116. To answer these questions, according to Locke, we must consider whether the knowledge, or the proposition which expresses it, concerns substances, _i.e._, ‘the co-existence of ideas in nature,’ on the one hand; or, on the other, either the properties of a mathematical figure or ‘moral ideas.’ If it is of the latter sort, the agreement of the ideas in the mind is itself their agreement in reality, since the ideas themselves are archetypes. (Book IV. chap. iv. secs. 6, 7.) It is only when the ideas are ectypes, as is the case when the proposition concerns substances, that the doubt arises whether the agreement between them represents an agreement in reality. The distinction made here virtually corresponds to that which appears in the chapters on the reality and adequacy of ideas in the Second Book, and again in those on ‘names’ in the Third. There the ‘complex ideas of modes and relation’ are pronounced necessarily real adequate and true, because, ‘being themselves archetypes, they cannot differ from their archetypes.’ (Book II. chap. XXX. sec. 4.) [1] With them are contrasted simple ideas and complex ideas of substances, which are alike ectypes, but with this difference from each other, that the simple ideas cannot but be faithful copies of their archetypes, while the ideas of substances cannot but be otherwise. (Book II. chap. xxxi. secs. 2, 11, &c.) Thus, ‘the names of simple ideas and substances, with the abstract ideas in the mind which they immediately signify, intimate also some real existence, from which was derived their original pattern. But the names of mixed modes terminate in the idea that is in the mind.’ (Book III. chap. iv. sec. 2.) ‘The names of simple ideas and modes,’ it is added, ‘signify always the real as well as nominal essence of their species’--a statement which, if it is to express Locke’s doctrine strictly, must be confined to names of simple ideas, while in respect of modes it should run, that ‘the nominal essence which the names of these signify is itself the real.’ [1] cf. Book II. chap. xxxi. sec. 3, and xxxii. sec. 17. ... when the knowledge concerns substances. In this case general truth must be merely verbal. Mathematical truths, since they concern not substances, may be both general and real.