As for the trustworthiness of memory, there are two things to be said. Taken as a whole, memory is one of the independent sources of our knowledge; that is to say, there is no way of arriving at the things we know through memory by any argument wholly derived from things known otherwise. But no single memory is obliged to stand alone, because it fits, or does not fit, into a system of knowledge about the past based upon the sum-total of memories. When what is remembered is a perception by one or more of the public senses, other people may corroborate it. Even when it is private, it may be confirmed by other evidence. You may remember that you had a toothache yesterday, and that you saw the dentist to-day; the latter fact may be confirmed by an entry in your diary. All these make a consistent whole, and each increases the likelihood of the other. Thus we can test the truth of any particular recollection, though not of memory as a whole. To say that we cannot test the truth of memory as a whole is not to give a reason for doubting it, but merely to say that it is an independent source of knowledge, not wholly replaceable by other sources. We know that our memory is fallible, but we have no reason to distrust it on the whole after sufficient care in verification has been taken. The causation of particular acts of recollection seems to be wholly associative. Something in the present is very like something in the past, and calls up the context of the past occurrence in the shape of images or words; when attention falls upon this context, we believe that it occurred in the past, not as mere images, and we then have an act of recollection. There are many difficult problems connected with memory which I have not discussed, because they have an interest which is more purely psychological than philosophical. It is memory as a source of knowledge that specially concerns the philosopher. CHAPTER XIX THE INTROSPECTIVE ANALYSIS OF PERCEPTION We have considered perception already from the behaviourist standpoint, and also from that of physics. In the present chapter we are to consider it from the standpoint of self-observation, with a view to discovering as much as we can about the intrinsic character of the event in us when we perceive. I shall begin with certain traditional doctrines as to mental events, and shall thence pass to the doctrines that I wish to advocate. The words “mind” and “matter” are used glibly, both by ordinary people and by philosophers, without any adequate attempt at definition. Philosophers are much to blame for this. My own feeling is that there is not a sharp line, but a difference of degree; an oyster is less mental than a man, but not wholly un-mental. And I think “mental” is a character, like “harmonious” or “discordant”, that cannot belong to a single entity in its own right, but only to a system of entities. But before defending this view, I wish to spend some time on the theories that have been current in the past.