[1] All knowledge is certain according to Locke (Cf. IV. chap. vi. sec. 13, ‘certainty is requisite to knowledge’), though the knowledge must be expressed before the term ‘certainty’ is naturally applied to it. (Book IV. chap. vi. sec. 3.) ‘Certainty of knowledge’ is thus a pleonastic phrase, which only seems not to be so because we conceive knowledge to have a relation to things which Locke’s definition denies it, and by ‘certainty,’ in distinction from this, understand its relation to the subject.
‘Certainty of truth’ is, in like manner, a pleonastic phrase, there being no difference between the definition of it (Book IV. chap. vi. sec. 3) and that of ‘truth’ simply, given in Book IV. chap. V. sec. 2.
The knowledge expressed by a proposition, though certain, may not be real …
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.
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 but ‘playing with sounds.’ It may serve to remind an opponent of a definition which he has made but is forgetting, but ‘carries no knowledge with it but of the signification of a word, however certain it be.’ (Book IV. chap. viii. secs. 5 & 9.) Yet there is a real gold, outside the mind, of which the complex idea of gold in the mind must needs try to be a copy, though the conditions of real existence are such that no ‘complex idea in the mind’ can possibly be a copy of it. Thus the verbal truth, which general propositions concerning substances express, is under a perpetual doom of being really untrue. The exemption of mathematical and moral knowledge from this doom remains an unexplained mercy. Because merely mental, such knowledge is real—there being no reality for it to _mis_represent—and yet not trifling. The proposition that ‘the external angle of all triangles is bigger than either of the opposite internal angles,’ has that general certainty which is never to be found but in our ideas, yet ‘conveys instructive real knowledge,’ the predicate being ‘a necessary consequence of precise complex idea’ which forms the subject, yet ‘not contained in it.’ (Book IV. chap. viii. sec. 8.) [3] The same might be said apparently, according to Locke’s judgment (though he is not so explicit about this), of a proposition in morals, such as ‘God is to be feared and obeyed by man.’ (Book IV. chap. xi. sec. 13.) [4] But how are such propositions, at once abstract and real, general and instructive, to be accounted for? There is no ‘workmanship of the mind’ recognised by Locke but that which consists in compounding and abstracting (i.e., separating) ideas of which ‘it cannot originate one.’ The ‘abstract ideas’ of mathematics, the ‘mixed modes’ of morals, just as much as the ideas of substances, must be derived by such mental artifice from a material given in simple feeling, and ‘real’ because so given. Yet, while this derivation renders ideas of substances unreal in contrast with their real ‘originals,’ and general propositions about them ‘trifling,’ because, while ‘intimating an existence,’ they tell nothing about it, on the other hand it actually constitutes the reality of moral and mathematical ideas. Their relation to an original disappears; they are themselves archetypes, from which the mind, by its own act, can elicit other ideas not already involved in the meaning of their names. But this can only mean that the mind has some other function than that of uniting what it has ‘found’ in separation, and separating again what it has thus united—that it can itself originate.
[1] See above, paragraph 25.
[2] Book IV. chap. xi. sec. 13, xii. 9, &c.
[3] Just as according to Kant such a proposition expresses a judgment ‘synthetical,’ yet ‘á-priori.’