To answer the last clause in your post, Logicon, that is a tough question. Let me put it this way: before Descartes, philosophy and laymen alike had a bit different conception on the relationship of subject and object. Kantian philosophy couldn't have happened in the 1500's.

The difference between Kant and Plato is that Kant is an "activist," he practises a critical theory.

Plato - he started out with epistemology, it was the main feature throughout, in my opinion. He asked, first "What is good? What is evil?" and after that (for example, in Theaitetos): "What is knowledge?"

And if that question is presented, it is epistemology after that! :-)

But Plato's problem was not that our "forms of Anschauung" and "categories of Verstand" organise our sense perception - quite the contrary, he found sense perception fallible due to the fact it showed change, no clear limits, chaos. In addition to this, there was the question of how we use our (universal) concepts. And the "theory of ideas" is an answer, or a brilliant attempt to answer, both of these questions. Kant just thought we cannot ever separate, or "turn off," the elements of our mind.

Don't you think?