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.
298. We are thus brought to the third statement, according to which philosophical relation, instead of being an idea acquired upon comparison, is itself the comparison. A comparison of ideas may seem not far removed from the simple sequence of resembling ideas; but if we examine the definition of cause, as stated above, which with Hume corresponds to the view of the relation of cause and effect as a ‘philosophical’ one, we find that the relation in question is neither a comparison of the related objects nor an idea which arises upon such comparison. According to his statement a comparison is indeed necessary to give us an idea of the relation—a comparison, however, not of the objects which we reckon severally cause and effect with each other, but (a) of each of the two objects with other like objects, and (b) of the relation of precedency and contiguity between the two objects with that previously observed between the like objects. Now, unless the idea of relation between objects in the way of cause and effect is one that consists in, or is acquired by, comparison of those objects, the fact that another sort of comparison is necessary to constitute it does not touch the question of its possibility. However we come to have it, however reducible to impressions the objects may be, it is not only other than the idea of either object taken singly; it is not, as an idea of resemblance might be supposed to be, constituted by the joint presence or immediate sequence upon each other of the objects. Here, then, is an idea which is not taken either from an impression or from a compound of impressions (if such composition be possible), and this idea is ‘the source of all our reasonings concerning matters of fact.’
The comparison is between present and past experience of succession of objects.
299. The modern followers of Hume may perhaps seek refuge in the consideration that though the relation of cause and effect between objects is not one in the way of resemblance or one of which the idea is given by comparison of the objects, it yet results from comparisons, which may be supposed to act like chemical substances whose combination produces a substance with properties quite different from those of the combined substances, whether taken separately or together. Some anticipation of such a solution, it may be said, we find in Hume himself, who is aware that from the repetition of impressions of sense and their ideas new, heterogeneous, impressions—those of ‘reflection’—are formed. Of this more will be said when we come to Hume’s treatment of cause and effect as a ‘natural relation.’ For the present we have to enquire what exactly is implied in the comparisons from which this heterogeneous idea of relation is derived. If we look closely we shall find that they presuppose a consciousness of relations as little reducible to resemblance, i.e. as little the result of comparison, as that of cause and effect itself. It has been already noticed how Hume treats the judgment of proportion between figures as a mere affair of sense, because such relation depends entirely on the ideas compared, without reflecting that the existence of the figures presupposes those relations of space to which, because (as he admits) they do not depend on the comparison of ideas, the only excuse for reckoning any relation sensible does not apply. In the same way he contents himself with the fact that the judgment of cause and effect implies a comparison of present with past experience, and may thus be brought under his definition of ‘philosophical relation,’ without observing that the experiences compared are themselves by no means reducible to comparison. We judge that an object, which we now find to be precedent and contiguous to another, is its cause when, comparing present experience with past, we find that it always has been so. That in effect is Hume’s account of the relation, ‘considered as a philosophical one:’ and it implies that the constitution of the several experiences compared involves two sorts of relation which Hume admits not to be derived from comparison, (a) relation in time and place, (b) relation in the way of identity.
Observation of succession already goes beyond sense.
300. As to relations in time and space, we have already traced out the inconsistencies which attend Hume’s attempt to represent them as compound ideas. The statement at the beginning of Part III., that they are relations not dependent on the nature of compared ideas, is itself a confession that such representation is erroneous. If the difficulty about the synthesis of successive feelings in a consciousness that consists merely of the succession could be overcome, we might admit that the putting together of ideas might constitute such an idea of relation as depends on the nature of the combined ideas. But no combination of ideas can yield a relation which remains the same while the ideas change, and changes while they remain the same. Thus, when Hume tells us that ‘in none of the observations we may make concerning relations of time and place can the mind go beyond what is immediately present to the senses, to discover the relations of objects’ [1] the statement contradicts itself. Either we can make no observation concerning relation in time and place at all, or in making it we already ‘go beyond what is immediately present to the senses,’ since we observe what is neither a feeling nor several feelings put together. If then Hume had succeeded in his reduction of reasoning from cause or effect to observation of this kind, as modified in a certain way by habit, the purpose for which the reduction is attempted would not have been attained. The separation between perception and inference, between ‘intuition’ and ‘discourse,’ would have been got rid of, but inference and discourse would not therefore have been brought nearer to the mere succession of feelings, for the separation between feeling and perception would remain complete; and that being so, the question would inevitably recur—If the ‘observation’ of objects as related in space and time already involves a transition from the felt to the unfelt, what greater difficulty is there about the interpretation of a feeling as a change to be accounted for (which is what is meant by inference to a cause), that we should do violence to the sciences by reducing it to repeated observation lest it should seem that in it we ‘go beyond’ present feeling?
[1] P. 376. [Book I, part III., sec. II.]