[1] pp. 533 and 534 [Book I., part IV., sec. VI.]
… in regard to relations. His account of these.
206. His position in regard to ideas of relation cannot be so summarily exhibited. It is from its ambiguity, indeed, that his system derives at once its plausibility and its weakness. In the first place, it is necessary, according to him, to distinguish between ‘natural’ and ‘philosophical relation.’ The latter is one of which the idea is acquired by the comparison of objects, as distinct from natural relation or ‘the quality by which two ideas are connected together in the imagination, and the one naturally’ (i.e. according to the principle of association) ‘introduces the other’. [1] Of philosophical relation—or, according to another form of expression, of ‘qualities by which the ideas of philosophical relation are produced’—seven kinds are enumerated; viz. ‘resemblance, identity, relations of time and place, proportion in quantity and number, degrees in quality, contrariety, and causation’. [2] Some of these do, some do not, apparently correspond to the qualities by which the mind is naturally ‘conveyed from one idea to another;’ or which, in other words, constitute the ‘gentle force’ that determines the order in which the imagination habitually puts together ideas. Freedom in the conjunction of ideas, indeed, is implied in the term ‘imagination,’ which is only thus differenced from ‘memory;’ but, as a matter of fact, it commonly only connects ideas which are related to each other in the way either of resemblance, or of contiguity in time and place, or of cause and effect. Other relations of the philosophical sort are the opposite of natural. Thus, ‘distance will be allowed by philosophers to be a true relation, because we acquire an idea of it by the comparing of objects; but in a common way we say, “that nothing can be more distant than such or such things from each other; nothing can have less relation”’ (ibid.).
[1] p. 322 [Book I, part I., sec. V.]
[2] ibid., and p. 372 [Book I., part III., sec. I.]
It corresponds to Locke’s account of the sorts of agreement between ideas.
207. Hume’s classification of philosophical relations evidently serves the same purpose as Locke’s, of the ‘four sorts of agreement or disagreement between ideas,’ in the perception of which knowledge consists; [1] but there are some important discrepancies. Locke’s second sort, which he awkwardly describes as ‘agreement or disagreement in the way of relation,’ may fairly be taken to cover three of Hume’s kinds; viz. relations of time and place, proportion in quantity or number, and degrees in any quality. About Locke’s first sort, ‘identity and diversity,’ there is more difficulty. Under ‘identity,’ as was pointed out above, he includes the relations which Hume distinguishes as ‘identity proper’ and ‘resemblance.’ ‘Diversity’ at first sight might seem to correspond to ‘contrariety;’ but the latter, according to Hume’s usage, is much more restricted in meaning. Difference of number and difference of kind, which he distinguishes as the opposites severally of identity and resemblance, though they come under Locke’s ‘diversity,’ are not by Hume considered relations at all, on the principle that ‘no relation of any kind can subsist without some degree of resemblance.’ They are ‘rather a negation of relation than anything real and positive.’ ‘Contrariety’ he reckons only to obtain between ideas of existence and non-existence, ‘which are plainly resembling as implying both of them an idea of the object; though the latter excludes the object from all times and places in which it is supposed not to exist’. [2] There remain ‘cause and effect’ in Hume’s list; ‘co-existence’ and ‘real existence’ in Locke’s. ‘Co-existence’ is not expressly identified by Locke with the relation of cause and effect, but it is with ‘necessary connection.’ It means specially, it will be remembered [3], the co-existence of ideas, not as constituents of a ‘nominal essence,’ but as qualities of real substances in nature; and our knowledge of this depends on our knowledge of necessary connection between the qualities, either as one supposing the other (which is the form of necessary connection between primary qualities), or as one being the effect of the other (which is the form of necessary connection between the ideas of secondary qualities and the primary ones). Having no knowledge of necessary connection as in real substances, we have none of ‘co-existence’ in the above sense, but only of the present union of ideas in any particular experiment. [4] The parallel between this doctrine of Locke’s and Hume’s of cause and effect will appear as we proceed. To ‘real existence,’ since the knowledge of it according to Locke’s account is not a perception of agreement between ideas at all, it is not strange that nothing should correspond in Hume’s list of relations.
[1] See above, paragraph 25 and the passages from Locke there referred to.
[2] p. 323 [Book I, part I., sec. V.]
[3] See above, paragraph 122.