§ 8. _The Problem of Relation._—More perplexing than the problem of Substance and its Qualities is the question to which the pre-scientific assumption that the world consists of a number of _interrelated_ things gives rise. This problem of Relation becomes still more prominent when reflection upon the problem of Substance and Quality has made it manifest that what we call the qualities of things are one and all dependent upon their relations either to our perceptive organs or to other things. Put quite simply the problem is as follows: Things stand in a variety of relations to one another, and what we commonly call the qualities of each are dependent on (_a_) its modes of relation to other things, (_b_) its relation to our percipient organism. Again, the various qualities of one thing stand in relation among themselves. To begin with, they all exhibit the relations of identity and difference. They all so far possess a common nature as to be capable of being compared in respect of the special ways in which they manifest that nature, and are thus so far identical; again, they can be discriminated and distinguished, and are so far in the relation of difference. Further, the qualities of one thing are interconnected, as we have already seen, by various special laws or modes of relation, which exhibit the changes in the behaviour of the thing corresponding to changes in the surrounding circumstances. Thus Phenomenalism, when it has banished the notion of a substantial unity in things, has to identify the world of things, as we have already seen, with qualities in relation to one another. But now the question arises, How are we to understand the conception of qualities in relation? Can we, on the one hand, reduce all qualities to relations or all relations to qualities, or, on the other, can we form an intelligible idea of the way in which a single whole or system can be formed by the union of the two? There are, of course, other questions of great though relatively secondary importance connected with the problem of relation, _e.g._, the question as to the number of ultimately irreducible kinds of relation, but the scope of the present work will permit of nothing beyond a brief discussion of the central difficulty. We will take the various alternatives in order. (1) Philosophers have often been tempted to evade the difficulty of showing how qualities _and_ relations together can make up a system by suppressing one member of the antithesis altogether. Thus it has been maintained, on the one hand, that the world of real things consists entirely of simple unrelated qualities, and that what we call relations between these qualities are merely our own subjective ways of apprehending them. On the other hand, it has been suggested that there may be nothing in the real world except relations, and that what we call qualities of various kinds are nothing but forms of relation. But neither of these views seems seriously tenable.