127. The dominant notion of the self in Locke is that of the inward substance, or ‘substratum of ideas,’ co-ordinate with the outward, ‘wherein they do subsist, and from which they do result.’ ‘Sensation convinces that there are solid extended substances, and reflection that there are thinking ones’ (Book II. chap, xxiii. sec. 29). We have already seen how, without disturbance from his doctrine of the fictitiousness of universals, he treats the simple idea as carrying with it the distinction of outward and inward, or relations severally to a ‘thing’ and to a ‘mind.’ It reports itself ambiguously as a quality of each of these separate substances. It is now, or was to begin with, the result of an outward thing ‘actually operating upon us;’ for ‘of simple ideas the mind cannot make one to itself:’ on the other hand, it is a ‘perception,’ and perception is an ‘operation of the mind.’ In other words it is at once a modification of the mind by something of which it is consciously not conscious, and a modification of the mind by itself--the two sources of one and the same modification being each determined only as the contradictory of the other. Thus, when we come to probe the familiar metaphors under which Locke describes Reflection, as a ‘fountain of ideas’ other than sensation, we find that the confusions which we have already explored in dealing with the ideas of sensation recur under added circumstances of embarrassment. Not only does the simple idea of reflection, like that of sensation, turn out to be already complicated in its simplicity with the superinduced ideas of cause and relation, but the causal substance in question turns out to be one which, from being actually nothing, becomes something by acting upon itself; while all the time the result of this action is indistinguishable from that ascribed to the opposite, the external, cause. Of which substance is perception the effect?