But if matter has fared badly, mind has fared little better. The adjective “mental” is one which is not capable of any exact significance. There is, it is true, an important group of events, namely percepts, all of which may be called “mental”. But it would be arbitrary to say that there are no “mental” events except percepts, and yet it is difficult to find any principle by which we can decide what other events should be included. Perhaps the most essential characteristics of mind are introspection and memory. But memory in some of its forms is, as we have seen, a consequence of the law of conditioned reflexes, which is at least as much physiological as psychological, and characterises living tissue rather than mind. Knowledge, as we have found, is not easy to distinguish from sensitivity, which is a property possessed by scientific instruments. Introspection is a form of knowledge, but turns out, on examination, to be little more than a cautious interpretation of ordinary “knowledge”. Where the philosopher’s child at the Zoo says “There is a hippopotamus over there”, the philosopher should reply: “There is a coloured pattern of a certain shape, which may perhaps be connected with a system of external causes of the sort called a hippopotamus”. (I do not live up to this precept myself.) In saying that there is a coloured pattern, the philosopher is practising introspection in the only sense that I can attach to that term, _i.e._ his knowledge-reaction is to an event situated in his own brain from the standpoint of physical space, and is consciously avoiding physiological and other inference as far as possible. Events to which a knowledge-reaction of this sort occurs are “mental”; so are, presumably, other events resembling them in certain respects. But I do not see any way of defining this wider group except by saying that mental events are events in a living brain, or, better, in a region combining sensitivity and the law of learned reactions to a marked extent. This definition has at least the merit of showing that mentality is an affair of causal laws, not of the quality of single events, and also that mentality is a matter of degree.