69. Here again it is necessary to guard against the notion that Locke’s obvious _petitio principii_ might be avoided by a better statement without essential change in his doctrine of ideas. It is true that ‘a notice of the vicissitude of things’ includes that ‘invention of the understanding’ which it is supposed to suggest, but state the primary knowledge otherwise--reduce the vicissitude of things, as it ought to be reduced, in order to make Locke consistent, to the mere multiplicity of sensations--and the appearance of suggestion ceases. Change or ‘vicissitude’ is quite other than mere diversity. It is diversity relative to something which maintains an identity. This identity, which ulterior analysis may find in a ‘law of nature,’ Locke found in ‘things’ or ‘substances.’ By the same unconscious subreption, by which with him a sensible thing takes the place of sensation, ‘vicissitude of things’ takes the place of multiplicity of sensations, carrying with it the observation that the changed state of the thing is due to something else. The mere multiplicity of sensations could convey no such ‘observation,’ any more than the sight of counters in a row would convey the notion that one ‘received its existence’ from the other. Only so far as the manifold appearances are referred, as its vicissitudes, to something which remains one, does any need of accounting for their diverse existence, or in consequence any observation of its derivation ‘from some other being,’ arise. Locke, it is true, after stating that it is upon a notice of the vicissitude of things that the observation in question rests, goes on to speak as if an _origination_ of substances, which is just the opposite of their vicissitude, might be observed; and the second instance of production which he gives--that of ashes upon the burning of wood--seems intended for an instance of the production of a substance, as distinct from the production of a quality. He is here, however, as he often does, using the term ‘substance’ loosely, for ‘a certain collection of simple ideas,’ without reference to the ‘substratum wherein they do subsist,’ which he would have admitted to be ultimately the same for the wood and for the ashes. The conception, indeed, of such a substratum, whether vaguely as ‘nature,’ or more precisely as a ‘real constitution of insensible parts’ (Book III. chap. iii. secs. 18, &c.), governed all his speculation, and rendered to him what he here calls _substance_ virtually a _mode_, and its production properly a ‘vicissitude.’ Relation of cause has to be put into sensitive experience in order to be got from it.