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.
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 connected with the body which is speaking in the way (to be further investigated) in which thoughts and bodies are connected. All this is rather complicated, and cannot be admitted as part of any ultimate certainty. What Descartes really felt sure about was a certain occurrence, which he described in the words “I think”. But the words were not quite an accurate representation of the occurrence; indeed, words never can escape from certain grammatical and social requirements which make them say at once more and less than we really mean. I think we ought to admit that Descartes was justified in feeling sure that there was a certain occurrence, concerning which doubt was impossible; but he was not justified in bringing in the word “I” in describing this occurrence, and it remains to be considered whether he was justified in using the word “think”.
In using a general word such as “think”, we are obviously going beyond the datum, from a logical point of view. We are subsuming a particular occurrence under a heading, and the heading is derived from past experience. Now all words are applicable to many occurrences; therefore all words go beyond any possible datum. In this sense, it is impossible ever to convey in words the particularity of a concrete experience; all words are more or less abstract. Such, at least, is a plausible line of argument, but I am by no means sure that it is valid. For example, the sight of a particular dog may make the general word “dog” come into your mind; you then know that it is a dog, but you may not notice what sort of dog it is. In this sense, the knowledge with which we start is abstract and general; that is to say, it consists of a learned reaction to a stimulus of a certain sort. The reactions, at any rate in so far as they are verbal, are more uniform than the stimuli. A witness might be asked “Did you see a dog?” “Yes.” “What sort of dog?” “Oh, just an ordinary dog; I don’t remember more about it.” That is to say, the witness’s reaction consisted of the generalised word “dog” and no more. One is almost reminded of quantum phenomena in the atom. When light falls on a hydrogen atom, it may make the electron jump from the first orbit to the second, or to the third, etc. Each of these is a generalised reaction to a stimulus which has no corresponding generality. So dogs and cats have each their individual peculiarities, but the ordinary inobservant person responds with the generalised reaction “dog” or “cat”, and the particularity of the stimulus leads to no corresponding particularity in the knowledge-reaction.
To return to Descartes and his thinking: it is possible, according to what we have just said, that Descartes knew he was thinking with more certainty than he knew what he was thinking about. This possibility requires that we should ask what he meant by “thinking”. And since, for him, thinking was the primitive certainty, we must not introduce any external stimulus, since he considered it possible to doubt whether anything external existed.
Descartes used the word “thinking” somewhat more widely than we should generally do nowadays. He included all perception, emotion, and volition, not only what are called “intellectual” processes. We may perhaps with advantage concentrate upon perception. Descartes would say that, when he “sees the moon”, he is more certain of his visual percept than he is of the outside object. As we have seen, this attitude is rational from the standpoint of physics and physiology, because a given occurrence in the brain is capable of having a variety of causes, and where the cause is unusual common sense will be misled. It would be theoretically possible to stimulate the optic nerve artificially in just the way in which light coming from the moon stimulates it; in this case, we should have the same experience as when we “see the moon”, but should be deceived as to its external source. Descartes was influenced by an argument of this sort, when he brought up the possibility of a deceitful demon. Therefore what he felt certain about was not what he had initially felt certain about, but what remained certain after an argument as to the causes of perception. This brings us to a distinction which is important, but difficult to apply; the distinction between what we in fact do not doubt, and what we should not doubt if we were completely rational. We do not in fact question the existence of the sun and moon, though perhaps we might teach ourselves to do so by a long course of Cartesian doubt. Even then, according to Descartes, we could not doubt that we have the experiences which we have hitherto called “seeing the sun” and “seeing the moon”, although we shall need different words if we are to describe these experiences correctly.
The question arises: Why should we not doubt everything? Why should we remain convinced that we have these experiences? Might not a deceitful demon perpetually supply us with false memories? When we say “a moment ago I had the experience which I have hitherto called seeing the sun”, perhaps we are deceived. In dreams we often remember things that never happened. At best, therefore, we can be sure of our present momentary experience, not of anything that happened even half a minute ago. And before we can so fix our momentary experience as to make it the basis of a philosophy, it will be past, and therefore uncertain. When Descartes said “I think”, he may have had certainty; but by the time he said “therefore I am”, he was relying upon memory, and may have been deceived. This line of argument leads to complete scepticism about everything. If we are to avoid such a result, we must have some new principle.
In actual fact, we start by feeling certainty about all sorts of things, and we surrender this feeling only where some definite argument has convinced us that it is liable to lead to error. When we find any class of primitive certainties which never leads to error, we retain our convictions in regard to this class. That is to say, wherever we feel initial certainty, we require an argument to make us doubt, not an argument to make us believe. We may therefore take, as the basis of our beliefs, any class of primitive certainties which cannot be shown to lead us into error. This is really what Descartes does, although he is not clear about it himself.
Moreover, when we have found an error in something of which we were previously certain, we do not as a rule abandon entirely the belief which misled us, but we seek, if we can, to modify it so that it shall no longer be demonstrably false. This is what has happened with perception. When we think we see an external object, we may be deceived by a variety of causes. There may be a mirage or a reflection; in this case, the source of the error is in the external world, and a photographic plate would be equally deceived. There may be a stimulus to the eye of the sort that makes us “see stars”, or we may see little black dots owing to a disordered liver, in which case, the source of the error is in the body but not in the brain. We may have dreams in which we seem to see all sorts of things; in this case, the source of the error is in the brain. Having gradually discovered these possibilities of error, people have become somewhat wary as to the objective significance of their perceptions; but they have remained convinced that they really have the perceptions they thought they had, although the common-sense interpretation of them is sometimes at fault. Thus, while retaining the conviction that they are sure of something, they have gradually changed their view as to what it is that they are sure of. Nothing is known that tends to show error in the view that we really have the percepts we think we have, so long as we are prudent in interpreting them as signs of something external. That is the valid basis for Descartes’ view that “thought” is more certain than external objects. When “thought” is taken to mean the experiences which we usually regard as percepts of objects, there are sound reasons for accepting Descartes’ opinion to this extent.