Immaterial Aspects of Thought by James Rosshttp://www3.nd.edu/~afreddos/courses/43151/ross-immateriality.pdf
ANIMAL cognition and desire, from the appetite of a clam to the optical systems of vultures and frigate birds, is supposed to have neurobiological explanations resultant from, if not reducible to, universal laws of physics. That is a minimal and modest project for epistemology naturalized, one to be assisted by specialized sciences.
There is a larger and bolder project of epistemology naturalized, namely, to explain human thought in terms available to physical science, particularly the aspects of thought that carry truth values, and have formal features, like validity or mathematical form. That project seems to have hit a stone wall, a difficulty so grave that philosophers dismiss the underlying argument, or adopt a cavalier certainty that our judgments only simulate certain pure forms and never are real cases of, e.g., conjunction, modus ponens, adding, or genuine validity. The difficulty is that, in principle, such truth-carrying thoughts2 cannot be wholly physical (though they might have a physical medium),3 because they have features that no physical thing or process can have at all.