So I've been parsing Kant's aesthetics. The main source of it is the Critique of Judgement, the third section of Kant's massive critical project (the first two parts being the more well-known Critiques of pure reason and practical reason, that is, ethics). To put it plainly, the half of the Critique of Judgement dealing with aesthetics is not exactly the watertight freighter you might expect from the author of the Critique of Pure Reason. Kant spends a lot of ink distinguishing between "free beauty," that is, beauty that is perceived without any intermediary concepts, and "dependent beauty," considering something beautiful (or having aesthetic merit) based on comparison with some pre-existing concept in the subject's mind. Only a perception of free beauty qualifies as a true aesthetic judgement; if there's an intervening concept, then the subject is merely judging what is agreeable or functionally good. But, of course, only the perceiving subject knows whether their judgement is concept-free and therefore aesthetically valid, and Kant admits that the perceiving subject is an unreliable witness, often unaware that a perception of beauty is based on a concept. Which, of course, makes it tricky to tell whether an aesthetic judgement can be universally valid, which is Kant's ultimate goal. How can an aesthetic judgement be universal if a) only the individual knows for sure whether it's a true aesthetic judgement in the first place, and b) not even then? Nonetheless, Kant goes on to assert that aesthetic judgements can, in fact, be universally valid, basically by engaging in a little rhetorical second-dealing and hoping his sleight-of-hand is good enough that you don't really notice.
This is, of course, a bare-bones summary. But throughout the Critique of Aesthetic Judgement, one gets a definite sense that Kant's heart really isn't in this one to the extent it was in the first two Critiques. Part of this is quite possibly due to Kant's own aesthetic preferences—he wasn't a painting/sculpture/music guy, he was a literature/poetry guy, which gets him in a bit of a pickle regarding that difference between free and dependent beauty. (Music without words, he notes on more than one occasion, is a prime example of something perceived as free beauty, meaning it's happy hunting ground for true aesthetic judgements, yet he ranks it far below poetry, in spite of poetry's necessarily dependent status, reliant on the intervening concept of language. Hmmmm.) Kant is far too good a philosopher to traffic in the usual 18th-century aesthetic concept of "rummaging among the details of individual subjectivity for the grounds of the aesthetic," as James Kirwan puts it, but he doesn't really come up with anything solid in its place, even as Enlightenment habit causes him to maintain the possibility of a universally valid judgement.
If Kant's purpose in the Critiques was to uphold Enlightenment rationality, then it's hard not to think that he might have been better off quitting after the first two. It's remarkable how much of the Critique of Judgement reads like a man walking up to the edge of Romanticism but not crossing the line—not because that's what Kant was doing, but because, like a blanket laid out for a picnic, the fuzzy portions of his argument are so inviting. Aesthetics was a primary front across which the Romantics would assault the Enlightenment. It's as if Kant built his formidable fortress of pure and practical reason, and then, with aesthetics, inadvertently told everyone where the spare key was hidden.
Mary Mothershill, writing in A Companion to Aesthetics, speculates why Kant felt the need to delve into the aesthetic wilderness:
... Kant's motive in the third Critique is not to bridge gaps and achieve unity; the distinctions insisted on in the first two Critiques are a priori and necessary, not to be overridden. His wish is, rather, to make the whole system less austere and more congenial. That, one might argue, is a retrograde step: it is not the philosopher's job, any more than it is the scientist's, to come up with results that are attractive and inspiring.But there could be another reason—as a good Enlightenment philosopher, Kant may have been driven to come up with some account of aesthetics simply to complete his system. The rationalist in Kant was impelled to finish the house he had framed even though he didn't have much interest in interior decoration.
The thing is, Kant's basic aesthetic insight points down a really interesting path. As Kirwan explains (clearer than Kant does), in Kant's aesthetics, an aesthetic judgement is not something you do, it's something that happens to you, and the philosophical circle to be squared is in knowing that such a judgement is, in fact, happening. Aesthetics doesn't originate with the subject, but it isn't anything intrinsic in the object perceived, either—it is, instead, the mind's reaction to an influx of sense-data that's too much to think about all at once. Therein lies the difference between the Enlightenment and the Romantics: Kant pools that excess into the concept-stocked pond of dependent beauty, but the Romantics let it overflow all the way to the mind's horizon, where, if you look hard enough, you might catch a glimpse of the Divine.