We are concerned in these chapters with what we can know about other men by merely observing their behaviour. In this chapter, I propose to consider everything that would commonly be called “memory”, in so far as it can be made a matter of external observation. And perhaps it may be as well, at this point, to state my own view of the question of “behaviourism”. This philosophy, of which the chief protagonist is Dr. John B. Watson, holds that everything that can be known about man is discoverable by the method of external observation, _i.e._ that none of our knowledge depends, essentially and necessarily, upon data in which the observer and the observed are the same person. I do not fundamentally agree with this view, but I think it contains much more truth than most people suppose, and I regard it as desirable to develop the behaviourist method to the fullest possible extent. I believe that the knowledge to be obtained by this method, so long as we take physics for granted, is self-contained, and need not, at any point, appeal to data derived from introspection, _i.e._ from observations which a man can make upon himself but not upon any one else. Nevertheless, I hold that there are such observations and that there is knowledge which depends upon introspection. What is more, I hold that data of this kind are required for a critical exposition of physics, which behaviourism takes for granted. I shall, therefore, after setting forth the behaviourist view of man, proceed to a scrutiny of our knowledge of physics, returning thence to man, but now as viewed from within. Then, finally, I shall attempt to draw conclusions as to what we know of the universe in general.