During a helpful conversation with Amber Carpenter today I realized that an argument from Kant that I've been interested in for a while may have an analogue in Plato.
The Kantian version of the argument is that experience cannot provide grounds for (certain of) our most fundamental concepts, since we need those concepts to be in place before we can have experiences of the right kind (this is most prominent in the Transcendental Aesthetic). The Platonic analogue (I'm looking at Phaedo 74c-75c here) is that experience cannot provide grounds for our grasp of (certain) forms, since whenever we experience something(s) as possessing the relevant quality or standing in the relevant relation we compare it/them (unfavourably) with the form itself, which wouldn't be possible unless we already knew the form.
Plato seems to take it that the forms in question are real, although they cannot be empirically known. Whereas Kant, believing that certain basic concepts cannot be empirically grounded, concludes that they do not correspond to (transcendentally) real features of the world.
(For the record, I myself am tempted to think both that our basic concepts are empirically grounded and that they reflect real structural features of the independent world.)