56. In short, with Locke the simple idea, the perception of existence corresponding to the idea, and the judgment of identity, are absolutely merged, and in mutual involution, sometimes under one designation, sometimes under another, are alike presented as the beginning of knowledge. As occasion requires, each does duty for the other. Thus, if the ‘reality of knowledge’ be in question, the simple idea, which is given, is treated as involving the perception of existence, and the reality is established. If in turn this perception is distinguished from the simple idea, and it is asked whether the correspondence between idea and existence is properly matter of knowledge, the simple idea has only to be treated as involving the judgment of identity, which again involves that of existence, and the question is answered. So in the context under consideration (Book IV. chap. ii. sec. 14), after raising the question as to the existence of a thing corresponding to the idea, he answers it by the counter question, ‘whether anyone is not invincibly conscious to himself of a different perception, when he looks on the sun by day, and thinks on it by night; when he actually tastes wormwood, or smells a rose, or only thinks on that savour or odour? We as plainly find the difference there is between any idea revived in our minds by our own memory, and actually coming into our minds by our senses, as we do between any two distinct ideas.’ The force of the above lies in its appeal to the perception of identity, or--to apply the language in which Locke describes this perception--the knowledge that the idea which a man calls the smell of a rose is the very idea it is. [1] The mere difference in liveliness between the present and the recalled idea, which, as Berkeley and Hume rightly maintained, is the only difference between them as mere ideas, cannot by itself constitute the difference between the knowledge of the presence of a thing answering to the idea and the knowledge of its absence. It can only do this if the more lively idea is _identified_ with past lively ideas as a representation of one and the same thing which ‘agrees with itself’ in contrast to the multiplicity of the sensations, its signs. Only in virtue of this identification can either the liveliness of the idea show that the thing--the sun or the rose--is there, or the want of liveliness that it is not, for without it there would be no thing to be there or not to be there. It is because this identification is what Locke understands by the first sort of perception of agreement between ideas, and because he virtually finds this perception again in the simple idea, that the simple idea is to him the index of reality. But if so, the idea in its primitive simplicity is the sign of a thing that is ever the same in the same relations, and we find the ‘workmanship of the mind,’ its inventions of substance, cause, and relation, in the very rudiments of knowledge. [1] See above, paragraph 25.