We must further remark that the serious pursuit of any kind of science implies not only that there are truths, but that some of them, at least, can be known by man.  Hence there arises a problem which is not quite the same as that of logic.  What is the relation we mean to speak of when we talk of ‘knowing’ something, and what conditions must be fulfilled in order that a proposition may not only be true but be known by us to be true?  The very generality of this problem marks it out as one which belongs to what I have been all along calling Philosophy. (We must be careful to note that the problem does not belong to the ’special science’ of psychology.  Psychology aims at telling us how particular thoughts and trains of thought arise in an individual mind, but it has nothing to say on the question which of our thoughts give us ‘knowledge’ and which do not.  The ‘possibility of knowledge’ has to be presupposed by the psychologist as a pre-condition of his particular investigations exactly as it is presupposed by the physicist, the botanist, or the economist.) The study of the problem ’what are the conditions which must be satisfied whenever anything at all is known’ is precisely what Kant meant by Criticism, though the raising of the problem in this definite form is not due to Kant but goes back to Plato, who made it the subject of one of his greatest dialogues, the Theaetetus.  The simplest way to make the nature and importance of the problem clear is perhaps the way Mr. Russell adopts in the Problems of Philosophy—­to give a very rough statement of Kant’s famous solution.

Kant held that careful analysis shows us that any piece of knowledge has two constituents of very diverse origin.  It has a matter or material constituent consisting, as Kant held, of certain crude data supplied by sensation, colours, tones of varying pitch and loudness, odours, savours, and the like.  It has also a form or formal constituent.  Our data, when we know anything at all, are arranged on some definite principle of order.  When we recognize an object by the eye or a tune by the ear, we do not apprehend simply so much colour or sound, but colours spread out and forming a pattern or notes following one another in a fixed order. (If you reverse the movement of a gramophone, you get the same notes as before, but you do not get the same tune.) Further, Kant thought it could be shown that the data of our knowledge are a disorderly medley and come to us from without, being supplied by things which exist and are what they are equally whether any one perceives them or not, but the element of form, pattern, or order is put into them by our own minds in the act of knowing them.  Our minds are so constructed that we can only perceive things or think of them as connected by certain definite principles of orderly arrangement.  This, he thought, explains the indubitable fact that we can sometimes know universal