In an analogy, we compare one thing with another. We might describe a person as being like a fox, a prickly rose, a robot, a hurricane. Love is said to be a disease, a game, a drug, a heatwave, and “a smoke made with the fume of sighs” (Shakespeare’s Romeo and Juliet). Less poetically, a physicist might compare an atom to the solar system—electrons revolve around a nucleus at the center like planets go around the sun.

In this chapter, we focus on the use of analogy in explanation and argument.1 The first point to note is that words such as similar and like have incomplete meaning. Saying that two things are similar has a concrete meaning only with respect to some standard of comparison. Pick any two objects, and they are bound to be similar in some way. A washing machine is like a pigeon—they both occupy space, produce waste, and are noisy. An informative analogy should make it clear how two things are similar—that is, what their common properties are. Former U.S. President Ronald Reagan once said “Government is like a baby” which on its own is rather incomprehensible. But then he added, “an alimentary canal with a big appetite at one end and no sense of responsibility at the other,” which immediately made the remark a witty and memorable one about government overspending.

Identifying common properties is very important in analogical arguments, where we argue that because X is similar to Y, something that is true of X is also true of Y. Here are ...

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