A Treatise of Human Nature / Being an Attempt to Introduce the Experimental Method Into Moral Subjects; and Dialogues Concerning Natural Religion

by David Hume

Such restriction, if maintained, would render the testimony unmeaning.

65. In the very sentence, however, where Locke restricts the testimony of sensation to existence present along with it, he uses language inconsistent with this restriction. The particular existence which he instances as ‘testified to’ is that of ‘such a collection of simple ideas as is wont to be called man.’ But these ideas can only be present in succession. (See Book II. chap. vii. sec. 9, and chap. xiv. sec. 3.) Even the surface of the man’s body can only be taken in by successive acts of vision; and, more obviously, the states of consciousness in which his qualities of motion and action are presented occupy separate times. If then sensation only testifies to an existence present along with it, how can it testify to the co-existence (say) of an erect attitude, of which I have a present sight, with the risibility which I saw a minute ago? How can the ‘collection of ideas wont to be called man,’ as co-existing, be formed at all? and, if it cannot, how can the present existence of an object so-called be testified to by sense any more than the past? The same doctrine, which is fatal to the supposition of ‘a necessary connexion between the man’s existence a minute since and his existence now,’ is in fact fatal to the supposition of his existence as a complex of qualities at all. It does not merely mean that, for anything we know, the man may have died. Of course he may, and yet there may be continuity of existence according to natural laws, though not one for which we have the testimony of present sense, between the living body and the dead. What Locke had in his mind was the notion that, as existence is testified to only by present sensation, and each sensation is merely individual and momentary, there could be no testimony to the continued existence of anything. He could not, however, do such violence to the actual fabric of knowledge as would have been implied in the logical development of this doctrine, and thus he allowed himself to speak of sense as testifying to the co-existence of sensible qualities in a thing, though the individual sensation could only testify to the presence of one at a time, and could never testify to their nexus in a common cause at all. This testimony to co-existence in a present thing once admitted, he naturally allowed himself in the further assumption that the testimony, on its recurrence, is a testimony to the same co-existence and the same thing. The existence of the same man (he evidently supposes), to which sensation testified an hour ago, may be testified to by a like sensation now. This means that resemblance of sensation becomes identity of a thing—that like sensations occurring at different times are interpreted as representing the same thing, which continuously exists, though not testified to by sense, between the times.

But it is not maintained: the testimony is to operation of permanent identical things.

66. In short, as we have seen the simple idea of sensation emerge from Locke’s inquiry as to the beginning of knowledge transformed into the judgment, ‘I have an idea different from other ideas which I did not make for myself,’ so now from the inquiry as to the correspondence between knowledge and reality it emerges as the consciousness of a thing now acting upon me, which has continued to exist since it acted on me before, and in which, as in a common cause, have existed together powers to affect me which have never affected me together. If in the one form the operation of thought in sense, the ‘creation of the understanding’ within the simple idea, is only latent or potential, in the other it is actual and explicit. The relations of substance and quality, of cause and effect, and of identity—all ‘inventions of the mind’—are necessarily involved in the immediate, spontaneous testimony of passive sense.

Locke’s treatment of relations of cause and identity.

67. It will be noticed that it is upon the first of these, the relation of substance and quality, that our examination of Locke’s Essay has so far chiefly gathered. In this it follows the course taken by Locke himself. Of the idea of substance, eo nomine, he treats at large: of cause and identity (apart from the special question of personal identity) he says little. So, too, the ‘report of the senses’ is commonly exhibited as announcing the sensible qualities of a thing rather than the agency of a cause or continuity of existence. The difference, of course, is mainly verbal. Sensible qualities being, as Locke constantly insists, nothing but ‘powers to operate on our senses’ directly or indirectly, the substance or thing, as the source of these, takes the character of a cause. Again, as the sensible quality is supposed to be one and the same in manifold separate cases of being felt, it has identity in contrast with the variety of these cases, even as the thing has, on its part, in contrast with the variety of its qualities. Something, however, remains to be said of Locke’s treatment of the ideas of cause and identity in the short passages where he treats of them expressly. Here, too, we shall find the same contrast between the given and the invented, tacitly contradicted by an account of the given in terms of the invented.

That from which he derives idea of cause pre-supposes it.

68. The relation of cause and effect, according to Locke’s general statement as to relation, must be something ‘not contained in the real existence of things, but extraneous and superinduced.’ (Book II. chap. xxv. sec. 8.) It is a ‘complex idea,’ not belonging to things as they are in themselves, which the mind makes by its own act. (Book II. chap xii. secs. 1, 7, and chap. xxv. sec. 1.) Its origin, however, is thus described:—‘In the notice that our senses take of the constant vicissitude of things, we cannot but observe that several particular, both qualities and substances, begin to exist; and that they receive this their existence from the due application and operation of some other being. From this observation we get our ideas of cause and effect. That which produces any simple or complex idea we denote by the general name cause; and that which is produced, effect. Thus, finding that in that substance which we call wax, fluidity, which is a simple idea that was not in it before, is constantly produced by the application of a certain degree of heat, we call the simple idea of heat, in relation to fluidity in wax, the cause of it, and fluidity the effect. So, also, finding that the substance, wood, which is a certain collection of simple ideas so-called, by the application of fire is turned into another substance called ashes, i.e. another complex idea, consisting of a collection of simple ideas, quite different from that complex idea which we call wood; we consider fire, in relation to ashes, as cause, and the ashes as effect.’ Here we find that the ‘given,’ upon which the relation of cause and effect is ‘superinduced’ or from which the ‘idea of it is got’ (to give Locke the benefit of both expressions), professedly, according to the first sentence of the passage quoted, involves the complex or derived idea of substance. The sentence, indeed, is a remarkable instance of the double refraction which arises from redundant phraseology. Our senses are supposed to ‘take notice of a constant vicissitude of things,’ or substances. Thereupon we observe, what is necessarily implied in this vicissitude, a beginning of existence in substances or their qualities, ‘received from the due application or operation of some other being.’ Thereupon we infer, what is simply another name for existence thus given and received, a relation of cause and effect. Thus not only does the datum of the process of ‘invention’ in question, i.e. the observation of change in a thing, involve a derived idea, but a derived idea which presupposes just this process of invention.

Rationale of this ‘petitio principii’.

69. Here again it is necessary to guard against the notion that Locke’s obvious petitio principii might be avoided by a better statement without essential change in his doctrine of ideas. It is true that ‘a notice of the vicissitude of things’ includes that ‘invention of the understanding’ which it is supposed to suggest, but state the primary knowledge otherwise—reduce the vicissitude of things, as it ought to be reduced, in order to make Locke consistent, to the mere multiplicity of sensations—and the appearance of suggestion ceases. Change or ‘vicissitude’ is quite other than mere diversity. It is diversity relative to something which maintains an identity. This identity, which ulterior analysis may find in a ‘law of nature,’ Locke found in ‘things’ or ‘substances.’ By the same unconscious subreption, by which with him a sensible thing takes the place of sensation, ‘vicissitude of things’ takes the place of multiplicity of sensations, carrying with it the observation that the changed state of the thing is due to something else. The mere multiplicity of sensations could convey no such ‘observation,’ any more than the sight of counters in a row would convey the notion that one ‘received its existence’ from the other. Only so far as the manifold appearances are referred, as its vicissitudes, to something which remains one, does any need of accounting for their diverse existence, or in consequence any observation of its derivation ‘from some other being,’ arise. Locke, it is true, after stating that it is upon a notice of the vicissitude of things that the observation in question rests, goes on to speak as if an origination of substances, which is just the opposite of their vicissitude, might be observed; and the second instance of production which he gives—that of ashes upon the burning of wood—seems intended for an instance of the production of a substance, as distinct from the production of a quality. He is here, however, as he often does, using the term ‘substance’ loosely, for ‘a certain collection of simple ideas,’ without reference to the ‘substratum wherein they do subsist,’ which he would have admitted to be ultimately the same for the wood and for the ashes. The conception, indeed, of such a substratum, whether vaguely as ‘nature,’ or more precisely as a ‘real constitution of insensible parts’ (Book III. chap. iii. secs. 18, &c.), governed all his speculation, and rendered to him what he here calls substance virtually a mode, and its production properly a ‘vicissitude.’