of self-contradiction makes great demands upon us; Hegel and Bradley imagined that we could know the nature of the universe by means of this principle alone. In this they were pretty certainly mistaken, but nevertheless a great deal of our “knowledge” depends upon this principle to a greater or less extent. Most of our knowledge is like that in a cookery book, maxims to be followed when occasion arises, but not useful at every moment of every day. Since knowledge may be useful at any time, we get gradually, through conditioning, a general desire for knowledge. The learned man who is helpless in practical affairs is analogous to the miser, in that he has become absorbed in a means. It should be observed, also, that knowledge is neutral as among different purposes. If you know that arsenic is a poison, that enables you equally to avoid it if you wish to remain in health, and to take it if you wish to commit suicide. You cannot judge from a man’s conduct in relation to arsenic whether he knows that it is a poison or not, unless you know his desires. He may be tired of life, but avoid arsenic because he has been told that it is a good medicine; in this case, his avoidance of it is evidence of _lack_ of knowledge. But to return to Columbus: surely, the reader will say, Columbus really did cross the Atlantic in 1492, and that is why we call this statement “knowledge”. This is the definition of “truth” as “correspondence with fact”. I think there is an important element of correctness in this definition, but it is an element to be elicited at a later stage, after we have discussed the physical world. And it has the defect--as pragmatists have urged--that there seems no way of getting at “facts” and comparing them with our beliefs: all that we ever reach consists of other beliefs. I do not offer our present behaviouristic and pragmatic definition of “knowledge” as the only possible one, but I offer it as the one to which we are led if we wish to regard knowledge as something causally important, to be exemplified in our reactions to stimuli. This is the appropriate point of view when we are studying man from without, as we have been doing hitherto.