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 degrees of doubtfulness, and we should like to know which of these two philosophers, if either, is right as to the region of minimum doubtfulness. Let us begin by examining Descartes’ view. “I think, therefore I am” is what he says, but this won’t do as it stands. What, from his own point of view, he should profess to know is not “_I_ think”, but “there is thinking”. He finds doubt going on, and says: There is doubt. Doubt is a form of thought, therefore there is thought. To translate this into “_I_ think” is to assume a great deal that a previous exercise in scepticism ought to have taught Descartes to call in question. He would say that thoughts imply a thinker. But why should they? Why should not a thinker be simply a certain series of thoughts, connected with each other by causal laws? Descartes believed in “substance”, both in the mental and in the material world. He thought that there could not be motion unless something moved, nor thinking unless some one thought. No doubt most people would still hold this view; but in fact it springs from a notion--usually unconscious--that the categories of grammar are also the categories of reality. We have already seen that “matter” is merely a name for certain strings of sets of events. It follows that what we call motion of matter really means that the centre of such a set of events at one time does not have the same spatial relations to other events as the connected centre at another time has to the connected other events. It does not mean that there is a definite entity, a piece of matter, which is now in one place and now in another. Similarly, when we say, “I think first this and then that”, we ought not to mean that there is a single entity “I” which “has” two successive thoughts. We ought to mean only that there are two successive thoughts which have causal relations of the kind that makes us call them parts of one biography, in the same sort of way in which successive notes may be parts of one tune; and that these thoughts are