Kant, indeed, laid the foundations of philosophy afresh. With characteristic insight, he went to the very root problem of all, and challenged human thought itself. Before we can know anything, we must first of all demand the credentials of the instrument by which knowledge is gained. Before asking, _What_ do I know? the preliminary question should be, _How_ do I know? Otherwise we cannot say whether we are in a position to give any answer to those ultimate problems, the answers to which constitute philosophy. It is far from easy to present Kant's criticism of knowledge at once simply and accurately. This philosopher has a not undeserved reputation for obscurity, and had he written in any other language than German, he would perhaps have found no readers. THE PROBLEM OF KNOWLEDGE.--It had already been realised by the predecessors of Kant that what is called "sense-experience" is a less simple process than it seems, and that our senses cannot be said to reveal to us any object as it actually _is_. John Locke himself was not the first to point out that the so-called "secondary qualities" of any material object (i.e. colour, taste, etc.) are produced just as much by the person who perceives, as by the object which is perceived. Galileo, Descartes, and Hobbes, besides others, had been aware of this fact, which indeed becomes evident to the most superficial analysis of sense-experience. The "primary qualities," i.e. density, extension, etc., continued to be regarded as subsisting _in_ the objects themselves, and independently of any perceiving consciousness. But even this view did not prove permanent, and it was the episcopal philosopher, George Berkeley (1685-1753) who demonstrated in his _New Theory of Vision_ that not even _these_ qualities could rightly be regarded as subsisting independently. Thus it had already been realised, long before Kant wrote his _Critique of Pure Reason_ (published, 1781), that our senses are far from revealing to us things as they _are_; it is only the _appearances_ of things and not the _things themselves_ that the senses present to us. Indeed, as is well known, the Scotch philosopher David Hume (1711-1776), who was a master in the art of raising problems, extended this line of criticism until it reached to pure scepticism. He put the question, If all our knowledge is derived from sense-experience, and _if_ sense-experience only supplies us with appearance and not reality, what degree of trustworthiness can there be in human knowledge? And he was not afraid to give the logical answer--None. Hume may thus be said to have brought things to an impasse. As a matter of fact, what he had done was to refute Locke's theory of knowledge (i.e. that it is derived entirely from sense-experience) by means of a _reductio ad absurdum_.