Examination of Hume’s language about them. 297. This definition, indeed, is not stated so exactly or so uniformly as might be wished. In different passages ‘philosophical relation’ appears as that in respect of which we compare any two ideas; as that of which we acquire the idea by comparing objects, [1] and finally (in the context of the passage last quoted) as itself the comparison. [2] The real source of this ambiguity lies in that impossibility of regarding an object as anything apart from its relations, which compels any theory that does not recognize it to be inconsistent with itself. It is Locke’s cardinal doctrine that real ‘objects’ are first given as simple ideas, and that their relations, unreal in contrast with the simple ideas, are superinduced by the mind--a doctrine which Hume completes by excluding all ideas that are not either copies of simple feelings or compounds of these, and by consequence ideas of relation altogether. The three statements of the nature of philosophical relation, given above, mark three stages of departure from, or approach to, consistency with this doctrine. The first, implying as it does that relation is not merely a subjective result in our minds from the comparison of ideas, but belongs to the ideas themselves, is most obviously inconsistent with it according to the form in which it is presented by Locke; but the second is equally incompatible with Hume’s completion of the doctrine, for it implies that we so compare ideas as to acquire an idea of relation other than the ideas put together--an idea at once open to Hume’s own challenge, ‘Is it a colour, sound, smell, &c.; or is it a passion or emotion?’ [1] Cf. Part I. 5. [2] P. 464. [Book I, part III., sec. XIV.] Philosophical relation consists in a comparison, but no comparison between cause and effect.