Bealer has a number of reasons for thinking that a naturalistic epistemology is self-undermining. Let me focus on one of these. (I've tried to take on all of them in the first chapter of Knowledge and Its Place in Nature.)

I do think that it is legitimate to talk of goals and functions in nature, and that these things can be made sense of in naturalistic terms. There is nothing at all contrary to naturalism in the idea of goal-directed systems.

It's not just that there is a cooperative spirit of investigation there, where we all recognise that we are engaged in a common project of inquiry. It's also that the philosophers are well-versed in the relevant empirical data, and the scientists are well-versed in the more abstract issues which are typically the central focus of philosophical work.

I was often asked how one could even make sense of this. Isn't the category of knowledge something that we project upon the world, rather than something that we discover in it?

Chemists in earlier centuries were quite interested in the nature of acids. They had no interest in analysing their concept of acid. After all, they knew that their understanding of acids was at a fairly primitive level, and what they wanted to do was understand something about the world better - the nature of acidity - not something about their own concepts.

The great philosophers of the 17th and 18th centuries did not think that epistemological questions floated free of questions about how the mind works. Those philosophers took a stand on all sorts of questions which nowadays we would classify as questions of psychology, and their views about psychological questions shaped their views about epistemology, as well they should have.

But there is no doubt that my own views on this are, in quite a number of ways, very different from those of Quine.