.

Making assumptions

An assumption is a claim that is taken to be true—without the need of proof. Most arguments are based to some extent on assumptions, since writers rarely have the time and space to prove all the conceivable claims on which an argument is based. For example, someone arguing about the best means of limiting population growth in developing countries might assume that the goal of limiting population growth is worthwhile. For most audiences, there would be no need to articulate this assumption or to defend it.

There is a danger, however, in failing to spell out and prove a claim that is clearly controversial. Consider the following short argument, in which a key claim is missing.

The writer seems to be assuming that the death penalty deters violent criminals and that it is a fair punishment—and that most audiences will agree. These are not reasonable assumptions; the writer will need to state and support both claims.

When a missing claim is an assertion that few would agree with, we say that a writer is guilty of a non sequitur (Latin for “it does not follow”).

Does it take more than sleep to be a successful student? Few people would agree with the missing claim—that people with good sleep habits always make successful students.

Deducing conclusions (deductive reasoning)

When we deduce a conclusion, we put things together, like any good detective. We establish that a general principle is true, that a specific case is an example of that principle, and that therefore a particular conclusion about that case is a certainty.

Deductive reasoning can often be structured in a three-step argument called a syllogism. The three steps are the major premise, the minor premise, and the conclusion.

The major premise is a generalization. The minor premise is a specific case. The conclusion follows from applying the generalization to the specific case.

Deductive arguments break down if one of the premises is not true or if the conclusion does not logically follow from the premises. In the following argument, the major premise is very likely untrue.

The conclusion is true only if the premises are true. If the police sometimes give tickets for driving less than five miles per hour over the limit, Dominic cannot safely conclude that he will avoid a ticket.

In the following argument, both premises might be true, but the conclusion does not follow logically from them.

The fact that Jay ran the race is no guarantee that he is a member of the club. Presumably, many runners are non members.

Assuming that both premises are true, the following argument holds up.