Arguments by analogy rest on a comparison between two cases. They examine a known case, and extend their findings there to an unknown case. Thus we might reason that because we find it difficult to forgive a girlfriend or boyfriend who cheated on us (a known case), it must be extremely difficult for someone to forgive a spouse who has had an affair (an unknown case).

This kind of argument relies on the cases compared being similar. The argument is only as strong as that comparison. If the two cases are dissimilar in important respects, then the argument commits the weak analogy fallacy.

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