280. It appears then that we have an idea of ‘distance unfilled with any coloured or solid object.’ To speak of this distance as ‘imaginary’ or fictitious can according to Hume’s principles make no difference, so long as he admits, which he is obliged to do, that we actually have an idea of it; for every idea, being derived from an impression, is as much or as little imaginary as every other. And not only have we such an idea, but Hume’s account of the ‘relations’ between it and the idea of extension implies that, as ideas of distance, they do not differ at all. But the idea of ‘distance unfilled with any coloured or solid object’ is the idea of vacuum. It follows that the idea of extension does not differ from that of vacuum, except so far as it is other than the idea of distance. But it is from the consideration of distance that Hume himself expressly derives it; [1] and so derived, it can no more differ from distance than an idea from a corresponding impression. Thus, after all, he has to all intents and purposes to admit the idea of vacuum, but saves appearances by refusing to call it extension—the sole reason for such refusal being the supposition that every idea, and therefore the idea of extension, must be a datum of sense, which the admission of an idea of ‘invisible and intangible distance’ already contradicts.
[1] Part II. § 3, sub. inst.
By a like device that he is able to explain the appearance of our having such ideas as Causation and Identity.
281. We now know the nature of that preliminary manipulation which ‘impressions and ideas’ have to undergo, if their association is to yield the result which Hume requires—if through it the succession of feelings is to become a knowledge of things and their relations. Such a result was required as the only means of maintaining together the two characteristic positions of Locke’s philosophy; that, namely, the only world we can know is the world of ‘ideas,’ and that thought cannot originate ideas. Those relations, which Locke had inconsistently treated at once as intellectual superinductions and as ultimate conditions of reality, must be dealt with by one of two methods. They must be reduced to impressions where that could plausibly be done: where it could not, it must be admitted that we have no ideas of them, but only ‘tendencies to suppose’ that we have such, arising from the association, through ‘natural relations,’ of the ideas that we have. So dexterously does Hume work the former method that, of all the ‘philosophical relations’ which he recognizes, only Identity and Causation remain to be disposed of by the latter; and if the other relations—resemblance, time and space, proportion in quantity and degree in quality—could really be admitted as data of sense, there would at least be a possible basis for those ‘tendencies to suppose’ which, in the absence of any corresponding ideas, the terms ‘Identity’ and ‘Causation’ must be taken to represent. But, as we have shown, they can only be claimed for sense, if sense is so far one with thought—one not by conversion of thought into sense but by taking of sense into thought—as that Hume’s favourite appeals to sense against the reality of intelligible relations become unmeaning. They may be ‘impressions,’ there may be ‘impressions of them,’ but only if we deny of the impression what Hume asserts of it, and assert of it what he denies—only if we understand by ‘impression’ not an ‘internal and perishing existence;’ not that which, if other than taste, colour, sound, smell or touch, must be a ‘passion or emotion ‘; not that which carries no reference to an object other than itself, and which must either be single or compound; but something permanent and constituted by permanently coexisting parts; something that may ‘be conjoined with’ any feeling, because it is none; that always carries with it a reference to a subject which it is not but of which it is a quality; and that is both many and one, since ‘in its simplicity it contains many different resemblances and relations.’
282. In the account just adduced of vacuum, the effect of that double dealing with ‘impressions,’ which we shall have to trace at large in Hume’s explanation of our language about Causation and Identity, is already exhibited in little. Just as, after the idea of pure space has been excluded because not a copy of any possible impression, we yet find an ‘idea,’ only differing from it in name, introduced as the basis of that tendency to suppose which is to take the place of the excluded idea, so we shall find ideas of relation in the way of Identity and Causation—ideas which according to Hume we have not—presupposed as the source of those ‘propensities to feign’ which he accounts for the appearance of our having them.
Knowledge of relation in way of Identity and Causation excluded by
Locke’s definition of knowledge.
283. The primary characteristic of these relations according to Hume, which they share with those of space and time, and which in fact vitiates that definition of ‘philosophical relation,’ as depending on comparison, which he adopts, is that they ‘depend not on the ideas compared together, but may be changed without any change in the ideas’. [1] It follows that they are not objects of knowledge, according to the definition of knowledge which Hume inherited, as ‘the perception of agreement or disagreement between ideas.’ A partial recognition of this consequence in regard to cause and effect we found in Locke’s suspicion that a science of nature was impossible—impossible because, however often a certain ‘idea of quality and substance’ may have followed or accompanied another, such sequence or accompaniment never amounts to agreement or ‘necessary connexion’ between the ideas, and therefore never can warrant a general assertion, but only the particular one, that the ideas in question have so many times occurred in such an order. ‘Matters of fact,’ however, which no more consist in agreement of ideas than does causation, are by Locke treated without scruple as matter of knowledge when they can be regarded as relations between present sensations. Thus the ‘particular experiment’ in Physics constitutes knowledge—the knowledge, for instance, that a piece of gold is now dissolved in aqua regia; and when ‘I myself see a man walk on the ice, it is knowledge.’ In such cases it does not occur to him to ask, either what are the ideas that agree or how much of the experiment is a present sensation. [2] Nor does Hume commonly carry his analysis further. After admitting that the relations called ‘identity and situation in time and place’ do not depend on the nature of the ideas related, he proceeds: ‘When both the objects are present to the senses along with the relation, we call this perception rather than reasoning; nor is there in this case any exercise of the thought or any action, properly speaking, but a mere passive admission of the impressions through the organs of sensation. According to this way of thinking, we ought not to receive as reasoning any of the observations we may make concerning identity and the relations of time and place; since in none of them the mind can go beyond what is immediately present to the senses, either to discover the real existence or the relations of objects’. [3]
[1] P. 372. [Book I, part III., sec. I.]
[2] Above, §§ 122 & 123.
[3] P. 376. [Book I, part III., sec. II.]