Analogical reasoning is one of the most common methods by which human beings attempt to understand the world and make decisions. We seem to have evolved with a propensity to find patterns in things and events, even when no such patterns exist.
Suppose, for example, that I am thinking about buying a new washing machine. I’m very likely to speak with other people who have recently bought new washing machines, noting their experiences with various makes, models, and dealers. If I discover that three of my friends have recently bought a particular brand that all three have been delighted with, then I might conclude by analogy that if I buy the same brand, I will be delighted too. Yet it is possible that some models of the same brand of washing machine are sufficiently different that the analogy is misleading.
The Argument from analogy is a special type of inductive argument, whereby perceived similarities between two or more things are used as a basis to infer some further similarity that has yet to be observed. A typical structure or form of the argument is:
Premise 1: P and Q are similar in respect to properties a, b, and c.
Premise 2: P has been observed to have further property x.
Conclusion: Therefore, Q probably has property x also.
Of course, the premises do not claim that P and Q are identical, only that they are similar. The argument may provide us with good evidence for the conclusion, but the conclusion does not follow as a matter of logical necessity. Determining the strength of the argument requires that we take into consideration more than just its form – the content of the premises must also come under scrutiny. An argument from analogy with insufficient inductive strength is fallacious. This fallacy is related to the Faulty generalisation fallacy.