[1] Locke only states this explicitly of comparison, ‘an operation of the mind about its ideas, upon which depends all that large tribe of ideas, comprehended under relation.’ (Book II. chap. xi. sec. 4.) It is clear, however, that the same remark must apply to the ‘discernment of ideas,’ which is strictly correlative to comparison, and to their composition, which means that they are brought into relation as constituents of a whole. That these three processes are necessary to constitute the beginnings of knowledge, according to Locke, appears from Book II. chap. xi. sec. 15, taken in connection with what precedes in that chapter. As real existence, the simple idea carries with it ‘invented’ relation of cause. 52. Thus invested, it is already substance or symbolical of substance, not a mere feeling but a felt thing, recognised either under that minimum of qualification which enables us merely to say that it is ‘something,’ or (in Locke’s language) abstract substance, or under the greater complication of qualities which constitutes a ‘particular sort of substance’--gold, horse, water, &c. Real existence thus means substance. It is not the simple idea or sensation by itself that is real, but this idea as caused by a thing. It is the thing that is primarily the real; the idea only secondarily so, because it results from a power in the thing. As we have seen, Locke’s doctrine of the necessary adequacy, reality, and truth of the simple idea turns upon the supposition that it is, and announces itself as, an ‘ectype’ of an ‘archetype.’ But there is not a different archetype to each sensation; if there were, in ‘reporting’ it the sensation would do no more than report itself. It is the supposed single cause of manifold different sensations or simple ideas, to which a single name is applied. ‘If sugar produce in us the ideas which we call whiteness and sweetness, we are sure there is a power in sugar to produce those ideas in our minds. ... And so each sensation answering the power that operates on any of our senses, the idea so produced is a real idea (and not a fiction of the mind, which has no power to produce any single idea), and cannot but be adequate ... and so all simple ideas are adequate.’ (Book II. chap. xxxi. sec. 2.) The sugar, which is here the ‘archetype’ and the source of reality in the idea, is just what Locke elsewhere calls ‘a particular sort of substance,’ as the ‘something’ from which a certain set of sensations result, and in which, as sensible qualities, they inhere. Strictly speaking, however, according to Locke, that which inheres in the thing is not the quality, as it is to us, but a power to produce it. (Book II. chap. viii. sec. 28, and c. xxiii. 37.) Correlativity of cause and substance.