13.3.2: Argument from Analogy

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Bradley H. Dowden
California State University Sacramento

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Dear sir,

A woman's composing of music is like a dog's walking on its hind legs. It is not done well, but you are surprised to find it done at all.

Yours truly,

Mr. C. Pig

This joke uses an argument from analogy. The unfamiliar world of electricity can be explained by showing how electricity in a wire behaves analogously to water flowing through a pipe. Analogies help with description, too. We envision a rolling ball when we hear that presidential candidate Roosevelt had momentum going into the New Hampshire primary.

Analogies can be used in arguing. A woman without a man is like a fish without a bicycle. This joke would be making a radical feminist comment, because hidden between the lines is an argument for why women don't need men. The joke is intended to counter the conclusion of someone who would say that a woman without a man is like a fish out of water.

Here is a more serious example of an argument by analogy. Suppose that for several months a scientist gives experimental drug D to a variety of dogs confined to cages. A group of similar caged dogs do not receive the drug. The scientist then tests to see whether the dogs receiving drug D are more cardiovascularly fit than the ones not receiving the drug. The scientist checks blood pressure, stamina, and other physiological measures. The scientist's initial conclusion is that dogs that get the drug are no more cardiovascularly fit than the other dogs. The scientist's final conclusion is that, for humans, taking drug D will be no substitute for getting lots of exercise, as far as cardiovascular fitness is concerned. This argument uses what analogy? Let’s figure it out. Here is the argument in standard form:

Dogs are like humans in many ways.
Dogs cannot use drug D as a substitute for exercise.
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Humans cannot use drug D as a substitute for exercise.

The conclusion follows with probability. However, we could rewrite the first premise so that the conclusion follows with certainty:

Dogs are like humans when it comes to deciding whether drugs can be a substitute for exercise.
Dogs cannot use drug D as a substitute for exercise.
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Humans cannot use drug D as a substitute for exercise.

This argument is deductive. Which of the two ways of treating the argument is better? It is hard to tell and doesn't make much difference. The scientist is more likely to have intended inductive standards to apply; at least we shall assume this from now on. But what is more important to see is that both ways of analyzing the argument depend on accepting the analogy between people and dogs. If the analogy is unacceptable, the argument breaks down. Scientists get into serious disputes about whether testing drugs on rats, dogs, and rabbits gives reliable information about how these drugs will affect human beings. These disputes are about analogy.

To generalize, the simplest inductive arguments from analogy have the following form:

Characteristics are the same thing as properties or qualities. In the drug-testing example, A = dogs, B = humans, and C = the characteristic of not being able to use drug D as a substitute for exercise. If A's have characteristic C but B's do not, the analogy between A and B is a faulty analogy as far as C is concerned. The phrase “in several respects” is there to remind us that when we are assessing some piece of reasoning that uses an analogy, we always need to keep in mind which aspects of the analogy should be taken seriously and which should be ignored.

Analogies are often stated without using the words analogous to and like. Persuading a terrorist to defect is supposed to be analogous to converting the child from watching TV to doing her homework. The key to seeing the analogy is in noting the word akin. Is this a faulty analogy? The average reader is not in a position to tell. Only people who are familiar both with persuading a terrorist to defect and with raising children would be in a position to say However, notice that in this passage the analogy is not used to draw some conclusion, as it is in the earlier analogies we have discussed. The analogy is used merely to explain the process of persuading a terrorist. The passage contains an explanatory analogy but not an argument by analogy. If it were to contain an argument by analogy, it would probably say that because the conversion of the child requires such and such, therefore persuading a terrorist does, too.

Exercise \(\PageIndex{1}\)

Arguments from analogy have the following logical form: A is analogous to B in important ways. A has property C. So, B has property C, too. What would the letters A, B, and C represent in the following argument by analogy?

I am a vegetarian, and I believe it's morally wrong to cook live shrimp. After all, it would be wrong for someone to toss you into a pan of boiling water, wouldn't it?

Answer: A = people, B = shrimp, C = the characteristic of it being morally incorrect to cook them by tossing them alive into a pan of boiling water.

Advertising that uses testimonials often promotes an argument by analogy. Take the Hollywood beauty who testifies to the TV viewer: "I got a silicone breast implant from Dr. Wrigley, and I got the lead part in a commercial. His plastic surgery can help you, too."¹ You, the female viewer, are being asked implicitly to accept the analogy with your own situation and conclude that the surgery will get you what you want. But as a logical reasoner you will confront the analogy directly by thinking something like this: "That's fine for her, but I'm not trying to get a part in a commercial, so realistically what does her testimony have to do with me in my situation?"

By criticizing the analogy in the argument that the TV program encourages you to create, you are using the technique of pointing out the disanalogies. The disanalogies are the differences, the ways in which the two are not analogous. We point out disanalogies when we say, "Yes, they're alike, but not in the important ways." We are apt, also, to use this method in response to the analogy between people and shrimp by pointing out that we are not like shrimp in terms of sensitivity to pain, or intelligence, or moral worth.

A second method of attacking an argument by analogy is to extend the analogy. We do this when we find other ways the two things are similar and then draw obviously unacceptable conclusions from this similarity. For example, we can attack the argument that uses the analogy between people and dogs by saying, "Dogs are like people in other ways, too. For example, we both like to eat meat. Since dogs enjoy their meat raw, you won't mind eating your hamburger raw tonight, will you?" When the original advocate of the cardiovascular argument answers, "No, we aren't that much like dogs," you can respond with "I agree, so how can you be so sure we are like dogs when it comes to taking drug D?"

Let's now analyze a complicated argument by analogy. You might have had the honor of getting involved in the following unpleasant discussion with Mario about white women marrying black men. During the conversation, Mario said:

A dog breeder wouldn't think of mixing different breeds, so the human race should not be mongrelized by interracial breeding. You accept my argument, or aren't you logical? Of course you accept it; you aren't some kind of pervert. Besides, you are not a dog breeder, so you are in no position to doubt what I say.

Let's cool down and analyze this volcanic eruption. Mario's statement, "The human race should not be mongrelized by interracial breeding," is loaded language filled with negative connotations. A less loaded replacement would be, "The human race should not produce children of parents from different races." The argument is primarily based on an analogy. The analogy is between having puppies of different breeds and having children of different races. There are important disanalogies to notice. Our background knowledge tells us that the purpose of dog breeding is to improve and retain the characteristics of the breed. The purpose of having children is not normally to improve and retain the racial characteristics of each parent. Did your parents have you primarily for design purposes? A second difficulty with the analogy is that even if mixing breeds produces mongrels that are of lesser quality in terms of winning blue ribbons in dog shows, it doesn't follow that mixing races produces children who are of lesser quality. In most societies, the citizens do believe that races shouldn't mix and that when they do they produce children who are "inferior," but this belief is based only on custom; there is no biological reason to believe that such children are physically or mentally inferior to their parents.

Mario was also mistaken in saying that if you lack expert knowledge about dog breeding, you should not doubt his claim. Our criticism of his analogy was based on common sense, not on any expert knowledge. His threatening to label you a "pervert" and not "logical" if you reject his argument is itself just name calling or intimidation. From a logical-reasoning perspective these threats do nothing positive for his position. If Mario were your boss, his attacks might convince you not to say you disagree with him, but his reasons shouldn't actually convince you to agree with him.

Exercise \(\PageIndex{1}\)

Evaluate this argument by analogy from 1940:

Armies are like people. If you cut off the head, the body may thrash around a bit, but very soon it quits fighting. So, a good way to win this European war against the Nazis and Fascists would be to concentrate all our energies on killing Hitler and Mussolini.

Answer: There is no doubt that if you cut off someone's head, the person will soon stop fighting. The problem is whether there is a message here for how to win World War II against the German and Italian armies led by Hitler and Mussolini, respectively. To some extent armies are like people. They eat, they sleep, they move, they fight. On the other hand, to some extent armies are not like people. They are composed of more than one person, they can be in many places at once, and a new head can easily be appointed, and so forth. The most important disanalogy, however, is that the person without a head has to stop fighting, but an army without a supreme leader does not have to stop fighting. Maybe the two armies would stop fighting if their supreme leaders were killed, but the argument by analogy does not provide a strong reason for this conclusion. In short, a person without a head has no brains; an army without a head still has the brains of its officer corps and individual soldiers. A much better case could be made for killing the supreme leader if it could be shown that, throughout history, armies have stopped fighting when their supreme leaders have been killed.

¹ This testimonial commits the post hoc fallacy.