Probably Jane owns at least one item of woollen clothing

This is certainly an inductively forceful argument. Suppose in fact you have good reason to accept both PI and P2. Indeed, suppose you know with certainty that they are both true. Does it follow that you should accept C? Can you imagine a situation in which you know that PI and P2 are true, but in which you could reasonably reject C? Yes. For suppose that, in addition to your knowledge of PI and P2, you know that Jane is violently allergic to wool. In that case, you could quite reasonably expect C to be false. Someone who knew the truth of PI and P2, but nothing else about Jane, could reasonably expect C to be true, but not you. The argument would be rationally persuasive for them, but not for you.

What we say, in such a case, is that the argument is defeated for you by other evidence that you have; you have, in effect, a more compelling argument for the falsity of the first argument's conclusion (the concept of evidence is discussed further in Chapter 7). In particular:

To say that an inductively forceful argument is defeated for a person is to say: the person reasonably believes the premises, but, nevertheless, reasonably rejects the conclusion.

An inductively forceful argument whose premises you have reason to accept is rationally persuasive only if your total evidence does not defeat the argument for you. Note that the definition pertains only to inductive arguments. The reason for this will be clear in a moment. Thus our definition:

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