Reasoning from analogy

An analogy is a special form of reasoning, which has some similarities with reasoning from specific cases. Reasoning by analogy involves drawing an equally specific conclusion from specific premises via a comparison of like aspects. Good analogies avoid comparisons between items that have too many dissimilarities. For example:

Imagine a friend gave you a guinea pig to look after but forgot to tell you anything about what to feed it. You might say to yourself, 'I have a guinea pig and do not know what to feed it; but I do know that my rabbit eats carrots, and that rabbits and guinea pigs are similar. Hence, I can probably feed my guinea pig carrots as well'.

Such arguments take the following general form:

An analogy between X and Y (in the premises) supports a conclusion about Y by showing that the conclusion is true of X; and X and Y are similar in sufficient relevant respects and are not relevantly dissimilar.

You need to be careful to make sure that you are comparing things that are similar in a relevant way. Take the following example of reasoning:

Shaving cream is clearly similar in colour, texture, moistness, and body to whipped cream, and I know that whipped cream is delicious on fruit salad. Hence, shaving cream is delicious on fruit salad.

Do you see what is wrong? The two types of cream are similar, but they are definitely not similar in respect of the one main characteristic involved in fruit salad eating: how they taste. This question of relevance has been explored in more detail in chapter 6.

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