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, then it 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 may 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 that 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 at consists of other's 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. There is, however, within the behaviourist philosophy, one important addition to be made to our definition. We began this chapter with sensitivity but we went on to the consideration of learned reactions, where the learning depended upon association. But there is another sort of learning - at least it is prima facie another sort - which consists of increase in sensitivity. All sensitivity in animals and human beings must count as a sort of knowledge; that is to say, if an animal behaves in the presence of a stimulus of a certain kind, as it would not behave in the absence of that stimulus, then, in an  important sense, it had `knowledge' as regards the stimulus. Now it appears that practice - e.g. in music - very greatly increases sensitivity. We learn to react differently to stimuli which only differ slightly; what is more, we learn to react to differences. A violin player can react with great precision to an interval of a fifth, if the interval is very slightly greater or less, his behaviour in tuning is influenced by the difference from a fifth. And as we have already had occasion to notice, we become, through practice, increasingly sensitive to form. All this increased sensitivity must count as increase of knowledge.


But in saying this we are not saying anything inconsistent with our earlier definition of knowledge. Sensitivity is essential in choosing the right reaction in many cases. To take the cookery book again: when it says `take a pinch of salt', a good cook knows how much to take, which is an instance of sensitivity. Accurate scientific observation, which is of great practical importance, depends upon sensitivity. And so do many of our practical dealings with other people: if we cannot `feel' their moods, we shall be always getting at cross purposes.


The extent to which sensitivity is improved by practice is astonishing. Town-bred people do not know whether the weather is warm or cold until they read the weather reports in the paper. An entomologist perceives vastly more beetles in the course of a country walk than other people do. The subtlety with which connoisseurs can distinguish among wines and cigars is the despair of youths who wish to become men of the world. Whether this increase of sensitivity can be accounted for by the law of association, I do not know. In many cases, probably, it can but I think sensitiveness to form which is the essential element in the more difficult forms of abstract thought as well as in many other matters, cannot be regarded as derived from the law of association, but is more analogous to the development of a new sense. I should therefore include improvement in sensitivity as an independent element in the advancement of knowledge. But I do so with some hesitation.


The above discussion does not pretend to cover the whole of the ground that has to be covered in discussing the definition of `knowledge'. There are other points of view, which are also necessary to a complete consideration of the question. But these must wait until, after considering the physical world, we come to the discussion of man as viewed from within.

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