It will be remembered that throughout Part I, we agreed to consider only those facts about a man which can be discovered by external observation, and we postponed the question whether this excluded any genuine knowledge or not. The usual view is that we know many things which could not be known without self-observation, but the behaviourist holds that this view is mistaken. I might be inclined to agree wholly with the behaviourist but for the considerations which were forced upon us in Part II, when we were examining our knowledge of the physical world. We were then led to the conclusion that, assuming physics to be correct, the data for our knowledge of physics are infected with subjectivity, and it is impossible for two men to observe the same phenomenon except in a rough and approximate sense. This undermines the supposed objectivity of the behaviourist method, at least in principle; as a matter of degree, it may survive to some extent. Broadly speaking, if physics is true and if we accept a behaviourist definition of knowledge such as that of Chapter VIII, we ought, as a rule, to know more about things that happen near the brain than about things that happen far from it, and most of all about things that happen in the brain. This _seemed_ untrue because people thought that what happens in the brain is what the physiologist sees when he examines it; but this, according to the theory of Chapter XII, happens in the brain of the physiologist. Thus the _a priori_ objection to the view that we know best what happens in our brains is removed, and we are led back to self-observation as the most reliable way of obtaining knowledge. This is the thesis which is to be expanded and sustained in the present chapter. As every one knows, the certainty of self-observation was the basis of Descartes’ system, with which modern philosophy began. Descartes, being anxious to build his metaphysic only upon what was absolutely certain, set to work, as a preliminary, to doubt anything that he could make himself doubt. He succeeded in doubting the whole external world, since there might be a malicious demon who took pleasure in presenting deceitful appearances to him. (For that matter, dreams would have supplied a sufficient argument.) But he could not manage to doubt his own existence. For, said he, I am really doubting; whatever else may be doubtful, the fact that I doubt is indubitable. And I could not doubt if I did not exist. He summed up the argument in his famous formula: _I think, therefore I am_. And having arrived at this certainty, he proceeded to build up the world again by successive inferences. Oddly enough, it was very like the world in which he had believed before his excursion into scepticism.