117. But though the distinction between different kinds of knowledge in regard to reality cannot but rest on the same principle as that drawn between different kinds of ideas in the same regard, it is to be noticed that in the doctrine of the Fourth Book ‘knowledge concerning substances,’ in contrast with that in which ‘our thoughts terminate in the abstract ideas,’ has by itself to cover the ground which, in the Second and Third Book, simple ideas and complex ideas of substances cover together. This is to be explained by the observation, already set forth at large, [1] that the simple idea has in Locke’s Fourth Book become explicitly what in the previous books it was implicitly, not a feeling proper, but the conscious reference of a feeling to a thing or substance. Only because it is thus converted, as we have seen, can it constitute the beginning of a knowledge which is not a simple idea but a conscious relation between ideas, or have (what yet it must have if it can be expressed in a proposition) that capacity of being true or false, which implies ‘the reference by the mind of an idea to something extraneous to it.’ (Book II. chap, xxxii. sec. 4.) Thus, what is said of the ‘simple idea’ in the Second and Third Books, is in the Fourth transferred to one form of knowledge concerning substances, to that, namely, which consists in ‘particular experiment and observation,’ and is expressed in singular propositions, such as ‘this is yellow,’ ‘this gold is now solved in aqua regia.’ Such knowledge cannot but be real, the proposition which expresses it cannot but have _real_ certainty, because it is the effect of a ‘body actually operating upon us’ (Book IV. chap. xi. sec. 1), just as the simple idea is an ectype directly made by an archetype. It is otherwise with complex ideas of substances and with general knowledge or propositions about them. A group of ideas, each of which, when first produced by a ‘body,’ has been real, when retained in the mind as representing the body, becomes unreal. The complex idea of gold is only a nominal essence or the signification of a name; the qualities which compose it are merely ideas in the mind, and that general truth which consists in a correct statement of the relation between one of them and another or the whole--_e.g._, ‘gold is soluble in aqua regia’--holds merely for the mind; [2] but it is not therefore to be classed with those other mental truths, which constitute mathematical and moral knowledge, and which, just because ‘merely ideal,’ are therefore real. Its merely mental character renders it in Locke’s language a ‘trifling proposition,’ but does not therefore save it from being _really_ untrue. It is a ‘trifling proposition,’ for, unless solubility in aqua regia is included in the complex idea which the sound ‘gold’ stands for, the proposition which asserts it of gold is not certain, not a truth at all. If it is so included, then the proposition is