90. The nominal essence of a thing, then, according to this view, being no other than the ‘complex idea of a substance,’ is a copy of reality, just as the simple idea is. It is a picture or representation in the mind of a thing that does exist by ideas of those qualities that are discoverable in it.’ (Book II. chap. xxxi. secs. 6, 8.) It only differs from the simple idea (which is itself, as abstract, a nominal essence) [1] in respect of reality, because the latter is a copy or effect produced singly and involuntarily, whereas we may put ideas together, as if in a thing, which have never been so presented together, and, on the other hand, never can put together all that exist together. (Book II. chap. xxx. sec. 5, and xxxi. 10.) So far as Locke maintains this view, the difficulty about general propositions concerning real existence need not arise. A statement which affirmed of gold one of the qualities included in the complex idea of that substance, would not express merely an analysis of an idea in the mind, but would represent a relation of qualities in the existing thing from which the idea ‘has been taken.’ These qualities, as in the thing, doubtless would not be, as in us, feelings (or, as Locke should rather have said in more recent phraseology, possibilities of feeling), but powers to produce feeling, nor could any relation between these, as in the thing, be affirmed but such as had produced its copy or effect in actual experience. No coexistence of qualities could be truly affirmed, which had not been found; but, once found--being a coexistence of qualities and not simply a momentary coincidence of feelings--it could be affirmed as permanent in a general proposition. That a relation can be stated universally between ideas collected in the mind, no one denies, and if such collection ‘is taken from a combination of simple ideas _existing together constantly in things_’ (Book II. chap, xxxii. sec. 18), the statement will hold equally of such existence. Thus Locke contrasts mixed modes, which, for the most part, ‘being actions which perish in the birth, are not capable of a lasting duration,’ with ‘substances, which are the actors; and wherein the simple ideas that make up the complex ideas designed by the name have a lasting union.’ (Book III. chap. vi. sec. 42.) [1] Book II. chap. xxxi. sec. 12. But such real essence a creature of thought.