As real existence, the simple idea carries with it ‘invented’ relation of cause.
52. Thus invested, it is already substance or symbolical of substance, not a mere feeling but a felt thing, recognised either under that minimum of qualification which enables us merely to say that it is ‘something,’ or (in Locke’s language) abstract substance, or under the greater complication of qualities which constitutes a ‘particular sort of substance’—gold, horse, water, &c. Real existence thus means substance. It is not the simple idea or sensation by itself that is real, but this idea as caused by a thing. It is the thing that is primarily the real; the idea only secondarily so, because it results from a power in the thing. As we have seen, Locke’s doctrine of the necessary adequacy, reality, and truth of the simple idea turns upon the supposition that it is, and announces itself as, an ‘ectype’ of an ‘archetype.’ But there is not a different archetype to each sensation; if there were, in ‘reporting’ it the sensation would do no more than report itself. It is the supposed single cause of manifold different sensations or simple ideas, to which a single name is applied. ‘If sugar produce in us the ideas which we call whiteness and sweetness, we are sure there is a power in sugar to produce those ideas in our minds. … And so each sensation answering the power that operates on any of our senses, the idea so produced is a real idea (and not a fiction of the mind, which has no power to produce any single idea), and cannot but be adequate … and so all simple ideas are adequate.’ (Book II. chap. xxxi. sec. 2.) The sugar, which is here the ‘archetype’ and the source of reality in the idea, is just what Locke elsewhere calls ‘a particular sort of substance,’ as the ‘something’ from which a certain set of sensations result, and in which, as sensible qualities, they inhere. Strictly speaking, however, according to Locke, that which inheres in the thing is not the quality, as it is to us, but a power to produce it. (Book II. chap. viii. sec. 28, and c. xxiii. 37.)
Correlativity of cause and substance.
53. In calling a sensation or idea the product of a power, substance is presupposed just as much as in calling it a sensible quality; only that with Locke ‘quality’ conveyed the notion of inherence in the substance, power that of relation to an effect not in the substance itself. ‘Secondary qualities are nothing but the powers which substances have to produce several ideas in us by our senses, which ideas are not in the things themselves, otherwise than as anything is in its cause.’ (Book II. chap, xxiii. sec. 9.) ‘Most of the simple ideas, that make up our complex ideas of substances, are only powers … or relations to other substances (or, as he explains elsewhere, ‘relations to our perceptions,’ [1]), and are not really in the substance considered barely in itself.’ (Book II. chap, xxiii. sec. 37, and xxxi. 8.) That this implies the inclusion of the idea of cause in that of substance, appears from Locke’s statement that ‘whatever is considered by us to operate to the producing any particular simple idea which did not before exist, hath thereby in our minds the relation of a cause.’ (Book II. chap. xxvi. sec. 1.) Thus to be conscious of the reality of a simple idea, as that which is not made by the subject of the idea, but results from a power in a thing, is to have the idea of substance as cause. This latter idea must be the condition of the consciousness of reality. If the consciousness of reality is implied in the beginning of knowledge, so must the correlative ideas be of cause and substance.
[1] Book II. chap. xxi. sec. 3.
How do we know that ideas correspond to reality of things? Locke’s answer.
54. On examining Locke’s second rehearsal of his theory in the fourth book of the Essay—that ‘On Knowledge’—we are led to this result quite as inevitably as in the book ‘On Ideas.’ He has a special chapter on the ‘reality of human knowledge,’ where he puts the problem thus:—‘It is evident the mind knows not things immediately, but only by the intervention of the ideas it has of them. Our knowledge therefore is real only so far as there is a conformity between our ideas and the reality of things. But what shall be here the criterion? How shall the mind, when it perceives nothing but its own ideas, know that they agree with things themselves?’ (Book IV. chap. iv. sec. 3.) It knows this, he proceeds to show, in the case of simple ideas, because ‘since the mind can by no means make them to itself, they must be the product of things operating on the mind in a natural way. … Simple ideas are not fictions of our fancies, but the natural and regular productions of things without us, really operating upon us; and so carry with them all the conformity which is intended, or which our state requires, for they represent to us things under those appearances which they are fitted to produce in us; whereby we are enabled to distinguish the sorts of particular substances,’ &c. &c. (Book IV. chap. iv. sec. 4.) The whole force of this passage depends on the notion that simple ideas are already to the subject of them not his own making, but the product of a thing, which in its relation to these ideas is a ‘particular sort of substance.’ It is the reception of such ideas, so related, that Locke calls ‘sensitive knowledge of particular existence,’ or a ‘perception of the mind, employed about the particular existence of finite beings without us.’ (Book IV. chap. ii. sec. 14.) This, however, he distinguishes from two other ‘degrees of knowledge or certainty,’ ‘intuition’ and ‘demonstration,’ of which the former is attained when the agreement or disagreement of two ideas is perceived immediately, the latter when it is perceived mediately through the intervention of certain other agreements or disagreements (less or more), each of which must in turn be perceived immediately. Demonstration, being thus really but a series of intuitions, carries the same certainty as intuition, only it is a certainty which it requires more or less pains and attention to apprehend. (Book IV. chap. ii. sec. 4.) Of the ‘other perception of the mind, employed about the particular existence of finite beings without us,’ which ‘passes under the name of knowledge,’ he explains that although ‘going beyond bare probability, it reaches not perfectly to either of the foregoing degrees of certainty.’ ‘There can be nothing more certain,’ he proceeds, ‘than that the idea we receive from an external object is in our minds; this is intuitive knowledge. But whether there be anything more than barely that idea in our minds, whether we can thence certainly infer the existence of anything without us which corresponds to that idea, is that whereof some men think there may be a question made; because men may have such ideas in their minds, when no such thing exists, no such object affects their senses.’ (Book IV. chap. ii. sec. 14.)
It assumes that simple ideas are consciously referred to things that cause them.
55. It is clear that here in his very statement of the question Locke begs the answer. If the intuitive certainty is that ‘the idea we receive from an external object is in our minds,’ [1] how is it possible to doubt whether such an object exists and affects our senses? This impossibility of speaking of the simple idea, except as received from an object, may account for Locke’s apparent inconsistency in finding the assurance of the reality of knowledge (under the phrase ‘evidence of the senses’) just in that ‘perception’ which reaches not to intuitive or demonstrative certainty, and only ‘passes under the name of knowledge.’ In the passage just quoted he shows that he is cognizant of the distinction between the simple idea and the perception of an existence corresponding to it, and in consequence distinguishes this perception from proper intuition, but in the very statement of the distinction it eludes him. The simple idea, as he speaks of it, becomes itself, as consciously ‘received from an external object,’ the perception of existence; just as we have previously seen it become the judgment of identity or perception of the ‘agreement of an idea with itself,’ which is his first kind of knowledge.
[1] I do not now raise the question, What are here the ideas, which must be immediately perceived to agree or disagree in order to make it a case of ‘intuitive certainty’ or knowledge according to Locke’s definition. See below, paragraphs 59, 101, and 147.