What did he do? Well, for starters, there’s the whole noumenal/phenomenal distinction. See, our Kant thinks we can’t know the way things really are (i.e. the noumenon), but that we only know what we do by way our own acts of conceptualization. So he has to tell us how we take up sense impingement under spacial-temporal forms (the “transcendental aesthetic”, he likes to call it). And how, whenever we bundle these things up — yes, yes, into “objects of experience”, without which everything would be “still less than a dream” — we use those twelve categories of his.
What’s wrong with this? Are you kidding? Have you even taken a look at these categories? Well:
Of Quantity: Unity; Plurality; Totality
Of Quality: Reality; Negation; Limitation
Of Relation: Inherence and Subsistence; Causality and Dependence; Community
Of Modality: Possibility/Impossibility; Existence/Non-existence; Necessity/Contingence
Now put on your Kantian goggles and take another good look. There exist a plurality of categories, each discreet, that is, limited in function, all depending on incoming sense-impingements to have something to work with. To make any sense of the categories, we must apply them to themselves.
Now, either these categories are (a) real, noumenal, describing some part of the furniture of reality or (b) not. If (a), we are--through mere consideration of the categories themselves--afforded a glimpse into the noumenon, and see some fine specimens of Quantity, Quality, Relation and Modality. That is, we get a glimpse of the very sort that Kant rules impossible. The very sort of glimpse that, judged impossible, prompted him to dream up these same categories in the first place.
If (b), the categories themselves are a fiction of the mind, a mere conceptualization of conceptualization. In which case the Critique is just so much hot air.
Kant’s epistemology, then, is either false or fictional. Take your pick. And Kant’s epistemology grounds everything else he wrote. Without it we’re left with nothing but some fine cosmopolitan sentiments. Yes, time for bed indeed.