Chapter Fifteen: Arguments from Analogy

Arguments that make their point by means of similarities are impostors, and, unless you are on your guard against them, will quite readily deceive you.

—Plato

Analogies decide nothing, that is true, but they can make one feel more at home.

—Sigmund Freud, New Introductory Lectures on Psychoanalysis

Arguments from analogy declare that because two items are the same in one respect they are the same in another. As Freud notes, they can make you feel at home—and for that reason they can be especially persuasive.

During World War I, the Socialist Party distributed leaflets to recent draftees, urging them to oppose the draft. The draft, they contended, violated the constitutional amendment against involuntary servitude. Oliver Wendell Holmes, chief justice of the Supreme Court, argued that they did not have the right to circulate the leaflets during wartime. The right to free speech, he asserted, “would not protect a man in falsely shouting fire in a theater and causing a panic.” Since in both cases “the words used . . . create a clear and present danger,” he concluded, the right to free speech did not protect the Socialists in expressing ideas that might harm the war effort. The argument begins with something familiar—of course we don’t have the right to falsely shout fire in a theater—and invites us to conclude the same about something less familiar—under certain circumstances we don’t even have the right to explain to others our interpretation of the U.S. Constitution. (Holmes is often quoted as calling it “a crowded theater.” He didn’t, though it is probably what he had in mind.)

Arguments from analogy are almost always enticing because, by their very nature, they use two of the quick-and-dirty shortcuts in reasoning described in Chapter 1. By beginning with the familiar, they exploit our dependence on the vividness shortcut; and by presenting similarities between the familiar and the unfamiliar, they take advantage of our dependence on the similarity shortcut. They are custom-made for the way our minds naturally operate. This makes us especially susceptible to them and heightens the importance of being able to evaluate them effectively.

15.1 Correct Form for Arguments from Analogy

Analogies are often used merely for rhetorical effect. Acel Moore of the Philadelphia Inquirer, for example, writes: “Writing editorials is a lot like wearing a navy blue suit and standing in a rainstorm on a cold day and wetting your pants; it may give you a warm feeling for a minute, but no one else is going to notice.” Moore doesn’t attempt to establish any conclusion based on the similarity—he simply makes note of it. Don’t jump to the conclusion that an analogy introduces an argument unless there really is—at least implicitly—a conclusion.

When there is an argument from analogy, as in the preceding free speech argument, it can typically be clarified according to the following form:

A and B, as always, are used here as name letters. They name the two analogs^[1]—that is, the two things (or classes of things) that are said to be analogous. A, the basic analog, is the one that we are presumed to be more familiar with; in the free speech argument it is falsely shouting fire in a theater. B, the inferred analog, is the thing in question, the one that the argument draws a conclusion about; in the free speech argument it is expressing ideas that might harm the war effort.

We will continue to use F and G as property letters. F is the basic similarity, the property that the two analogs share, presumably without controversy. In the free speech argument, the basic similarity is that they create a clear and present danger. And G is the inferred similarity, the property that the inferred analog is purported to have on the grounds that the basic analog has it. Is not protected by the right to free speech is the inferred similarity in the free speech argument. Here is one good way to clarify the argument:

Variations on this model are common. The basic or inferred analog, for example, will sometimes include more than one item, as in this example:

Manatees must be mammals, since whales and dolphins, like manatees, are sea creatures that give live birth, and whales and dolphins are definitely mammals.

In this case, the basic analog—the content of A—is whales and dolphins.

Likewise, either the basic similarity or the inferred similarity may include more than one property, as in this example:

Manatees must be mammals, since whales, like manatees, are sea creatures that give live birth and that nourish their young on the mother’s milk, and whales are definitely mammals.

In this example, the basic similarity—the content of F—is sea creatures that give live birth and nourish their young on the mother’s milk.

Clarifying an argument from analogy is usually a straightforward matter. It is easiest to begin by identifying the analogs—the two items that the arguer is comparing; insert the one that is not in question into the A position as the basic analog, and the one that is in question into the B position, as the inferred analog. Then insert the basic similarity—the property the two analogs uncontroversially share—into both premises as F. Finally, insert the inferred similarity into the first premise and the conclusion, as G.

Arguments from analogy are sometimes enthymemes. When there is an implicit statement, it is usually the second premise, the one that establishes the basic similarity. This is because arguers often assume, rightly, that the similarity between two analogs is so obvious that it goes without saying. Suppose I say to a friend of mine, whose son is about to enter first grade, “Since John behaves respectfully towards his parents, he will surely treat his teachers with respect.” The basic analog is John’s parents, the inferred analog is John’s teachers, and the inferred similarity is are treated with respect by John. But what is the basic similarity? We must identify a relevant trait that parents and teachers have in common, namely, that they are authority figures to John. Here is the clarified argument. (Brackets, as usual, indicate that premise 2 is implicit, but we also must supply to premise 1 the part about authority figures.)

15.2 The Total Evidence Condition (1): Relevant Similarities

If an argument from analogy can be loyally paraphrased in the form described above, then it satisfies the correct form condition. But for an inductive argument to be logically strong it must not only satisfy the correct form condition; it must also satisfy the total evidence condition. As with frequency arguments and inductive generalizations, there are two parts to the total evidence condition for arguments from analogy: the basic similarity must be relevant, and any dissimilarities must be irrelevant. If an argument does poorly on either one of these conditions, it should be judged no better than logically weak.

Although analogical arguments are sometimes accused of committing the fallacy of false analogy (or the fallacy of faulty analogy), this fallacy is very much like the fallacy of hasty generalization. The existence of the named fallacy highlights the ease with which we can make mistakes in this sort of reasoning. But to accuse an argument from analogy of committing this fallacy says nothing about what has gone wrong with the argument. It is far better to explain more specifically how it is that some necessary condition for soundness has not been satisfied.

15.2.1 The Relevance of the Basic Similarity

Begin your deliberations about the total evidence question by asking, Is the basic similarity relevant? The more relevant it is, the stronger the logic of the argument might be. When you consider this question, forget about the two analogs and simply consider to what extent the basic similarity counts in favor of the inferred similarity. A television advertising campaign by a dairy company shows old but cheerful citizens of the Republic of Georgia eating yogurt; they have eaten yogurt all their lives, we are told, and they are now
well past the century mark—one woman is now 134! Eating yogurt, we
are encouraged to believe, could do the same for us. The first step in evaluating how well this argument satisfies the total evidence condition is to ignore the two analogs (citizens of Georgia and us) and ask whether the basic similarity—eating yogurt—counts in favor of the inferred similarity—a long life. There is no special reason to think so, and the argument doesn’t help by providing one. So the logic of the argument is very weak.

More commonly an argument from analogy satisfies the condition at least to some degree. One large state university published the following story in its alumni magazine:

A preliminary appraisal of the results of a major assessment of faculty and graduate programs conducted by the Conference Board of Associated Research Councils placed our institution second in the nation among public research universities and in the top five overall. “It is gratifying to see our faculty receive this national recognition of their superior research and teaching,” said the Chancellor. Even though the study focused on graduate programs, he pointed out that the results could also be applied to the undergraduate program as well, since the two programs share the same faculty.

The university’s graduate program is the basic analog and its undergraduate program the inferred analog. The basic similarity is that the university’s excellent faculty staffs them. And the inferred similarity is that the academic programs are excellent. Is the basic similarity relevant? That is, does having an excellent faculty count toward the excellent academic programs? Of course it does. So this argument easily clears the first hurdle of the total evidence condition. But it is too soon to conclude that the argument is logically strong; there is still a second total evidence hurdle to clear.

15.2.2 Relevant Similarities and the Fallacy of Equivocation

Suppose I say, “Einstein was smart, and he was able to revolutionize physics. The physics teacher I had in high school is smart, too, so he should be able to revolutionize physics.” The basic similarity is relevant to the inferred similarity—smart is better than stupid when it comes to revolutionizing physics. But there is smart, and then there is smart. Surely my high school physics teacher is not as smart as Einstein. Doesn’t that weaken the argument? Let’s clarify it and see:

Smart shows up in both premises. To ask whether my high school physics teacher is as smart as Einstein is to ask, in effect, whether the word means the same thing in each case. It is a general expression. Recalling our coverage of generality in Chapter 5, this means that it is an expression that allows for degrees (examples were fine, bald, brown, living together, incompatible, wrong, and evil). As we saw, generality is usually unproblematic. It becomes problematic, however, when the meaning of the expression shifts from one use to the next, and when the apparent success of the argument depends on that shift. In that case, the argument commits the fallacy of equivocation; the lesson from Chapter 5 is to eliminate the ambiguity.

Let’s eliminate the ambiguity by using the reasonable-premises approach in revising premise 2; in that case it is as follows:

2. My high school physics teacher is smart, though not as smart as Einstein.

While this is probably true, we now have a major problem with the logic of the argument—namely, it no longer satisfies the correct form condition, since the basic similarity, established in premise 1, is not asserted in premise 2. (The form is now something like this: 1. A is F and G; 2. B is sort of like F; ∴ C. B is G.) Let’s try revising it again, this time using the reasonable-logic approach. This gives us the following:

2. My high school physics teacher is just as smart as Einstein.

This nicely fixes the logical problem, but at the cost of what is pretty obviously a false premise. Either way, the argument is unsound.

The Oliver Wendell Holmes free speech argument, presented at the beginning of the chapter, provides a weightier example of the same problem. The basic similarity, creating a clear and present danger, certainly counts in favor of the inferred similarity of not being protected by the right to free speech. But is the danger caused by the wartime expression of potentially subversive ideas as clear and as present as the danger caused by the false shout of fire in a theater? If not, doesn’t this weaken the argument? Let’s take another look at Holmes’s clarified argument.

The phrase clear and present danger, like the term smart in the Einstein example, is a general term that seems to apply to a greater degree in premise 1 than in premise 2. It is plausible to suppose that this shift contributes to the apparent success of the argument, and thus that the argument commits the fallacy of equivocation. So we should revise our paraphrase of premise 2 to eliminate the ambiguity. On the one hand, we could paraphrase it to say that those who scattered the leaflets created a clear and present danger, though less clear and present than falsely shouting fire in a theater. The premise would probably be true, but we would have created the same logical difficulty described in the Einstein argument—the basic similarity is not the same in each premise. On the other hand, we could paraphrase it to say that they created a clear and present danger that is just as clear and present as falsely shouting fire in a theater. We have now satisfied the correct form condition but probably have a false premise.

The problem is one to look for whenever you are clarifying an argument from analogy.

15.3 The Total Evidence Condition (2): Irrelevant Dissimilarities

15.3.1 The Irrelevance of the Dissimilarities

The second total evidence question is Are there relevant dissimilarities? Preferably they are irrelevant, for the more relevant the dissimilarities, the weaker the logic of the argument. When you consider this question, forget about the basic similarity and concentrate on the two analogs. There are always innumerable ways in which they are dissimilar, but most or all of them will be irrelevant. What matters is to what extent any dissimilarity makes the basic analog a better candidate for the inferred property.

Consider, for example, the free speech argument. There are many dissimilarities. One of the activities happens in a theater, for example, while the other could happen anywhere; but this is irrelevant, since there is no reason to think that things said in a theater are less deserving of protection by the right to free speech than things said anywhere else. Or, for example, one of them is spoken aloud, while the other could be written down; but again, this is irrelevant, for there is no general reason to think that the spoken word is more worthy of free speech protection than the written word.

Some of the dissimilarities, however, are relevant. In the theater case, what is expressed is intentionally deceptive, while in the leaflet case, what is expressed seems to have been utterly sincere. This, taken by itself, certainly makes the theater case a better candidate for exemption from free speech protection, and thus it counts as a relevant dissimilarity. Furthermore, in the theater case, the action is sure to have a harmful result; but in the leaflet case, there is no assurance that anyone will pay any attention or, if they do, that they will be influenced (in fact, it was established that no one had been persuaded by the leaflet). This, too, makes the theater case a better candidate for lack of protection by the right to free speech.

In short, even if we forget that the phrase clear and present danger may be equivocal, the argument does not score well on the second portion of the total evidence condition. Its logic can be judged, at best, as fairly weak. Brilliant jurist that he was, I should note that Oliver Wendell Holmes relied, as he should have, on a good deal more than just this argument in support of his conclusion.

Let’s now return to the academic excellence argument. Here is the clarification:

There are many dissimilarities between the graduate and undergraduate programs of any large state university. Graduate courses, for example, are usually assigned higher catalog numbers than are undergraduate courses. But this is irrelevant; catalog numbers are not like scores flashed by Olympic judges, with higher numbers going to better courses. Another difference is that in large state universities the graduate students tend to have much more exposure to the faculty than do the undergraduate students—their classes are much smaller and are more frequently taught by the regular faculty members. This is relevant, since student exposure to faculty can contribute powerfully to academic excellence. The conclusion may still be true. But even though this argument does well on the first condition, it performs badly on the second and so its logic must be considered weak.

15.4 The Special Character of Arguments from Analogy

15.4.1 Arguments from Analogy as Logical Borrowers

As you may have noticed, every example of an argument from analogy worked out in this chapter has been declared logically weak and thus unsound. This is not an aberration. Although not all arguments from analogy are unsound, they do establish their conclusions far less often than any other sort of argument. Plato, in the lead quotation for this chapter, calls them “impostors.” Analogical arguments, unlike any other arguments we look at in this book, have a built-in logical shortcoming.

Let’s take another look at the logical form of arguments from analogy:

What is the source of logical strength for such an argument? Not the correct form condition; as with every other inductive argument, satisfying this condition merely qualifies the argument for any strength that might be conferred by the total evidence condition. Not the second part of the total evidence condition; the absence of relevant dissimilarities simply means there is no evidence to undermine whatever strength it has.^[2] This leaves the first part of the total evidence condition as the sole positive source of logical strength.

How does the first part of the total evidence condition provide logical strength? By virtue of the fact that the basic similarity counts in favor of the inferred similarity. But what does count in favor of mean here? The only meaning I know is a sound argument can be offered for it. So we can now see that logically strong analogical arguments derive their logical strength from another argument—the argument that can be offered from the inferred similarity to the basic similarity. We will call such an argument (an argument from F to G—see premise 1 of the form clarified above) a background argument. Stated simply: an analogical argument’s only logical strength is borrowed from a background argument.

Any other sort of argument can, in principle, lend its strength to an argument from analogy. For example, in the preceding chapter we looked briefly at the argument Every Japanese car I’ve ever owned has been well built, so that Toyota is probably well built. It could easily be clarified as an argument from analogy, clarified as follows:

If the similarity is relevant in this case, it is because the background argument is a logically strong inductive generalization that goes from my experience of Japanese cars (the basic similarity) to the conclusion that Japanese cars in general are well built (the inferred similarity). The argument from analogy is logical only if this generalization works. So it borrows its logical strength from an inductive generalization.

The next passage, from Science News, provides a second example of borrowed logic in an argument from analogy.

The concept of “vintage year” took on a new meaning this week when two scientists presented the first chemical evidence that wine existed as far back as about 3500 bc. They had noticed a red stain while piecing together jars excavated from an Iranian site. They compared the stain with a similar stain in an ancient Egyptian vessel known to have contained wine. The researchers scraped the reddish residue from the jars and analyzed the samples with infrared spectroscopy. Residues from the Iranian and Egyptian jars looked alike and were full of tartaric acid, a chemical naturally abundant only in grapes. “Those crystals are a signature for wine,” says one researcher.

The argument can be clarified thus:

In this case, if the similarity is relevant it is because the background argument is a sound explanatory argument (of a sort we will cover thoroughly in the next chapter) that establishes that the red stains (the basic similarity) have properties that are best explained as caused by wine (the inferred similarity). This argument’s logical strength is borrowed from an explanatory argument.

As a final example, arguments from analogy can even borrow their logical strength from deductive arguments. Consider the validity counterexamples of Chapter 10. In that chapter we started with an inverted—and invalid—Socrates argument:

We then offered as a validity counterexample this obviously invalid (because of true premises and false conclusion) Atlantic argument:

In this way we saw that the Socrates argument was invalid. Like any validity counterexample, the reasoning can be represented as an argument from analogy, clarified as follows:

Here the relevance of the similarity depends on a deductive background argument; for the way to argue that a certain form (the basic similarity) is invalid (the inferred similarity) is by use of this valid affirming the antecedent argument, which has a self-evidently true first premise:

In this case, the logical strength of the analogical argument is borrowed from a sound deduction.

By its very nature, then, when an analogical argument works it works on borrowed logic. The two analogs mainly serve to get in the way by providing a basis for relevant dissimilarities. It is the background argument, which ignores the analogs and is concerned solely with the basic and inferred similarities, that serves as the argument’s motor. In the end, the background argument cannot itself be some other argument from analogy, since the background argument would depend on a background argument (and so on).

There are two practical lessons here. First, if you can see what the background argument is, bring it to the foreground when you clarify the argument, abandoning the analogical form. The Toyota argument, for example, would be much easier to evaluate properly if clarified as a complex argument composed of an inductive generalization and a frequency argument (as illustrated in Chapter 14); and the Iranian jar argument, likewise, if paraphrased as an explanatory argument. Second, if you cannot see what the background argument is, you should normally resist the temptation to judge it as logically strong until you better understand the background argument. As noted at the beginning of the chapter, analogical arguments are custom-made for the way our minds work, which makes them extraordinarily persuasive. But their inherent reliance on logical borrowing also makes them very good at concealing logical defects. When a persuasive car salesman won’t let you open the hood to inspect the motor, it may be prudent to shop elsewhere.

15.4.2 Arguments from Analogy as Psychological Lenders

From a logical point of view, analogical arguments are borrowers. But from a psychological point of view, they often put other arguments deeply into their debt. They can hint as well as hide.

Look, for example, at the Iranian jar argument. The analogy between the two stains is what suggested to the researchers that the jar had once contained wine. This set in motion a research effort in which samples scraped from both jars were examined by infrared spectroscopy, revealing crystals that were “a signature for wine.” One could perhaps say that this new evidence converts the initial analogical argument from a merely suggestive one into a logically strong one, by showing just how relevant the basic similarity (same red stain) is to the inferred similarity (that it contained wine). But it would be much clearer to simply say that the background argument displaces the argument from analogy. Analogical reasoning has lent a powerful psychological boost to the research program by producing the suggestive idea. Still, any logical strength it gains from that research program is borrowed from the background argument—that is, from the explanatory argument about crystals developed by the researchers. Clarity is increased if the initial analogy drops out of any account of the logical support for the conclusion—as long as it remains as a central feature of the history of the discovery.^[3]

Analogical arguments can lend a valuable psychological boost to inquiry of every sort. Consider the free speech argument. Even if you are not persuaded by the proposed analogy between shouting fire and distributing leaflets, it is certainly suggestive. In particular, it suggests that you are wrong if you think that all expressions are protected. Further, it suggests a way of reasoning about which ones are not protected—namely, by thinking about the possible dangers caused by the speech in question. If that way of reasoning succeeds, the argument from analogy gets psychological credit for suggesting it, even if it gets no logical credit for supporting it.

Nineteenth-century philosopher John Stuart Mill aptly declared that good reasoners will consider any analogical argument as a “guidepost, pointing out the direction in which more rigorous investigations should be prosecuted.” Arguments from analogy brilliantly serve a necessary function in reasoning. We would be lost without good guideposts. But we should not confuse them with destinations.

15.5 Summary of Chapter Fifteen

Arguments from analogy typically contend that because two items are the same in one respect, they are the same in another respect. The basic analog is compared to the inferred analog; because they have the basic similarity in common, it is concluded that the inferred analog also has the inferred similarity.

The total evidence condition has two parts. First, the basic similarity must be relevant—that is, it must count toward the presence of the inferred similarity. Second, there must not be any dissimilarities that are relevant—that is, any dissimilarity between the two analogs must not make the basic analog a better candidate for the inferred property. The argument is logically weaker to the extent that it fails in either of these two areas.

Their only positive logical strength comes from the background argument that establishes that the inferred similarity follows from the basic similarity; thus, whatever logical success analogical arguments have is borrowed. This makes it especially important to pay close attention to the first part of the total evidence condition. On the other hand, analogical arguments play an important psychological role in suggesting lines of reasoning, and so should be cultivated for that purpose.

15.6 Guidelines for Chapter Fifteen

• Structure arguments from analogy, when it would be loyal to do so, by identifying four things—the basic and inferred analogs and the basic and inferred similarities—then inserting each into its proper place in the form. Remember that the second premise, which declares the basic similarity, is often implicit.
• In considering whether an argument from analogy has satisfied the total evidence condition, first ask, Is the basic similarity relevant? To answer this question, look at the extent to which the basic similarity counts in favor of the inferred similarity.
• When the basic similarity is described by a general term, consider whether its meaning shifts from one use to the next. If it shifts enough to affect the soundness of the argument, revise your clarification to eliminate the ambiguity.
• In considering whether an argument from analogy has satisfied the total evidence condition, ask next, Are any of the dissimilarities relevant? To answer this question, look at the extent to which any dissimilarity makes the basic analog a better candidate than the inferred analog for the inferred property.
• When you can clearly see the background argument, clarify it rather than the argument from analogy. When you cannot see the background argument, you should normally reserve final judgment about the strength of the argument’s logic.

15.7 Glossary for Chapter Fifteen

Analogs—the two things (or classes of things) that are said to be similar in an argument from analogy.

Argument from analogy—an argument that asserts that because two items are the same in one respect, they are the same in another respect. They can be represented by this form:

Background argument—an argument that shows that the inferred similarity (of an analogical argument) follows from the basic similarity—that is, an argument that shows that the basic similarity is relevant.

Basic analog—in an argument from analogy, the item that we are presumably more familiar with, which is presumably known to have both the basic and the inferred similarities.

Basic similarity—in an argument from analogy, the property that the two analogs share, presumably without controversy.

Fallacy of false analogy—the mistake of using an argument from analogy in which the basic similarity is not relevant or in which there are relevant dissimilarities between the basic and inferred analogs. Because this term says nothing about what precisely has gone wrong with the argument, it is better to explain more specifically how it is that some necessary condition for soundness has not been satisfied. Also called the fallacy of faulty analogy.

Inferred analog—in an argument from analogy, the item in question, about which the argument is drawing its conclusion.

Inferred similarity—in an argument from analogy, the property that the inferred analog is alleged to have because the basic analog has it.