"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 to narrow limits to the understanding, and ventured out beyond it on the wings of ideas, in the empty space of pure understanding. He did not observe that with all his efforts he made no advance - meeting no resistance that might, as it were, serve as a support upon which he could take a stand, to which he could apply his powers, and so set his understaning in motion."
--How Knowledge is Possible, Immanual Kant