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.