70. We thus find that it is only so far as simple ideas are referred to things--only so far as each in turn, to use Locke’s instance, is regarded as an appearance ‘in a substance which was not in it before’--that our sensitive experience, the supposed _datum_ of knowledge, is an experience of the vicissitudes of things; and again, that only as an experience of such vicissitude does it furnish the ‘observation from which we get our ideas of cause and effect.’ But the reference of a sensation to a sensible thing means its reference to a cause. In other words, the invented relation of cause and effect must be found in the primary experience in order that it may be got from it. [1] [1] Locke’s contradiction of himself in regard to this relation might be exhibited in a still more striking light by putting side by side with his account of it his account of the idea of power. The two are precisely similar, the idea of power being represented as got by a notice of the alteration of simple ideas in things without (Book II. chap. xxi. sec. 1), just as the idea of cause and effect is. Power, too, he expressly says, is a relation. Yet, although the idea of it, both as derived and as of a relation, ought to be complex, he reckons it a simple and original one, and by using it interchangeably with ‘sensible quality’ makes it a primary _datum_ of sense. Origin of the idea of identity according to Locke. 71. The same holds of that other ‘product of the mind,’ the relation of identity. This ‘idea’ according to Locke, is formed when, ‘considering anything as existing at any determined time and place, we compare it with itself existing at another time.’ ‘In this consists identity,’ he adds, ‘when the ideas it is attributed to, vary not at all from what they were that moment wherein we consider their former existence, and to which we compare the present; for we never finding nor conceiving it possible that two things of the same kind should exist in the same place at the same time, we rightly conclude that whatever exists anywhere, at any time, excludes all of the same kind, and is there itself alone. When, therefore, we demand whether anything be the same or no? it refers always to something that existed such a time in such a place, which it was certain at that instant was the same with itself, and no other; from whence it follows that one thing cannot have two beginnings of existence, nor two things one beginning; it being impossible for two things of the same kind to be or exist in the same instant in the very same place, or one and the same thing in different places. That, therefore, that had one beginning, is the same thing; and that which had a different beginning in time and place from that is not the same, but diverse.’ He goes on to inquire about the _principium individuationis_, which he decides is ‘existence itself, which determines a being of any sort to a particular time and place, incommunicable to two beings of the same kind ...