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. [4] Locke, Book IV. sec. iii. chap. xiv.; and above, paragraph 121 and 122.