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.