In a series of earlier posts I have been giving an evolutionary argument against rationalism. In the course of doing so I have appealed to Devitt’s abduction argument. But I also think we can give an a priori argument against rationalism. That is, an argument to the effect that the empirical method is the only way to know anything about reality.
Is it conceivable that there are absolutely no necessary truths? It would seem not. For that would mean that it was true in every possible world that there were no necessary truths but if that were the case then it would be a necessary truth that there are no necessary truths (since it is true in all possible worlds) and that would make it false that in every possible world that there are no necessary truths. So we derrive a contradiction from the assumption that every truth is contingent and so must conclude that there is at least one necessary truth.
But is it contradictory to think that there were only one necessary truth? Could it be necessary that everything (other than this statement) was contingent? My intuition tells me that it is a neccesaary fact about reality that everything (besides this fact) is contingent. If you take the time to clearly and distinctly think about matters you will come to see, by the light of reason, that everything is necessarily such that it is contingent. This is because each thing which is claimed to exist of necessity can be conceived not to be necessary. And the claim that it is either necessary that p or necessary that not p can be conceived to be false as well, as when I conceive that numbers exist as non-physical objects in some possible worlds and do not exist at all in others.
According to the rationalist it is conceivable, so possible, that there be just this one necessary truth. Since it is possible that it is necessary, it is necessary. So we can have a priori knowledge about the world, but it consists in this only: we must know the world via the empirical method.
Richard Brown 
Hi Richard,
Before assessing an argument such as this, I think it would be good to have some characterization of what you mean by “rationalism” and “conceivable”.
Have you ever read “In Defense of Pure Reason” by Laurence Bonjour? He develops a version of rationalism which he calls “moderate rationalism”, according to which a priori insight is fallible and can be corrected by subsequent empirical discoveries together with further rational reflection.
Also, you say:
“And the claim that it is either necessary that p or necessary that not p can be conceived to be false as well, as when I conceive that numbers exist as non-physical objects in some possible worlds and do not exist at all in others.”
I fail to see how that is a counterexample to “it is either necessary that p or necessary that not p”; if numbers exist as non-physical objects in some possible worlds and not in others their existence is contingent, and the statement “numbers necessarily exist as non-physical objects” is necessarily false. Or have I misunderstood your point?
Is it conceivable that there are absolutely no necessary truths? It would seem not. For that would mean that it was true in every possible world that there were no necessary truths but if that were the case then it would be a necessary truth that there are no necessary truths (since it is true in all possible worlds) and that would make it false that in every possible world that there are no necessary truths
I don’t know, this seems way too easy. There are what Kripke calls “non-normal worlds”, after all, in which nothing is necessary. These worlds figure in models for S6 (which is something like S2 + MMp), and they are also used in models for S2 and S3. See Hughes and Cresswell, Routledge, 1996). So there is this snag: if you want to offer a proof of what’s conceivable, you’ll need to choose some logic (implicitly or not) to work out the proof. In S2 and S6, it is no doubt possible that nothing is necessary. So you can’t choose either of those logics to prove that it is inconcievable that there are no necessary truths. But avoiding just those logics in which you don’t get the result you want looks pretty clearly like begging the question.
Hi Jason,
Thanks for the comment.
I agree about the definitional issue. I have been using the terms loosely. By rationalism I mean a view that claims that there are necessary truths about reality that can be discovered a priori solely by the use of reason. I haven’t read that Bonjour book, but I am thinking about getting it.
As for the counter-example bit. I think I put the point poorly. What I was trying to head off was the kind of rationalist who will admit that it is epistemically possible that there be no numbers, but that if it turns out that there are no numbers, that fact would be necessary. I was suggesting that neither be necessary, but I didn’t mean to be doing what you took me to be doing.
Mike,
Thanks for the comment. I am very sympathetic to the point that you are making. There are two things I want to say in response. First, I do think that the argument is question begging against those who accept logics that allow for non-normal worlds. But though it is question begging against those people, it is also a useful argument against those rationalists who accept S5 or any logic that doesn’t allow non-normal worlds. It is not question begging against them. I guess I was assuming that most rationalists intuit that S5 is the one-true logic (I mean I though Kripke was a bit more of a conventionalist about these things than most)…but maybe I should take into account a more modust rationalism like Jason suggests that Bonjour argues for.
Second, your point shows a more general problem for the rationalist. How is it possible for the rationalist to give any argument for accepting one logic over another? Any such argument will beg the question in just the way you suggest. The only way to pick between them is their empirical adequacy. BUt maybe this is just the modest version of rationalism? If so, I wouldn’t have any problem with it, since it seems identical to classical empiricism.
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