In his book Plotinus on Intellect, Eyjófur Emilsson goes a long way to give an coherent account of the Intellect’s self-knowledge. Near the end he notes the probable impetus for Plotinus’s account being the skeptical dilemma that a person’s self-knowledge is impossible:
The argument in V.5.1 we just considered and the notion of truth in Intellect which ‘says what it is and is what it says’ is probably also prompted by sceptical considerations: it may be an attempt to block the kind of sceptical move which consists in insisting on a criterion for the validity of any proposed criterion (cf. Sextus Empiricus, PH 1, 166). Plotinus’ theory of divine thoughts is clearly meant to make such thoughts self-validating. In general it seems to be instructive to see Plotinus’ epistemological concerns—his contrast between cognition of images or impressions and knowledge of the things themselves as well as his insistence that genuine knowledge is identical with its object and true just in virtue of itself—in the light of sceptical considerations. His theory is so construed that it is supposedly impossible to put any wedge between Intellect and the object of its cognition.
(Eyjólfur Kjalar Emilsson, Plotinus on Intellect [Oxford: Clarendon Press, 2007], 170)
What we have here is a typical top-down move by Plotinus as opposed to appealing to some account of vividness (enargeia) of an impression (phantasia) the way the Stoics did. I find both approaches fascinating, especially—again—in light of Karamanolis’s book. I’d be interested to see if one can find a parallel development in Christian thought. I’m under the impression (but not the vivid kind) that one sees the likes of Clement of Alexandria making a Stoic-type move in his response to skeptical challenges, but I can’t say off the top of my head whether Origen or the Cappadocians prefer the bottom-up or top-down solution.