So I have been reading Devitt’s paper “No Place for the A Priori” where he lays out his case against a prori knowledge. His claim is that there is only one way of knowing, and that is the empirical way. His strategy in the paper is to first argue that the kinds of knowledge that people usually claim to be examples of the a priori (i.e. math, logic, and philosophy) can all be explained on the emprical model. This means that there are two options for how we come to have knowledge of (say) the logical truths and so we would then need to see if each option is equally viable. This lead him to the second part of his strategy, which is to argue that the notion of a priori knowledge is so mysterious and obscure that the relatively well worked out empirical model is to be preferred. He even goes so far as to suggest that if he is right, “it is not rational to believe in the a priori.” Oh irony of ironies!
What then is the empirical model? He says,
An answer starts from the metaphysical assumption that the worldly fact that p would make the belief that p true. The empirical justification of the belief is then to be found in its relationship to experiences that the worldly fact would cause. Justified beliefs are produced and/or sustained by experiences in a way that is appropriately sensitive to the way the world is. This is very brief and we shall return to the question later. Still it is hard to say much more.
So the empirical model looks like it boils down to the correspondance theory of truth. There is though a lot more to say. For instance ‘worldly fact that p’ is ambiguous as between (in Armstrong’s sense) a world of states of affairs (or, in Russell’s older terms ‘facts’), and a world of things. But we can leave that aside. His main claim is that even this sketchy characterization of the empirical model is more worked out than any account of a priori knowledge and can also account for our knowledge that seems to us a priori (math, logic, etc).
Now I am generally sympathetic to this claim, being a naturalist myself, but the way that he puts it certainly seems acceptable to some ‘a-prioraphiles’. Might not it be the case that the ‘worldly fact’ that modus ponens is a valid logical form play the kind of role that p does in the characterization above? Rational intuition is often characterized as a kind of ‘grasping’ with the mind. It is an intellectual kind of experience where one sees or appreciates some necessary fact about reality. Thus my beliefs about modus ponens would be ‘appropriately sensitive to the way the world is’. Of course, one couldn’t be a physicalist (like I am), again in Armstrong’s sense, but one could still be a naturalist…Or is that the a-priorafile can’t allow that modus ponens is a non-physical natural phenomena (i.e. a worldly phenomena that exists in space and time)?