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.