§ 73. =The Concept of Self.=—We said on p. 9 that the word _mind_ is used by the psychologist as an inclusive name for all the phenomena of the psychological world, that is to say, of the world _with man left in_. We then found, on p. 10, that the _man left in_ reduces to a functional nervous system. This means, of course, that there are as many psychological worlds as there are separate nervous systems; so that _the_ psychological world, which the psychologist tries to describe, is in reality an average or generalize world; though the observations upon which his descriptions rest are always made upon this or that particular world. The same thing holds of any science. A boy picks up a bit of jagged stone, and with a jerk of his wrist flips it across the road. No physicist could tell you the exact course described by that stone, and no physicist wants to. Physics deals with the ideal course of ideal projectiles hurled under fixed conditions; the boy and the jerk and the jagged stone are all generalised away into some mathematically smooth trajectory. The observations of physics, on the other hand, are made by men working under conditions that are not ideal, and using instruments that differ from the wrist and the stone only in degree, not in kind; the smooth curve is derived from data all of which have their margin of empirical error. Psychology, however, just because it has to do with a world in which man himself remains, is in a different case from the physical sciences; it has to take account of the =self=. _The concept of self is not solely psychological_; it is a common-sense concept; and like all the constructions of common sense it has three sides, philosophical, practical, and scientific. It is _philosophical_, in so far as it involves an attempt to explain or to rationalise the facts of observation; and it evidently does that; the notion of self is a way of explaining the continuity of memory and of conduct; I remember my past because I am I, and I behave in this way or that because it is ‘like me’ to do so. The concept is also _practical_; common sense rates a self as gifted or energetic or lazy or improvident; it is always valuing or estimating some Him or Her, some You or Me. It is further _scientific_, that is, psychological; for the self thus rated is some particular combination of talent, temperament and character, and the continuity which the self explains is some particular mental constitution, intellectual, emotive, active; one cannot at all define the ‘person’ or ‘individual’ of common sense without using psychological terms. So that psychology, if only in self-defence, must have its say in the matter, and must recast the self from its own point of view.