"The light dove, cleaving the air in her free flight, and feeling its resistance, might imagine that its flight would be still easier in empty space. It was thus that Plato left the world of the senses, as setting too narrow limits to the understanding, and ventured out beyond it on the wings of the ideas, in the empty space of the pure understanding. He did not observe that with all his efforts he made no advance...."
I don't know German, so of course I have no idea what the original passage as Kant composed it would sound or feel like to someone who does. But Kant's prose has a reputation for, um, Teutonic heaviness. Yet unless his translators have given him a huge gift, this passage shows that he had a light poetic side.
It seems to me to speak with great concision to the similarity/difference of Kant and Plato. They are both philosophers who distinguish between the apparently (or provisionally) real world on the one hand and the really real world on the other. They are both philosophers who think of the apparently real world as "this world," the one in which our problems and conflicts with one another arise. They both believe that in fundamental respects the solution to those problems, the solutions to the ultimate questions this world raises, consists in an appeal to the "other" world, the really real one.
So far so good, in terms at least of amity between these two figures. But it all leads us to the obvious tricky question: how do we distinguish what is really real from what is merely apparently real. On this point, Plato and Kant give exactly contrary answers. Plato says: the really real world is the one of which we have knowledge. Of the apparent world, we can only have opinions. Kant says precisely the reverse: the apparent world is the one we know, because our categories apply to it, and that is because it wouldn't appear to us at all if they didn't. The really real world is one we don't know (and, thus, the domain of faith).
Kant was expressing his own side of this argument in the above passage. We need the "resistance" of the world of sense in order to have any knowledge of anything at all.
The kind of knowledge that seems to Plato to tell us of the really real, for example the Pythagorean theorem, doesn't seem to Kant to be knowledge at all. Geometry becomes in his eyes a giant tautology, a closed system of "analytic" statements, telling us nothing about either of the two halves of Kant's dualism.