THE KANTIAN CRITICISM.--Kant says that it was Hume who "awoke him from his dogmatic slumbers." By this he meant that Hume made him realise that it was no use indulging in philosophic speculation generally, or listening to the speculations of others, until "the Problem of Knowledge" was satisfactorily solved. To this problem Kant applied himself. And recognising Locke to be the _fons et origo malorum_, he subjected his theory of human knowledge to a close analysis, and exposed it as being fallacious. Far from sense-experience being responsible for all our knowledge, Kant proved that important elements of knowledge are quite independent of sense-experience; especially was this so in the case of certain mathematical propositions. (Hence the question, How is pure mathematics possible? was put by Kant at the beginning of his philosophy.) But it is neither necessary nor desirable to enter into the arguments by means of which Kant proved his thesis, which was that the human mind contains in itself certain principles of knowledge (e.g. the idea of cause and effect, the ideas of mathematics, and so on) which it does not owe to sense-experience. KANT'S COPERNICAN HYPOTHESIS.--Kant called these principles of knowledge, _forms of thought_ or _categories_. The name, perhaps, is irrelevant to our purpose; all that we need to understand is that Kant turned the tables upon Locke. Locke said that the mind was a _tabula rasa_ which passively received impressions from outside. Kant said that the mind is nothing of the kind; it is not passive, but active; it does not "receive" whatever is offered, it "selects" what it wants; and _it imposes its own "forms of thought" upon the outside world_. Photography had not been invented at the time of this controversy, but Kant might have said: The mind is not a photographic plate receiving impressions from without, it rather resembles the lens which impressions must pass through, and be transformed by, before they can create a picture. Kant had, in fact, by this theory, instituted a revolution. His new dogma was: _The mind is the mould into which all our knowledge must be cast; and the constitution of our mind predetermines the shape that our knowledge takes._ Thus Kant had discovered that not only sensuous perception, but rational understanding also, has its forms and presuppositions. Just as we become aware of objects only by means of senses which perhaps hide or distort as much as they reveal; so also our rational knowledge is conditioned by the nature of our understanding, which dictates to reality the "forms" under which it can be understood and known.