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 everyone 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.
It is instructive to contrast this argument with Dr. Watson's. Dr. Watson, like Descartes, is sceptical of many things which others accept without question; and, like Descartes, he believes that there are some things so certain that they can be safely used as the basis of a startling philosophy. But the things which Dr. Watson regards as certain are just those which Descartes regarded as doubtful, and the thing which Dr. Watson most vehemently rejects is just what Descartes regarded as absolutely unquestionable. Dr. Watson maintains that there is no such thing as thinking. No doubt he believes in his own existence, but not because he thinks he can think. The things that strike him as absolutely indubitable are rats in mazes, time-measurements, physiological facts about glands and muscles, and so on. What are we to think when two able men hold such opposite views? The natural inference would be that everything is doubtful. This may be true, but there are
