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.