observation. Suppose, for example, that, for the first time in your life, you smell assafoetida. You say to yourself "That is a most unpleasant smell". Now unpleasantness is a matter of self-observation. It may be correlated with physiological conditions which can be observed in others, but it is certainly not identical with these, since people knew that things were pleasant and unpleasant before they knew about the physiological conditions accompanying pleasure and its opposite. Therefore when you say "That smell is unpleasant" you are noticing something that does not come into the world of physics as ordinarily understood. You are, however, a reader of psycho-analysis, and you have learned that sometimes hate is concealed love and love is concealed hate. You say to yourself, therefore: "Perhaps I really like the smell of assafoetida, but am ashamed of liking it". You therefore make your friends smell it, with the result that you soon have no friends. You then try children, and finally chimpanzees. Friends and children give verbal expression to their disgust: chimpanzees are expressive, though not verbal. All these facts lead you to state: "The smell of assafoetida is unpleasant". Although self-observation is involved, the result has the same kind of certainty, and the same kind of objective verification, as if it were one of the facts that form the empirical basis of physics.
(2) The second proposition, to the effect that the physical sciences are capable of affording an explanation of all the publicly observable facts about human behaviour, is one as to which it is possible to argue endlessly. The plain fact is that we do not yet know whether it is true or false. There is much to be said in its favour on general scientific grounds, particularly if it is put forward, not as a dogma, but as a methodological precept, a recommendation to scientific investigators as to the direction in which they are to seek for solution of their problems. But so long as much of human behaviour remains unexplained in terms of physical laws, we cannot assert dogmatically that there is no residue which is theoretically inexplicable by this method. We may say that the trend of science, so far, seems to render
