145. We have already found that Locke accepts each of the above as determinations of real existence, and that, though in spite of them he labours to maintain the real existence of outward things, he is so far faithful to them as to declare real essence unknowable. In answering the question as to ‘his own existence’ he wholly ignores them. He does not ask how the real existence of the thinking Ego sorts with his ordinary doctrine that the real is what would be in the world whether there were a mind or no; or its real identity, present throughout the particulars of experience, with his ordinary doctrine of the fictitiousness of ‘generals.’ A real existence of the mind, however, founded on the logical necessity of substantiation, rests on a shifting basis, so long as by the mind is understood a thinking thing, different in each man, to which his inner experience is referred as accidents to a substance. The same law of thought which compels such reference requires that the thinking thing in its turn, as that which is born grows and dies, be referred as an accident to some ulterior substance. ‘A fever or fall may take away my reason or memory, or both; and an apoplexy leave neither sense nor understanding, no, nor life.’ [1] Just as each outer thing turns out to be a ‘retainer to something else,’ so is it with the inner thing. Such a dependent being cannot be an ultimate substance; nor can any natural agents to which we may trace its dependence really be so either. The logical necessity of further substantiation would affect them equally, appearing in the supposition of an unknown something beyond, which makes them what they are. It is under such logical necessity that Locke, in regard to all the substances which he commonly speaks of as ultimate--God, spirit, body--from time to time gives warning of something still ulterior and unknowable, whether under the designation of substance or real essence (Book II. chap. xxiii. secs. 30 and 36). If, then, it will be said, substance is but the constantly-shifting result of a necessity of thought--so shifting that there is nothing of which we can finally say, ‘This is substance, not accident’--there can be no evidence of the ‘real existence’ of a permanent Ego in the necessary substantiation therein of my inner experience. [1] Locke, Book III. chap. vi. sec. 4. Sense in which the self is truly real.