A genealogy in philosophy is a dangerous undertaking. The historical roads are long, winding and overlapping, often doubling back and forth. However, I think a genealogy of epistemology, frought with danger though it is, can be reasonably established along the following lines.
Epistemic anxiety (EA) tends to be thought of as a fairly modern phenomena, generally emerging with Descartes. This anxiety, possibly more than anything else, defines modern philosophy. How do we know? What do we know? Do we, in fact, know things? This is not, however, a strictly modern attitude. As much as we like to think of epistemic anxiety as a post-cartesian condition, it is quite clearly something that afflicts the ancients.
I think we can establish something that looks like this: EA afflicted the ancients, did not afflict the medievals until the later medieval era, and reemerged with a vengeance by the dawn of the modern era. Consider Plato as a case study in the ancient era.
Plato is interesting in that he combines what we would call metaphysics and epistemology – if we were to divide his theory of knowledge in half, one half would be a modern-esque question of justification – what justifies a person in making a knowledge claim – and the other would be a near-kantian question – what must the world be like given the fact that we do know things? His epistemology is inseparable from his metaphysics, and though a large part of Plato’s writing is spent teasing out the question of justification (Socrates being the star of this particular show), just as much if not more time is spent on the metaphysical aspect of the question of knowledge, where this cashes out to his ‘theory of forms’. Here we have a metaphysical (we might today call it a ‘transcendental’) explanation of knowledge and the possibility of knowledge (taken to task by Aristotle, but that’s another story for another day).
Now, as we move to the medieval era, let us take Aquinas and William of Ockham as case studies in both non-skepticism and the beginning of skepticism.
What appears to happen is that by the medieval era, epistemology is separated from (but grounded in) metaphysics, and becomes wedded to psychology, Aquinas being the key example of this. As Fr. Copleston notes, it is futile to look for, in Aquinas, a proof of the certainty of knowledge or a rebuttal against subjective knowledge on favour of objective knowledge. The problem for Aquinas is how to justify and safeguard metaphysics, as opposed to justification of belief in the external world. Knowledge at this point in philosophical history seems to be simply given. Indeed, there are skeptics of knowledge of God – Scotus and Aquinas both argue that we can, in fact, have knowledge of God – but not skeptics of knowledge by itself. However, towards the end of the Middle Ages, loosely situated around William of Ockham, epistemic skepticism slowly begins taking shape – Gilson traces the twofold nature of this skepticism (epistemic and metaphysical, having to do with Ockham’s empiricism in both epistemology and causality, both of which were, if not entailed then strongly implied, by his nominalism) in ‘The Unity of Philosophical Experience’. Simply put, Gilson locates the error of Ockham in his proto-humean psychologism – that is, the mistaking of the’ description of our ways of knowing with the correct description of reality itself’, (p 71). Gilson argues that a consequence of psychologism is that, ‘Left without objective justification, human knowledge becomes a mere system of useful conventions, whose practical success remains a complete mystery to the minds of the very scientists who made it.’ (p. 72)
The space is thus cleared for the setting of the stage of modern epistemic anxiety – though some hundreds of years in the future, as Gilson astutely notes, once Ockham’s thought took root in the universities of Europe, medieval philosophy was on, ‘the straight road to skepticism’. (p. 72)