Logic

As a beginner in philosophy, I got the impression that philosophy is all about arguments. You put in statements (premises), use some rules of argumentation to manipulate these premises, and reach other statements (conclusions). Is there a way to argue for the rules of argumentation themselves? I mean, we use them all the time but how do we know that they are true? What kind of rules would we use to prove the rules of argumentation? Can we use the same rules? Thanks.

Comments

This is a difficult and somewhat contested question. Obviously, you cannot argue for the rules of argumentation except by arguing, and if there are rules of argumentation that must be followed if an argument is to be compelling, then one had better follow them. So there is, obviously, a kind of circularity in any such argument. But it is an unusual, and somewhat confusing, kind of circularity. The worst kind of circularity is what is called "begging the question". That is when you simply assume, perhaps tacitly, precisely what you want to prove. It's a bad kind of circularity because, if you assume that P, you can hardly help but reach the conclusion that P.

That's not what's happening in the case we're considering. Suppose we're trying to prove the logical principle of disjunctive syllogism, which says that, if "A or B" is true and "not A" is true, then B must be true. We might argue for it as follows.

Suppose that "A or B" is true and that "not A" is true. Since "A or B" is true, either A is true or B is true. Since "not A" is true, A is not true. So B must be true.

This argument uses the disjunctive syllogism. (It also uses principles about "or" and "not".) It does not, however, suppose as a premise that disjunctive syllogism is, in general, valid. In fact, all the argument needs is that this particular application of disjunctive syllogism is correct. Of course, if that application is not correct, then the argument proves nothing, but the argument is not simply circular.

Must every argument for a "rule of argumentation" use that very rule? No, certainly not. One can easily produce an argument for the disjunctive syllogism that does not use it. What is true, though, is that any such argument must use some such rules.

Many years ago a meteorologist told me that persistence forecasting compares favourably with other, more sophisticated rules for predicting the weather. When I asked the obvious question, she told me that persistence forcasting is the rule that says that the weather tomorrow will be the same as it was today. One thing (though not the first thing) that struck me about this reply was that it does seem perfectly coherent to argue that persistence forecasting is likely to be about as reliable in the future as it was observed to be in the past, even though persistence forecasting is itself a rule that says that that the future will be like the past. If you never checked its track record, you would have no reason to trust persistence forecasting; but if you did, you would.

(If you are familiar with the Humean problem of induction, this may suggest to you that it is possible to defend an inductive justification of induction after all. That's what it suggests to me.)

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