pleasant and certain others unpleasant is no part of physics. Finally, we come to imaginations, hallucinations, and dreams. In all these cases, we may suppose that there is an external stimulus, but the cerebral part of the causal chain is unusual, so that there is not in the outside world something connected with what we are imagining in the same way as in normal perception. Yet in such cases we can quite clearly know what is happening to us; we can, for example, often remember our dreams. I think dreams must count as “thought”, in the sense that they lie outside physics. They may be accompanied by movements, but knowledge of them is not knowledge of these movements. Indeed all knowledge as to movements of matter is inferential, and the knowledge which a scientific man should take as constituting his primary data is more like our knowledge of dreams than like our knowledge of the movements of rats or heavenly bodies. To this extent, I should say, Descartes is in the right as against Watson. Watson’s position seems to rest upon naive realism as regards the physical world, but naive realism is destroyed by what physics itself has to say concerning physical causation and the antecedents of our perceptions. On these grounds, I hold that self-observation can and does give us knowledge which is not part of physics, and that there is no reason to deny the reality of “thought”. CHAPTER XVII IMAGES In this chapter we shall consider the question of images. As the reader doubtless knows, one of the battle-cries of behaviourism is “death to images”. We cannot discuss this question without a good deal of previous clearing of the ground. What are “images” as conceived by their supporters? Let us take this question first in the sense of trying to know some of the phenomena intended, and only afterwards in the sense of seeking a formal definition. In the ordinary sense, we have visual images if we shut our eyes and call up pictures of scenery or faces we have known; we have auditory images when we recall a tune without actually humming it; we have tactual images when we look at a nice piece of fur and think how pleasant it would be to stroke it. We may ignore other kinds of images, and concentrate upon these, visual, auditory, and tactual. There is no doubt that we have such experiences as I have suggested by the above words; the only question is as to how these experiences ought to be described. Then we have another set of experiences, namely dreams, which feel like sensations at the moment, but do not have the same kind of relation to the external world as sensations have. Dreams, also, indubitably occur, and again it is a question of analysis whether we are to say that they contain “images” or not.