The other part of Descartes’ philosophy, namely, the emphasis upon methodical doubt, and consequently upon theory of knowledge, has been more fruitful. The beginning of a philosophic attitude is the realisation that we do not know as much as we think we do, and to this Descartes contributed notably. We have seen that he set to work to doubt all he could, but found he could not doubt his own existence, which he therefore took as the starting-point of his constructive system. He supposed that the most certain fact in the world is “I think”. This was unfortunate, since it gave a subjective bias to modern philosophy. As a matter of fact, “I” seems to be only a string of events, each of which separately is more certain than the whole. And “think” is a word which Descartes accepted as indefinable, but which really covers complicated relations between events. When is an event a “thought”? Is there some intrinsic characteristic which makes it a thought? Descartes would say yes, and so would most philosophers. I should say no. Take, _e.g._ a visual and an auditory sensation. Both are “thoughts” in Descartes’ sense, but what have they in common? Two visual sensations _have_ an indefinable common quality, viz. that which makes them visual. Two auditory sensations likewise. But a visual and an auditory sensation have in common, if I am not mistaken, no intrinsic property, but a certain capacity for being _known_ without inference. This amounts to saying that they are mnemic causes of a certain kind of event, called a cognition, and that they have moreover, a certain formal similarity to the cognition which they cause. Therefore, instead of taking the general “I think” as our basis, we ought to take the particular occurrences which are known without inference, among which sensations (or rather “perceptions”) will be included. These occurrences, as we have already seen, may be regarded with equal justice as physical and mental: they are parts of chains of physical causation, and they have mnemic effects which are cognitions. The former fact makes us call them physical, the latter mental, both quite truly. It is the particular events which are certain, not the “I think” which Descartes made the basis of his philosophy. It is not correct to regard the ultimate certainties as “subjective”, except in the sense that they are events in that part of space-time in which our body is--and our mind also, I should say.