Probably next years Cabernet Sauvignon harvest will be good

With suitable connecting premises added (for example: 'If next summer's weather is good for Cabernet Sauvignon grapes, then, probably, next year's Cabernet Sauvignon harvest will be good'), this becomes an inductively forceful argument, and P2 is true. Now suppose Jane knows that P2 is true. She also thinks that PI is true, but her reason is that she has the superstitious belief that whenever it rains on Winter solstice, the following summer will be hot (assume it really is unreasonable for her to accept PI; as we will see in more detail later, what are often called 'superstitious' beliefs are not always unreasonable). In that case, assuming it did rain on Winter solstice, she may think her belief in PI is reasonable or well-founded, but it is not. So in that case, having noticed its inductive forcefulness, she may well think that this argument is rationally persuasive for her, but she would be wrong. She may be persuaded, but she is not rationally persuaded.

Third, you can be mistaken about whether or not an argument is defeated for you. On the one hand, you might accept an argument for a given conclusion - thereby accepting the argument as rationally persuasive - without realizing that you have information sufficient to construct another argument that defeats the conclusion of the first argument. You might, for example, accept an argument for the conclusion that Mr Jones is at home, momentarily forgetting that you know that Mr Jones boarded a flight for Singapore yesterday. On the other hand, you might think that an argument is defeated for you when it isn't. You might, that is, mistakenly think you have good reasons to reject the conclusion of an argument that is inductively forceful and whose premises you accept.

Obviously one can give arguments for various purposes, including deceitful ones. But the rational, non-deceitful motivation for giving an argument is surely this: It is to give a sound argument which is rationally persuasive for oneself, and for its intended audience. Different arguments may be required for different audiences. We cannot always know with certainty whether an argument is sound, but that is the human predicament. It is simply a consequence of the fact that we do not always know with certainty which propositions are true and which false. If we did, we would never need arguments.

6 In saying that an argument is rationally persuasive for a person only if the person reasonably believes the premises, we are not requiring that the person have, at his or her disposal, further arguments with those premises as conclusions. What we are requiring is that the person be justified in accepting the premises. Justification is a wider concept than rational persuasiveness: if one has a rationally persuasive argument for a proposition then one is justified in accepting it, but one may be justified in accepting it by means other than argument. In particular, some beliefs, especially perceptual beliefs such as 'I see a dog in front of me', are often justified - reasonable - even though they are not inferred from other beliefs. We will return to this in the next chapter.

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