This distinctness and unity depend on distinctness and unity of interest. Thus different sensible qualities as severally presented to touch, sight, hearing, etc., are united in one thing because they have unity of interest, and on the perceptual level this interest is purely practical. The practical Interest lies in the power of the subject to act on the thing, and of the thing to act on the subject. But in all such activity the different senses cooperate so that the experiences they yield form part of one continuous whole. The visual appearance of the thing serves as a guide to the movements which lead to contact. Tactual and visual extension correspond point for point, and the practical value of the visual appearance lies in this correspondence. There is a similar practical relation between sounds or smells proceeding from an object, and its spacerelations as presented to sight and touch. By approaching the object the sounds and smells become intensified. Action on the object, though initially prompted by sound or smell, must by guided by sight and touch. In this way an object comes to exist for perceptual consciousness as the same in its diverse appearances to different senses. But this, so to speak, only accounts for the stuff of which things are made; it does not account for the division of this material into separate things. We have therefore to inquire why this or that group of sensible qualities is separated from its surroundings and treated as one thing. For ordinary common sense the world is mapped out into a plurality of these relatively independent units. Each of them emerges from its environment like an island from the sea. It is detached from its surroundings by its separateness and unity of interest. This interest is ordinarily of a practical kind; and the further we trace back the course of human development the more exclusively practical it becomes. It is true that for our highly complex consciousness the form of Thinghood has become very variable and fluctuating in its application. A stone is a single thing to a boy about to fling it at another boy. To the geologist examining its structure it may be several distinct things. It is nearly always possible to mentally break up what appears as one object into parts each of which has an identity and distinctness of its own. But we only do so in so far as the interest of the moment leads us to do it. The relativity and variability of our application of the category of Thinghood depends on this fluctuation of interest. In general however the division of the world into separate things is determined by more or less permanent and common interests of a practical nature. Thus if I were asked what things are in a room in which I happen to be lecturing I should say there was a blackboard, a desk, and so on. I should not begin to enumerate the dints and scratches on the blackboard, or the different planks in the flooring. I should be still less likely to mentally divide the uniform surface of the blackboard into different compartments and count each of these as a distinct thing. I should not do this unless I had a special interest to serve.

In more primitive stages of mental development, human interests are at once more exclusively practical in their nature and more limited in their range and less fluctuating. Hence for primitive man the division of the external world into separate units called things is more fixed and absolute. But the limit in this direction is reached in the perceptual consciousness. Animals distinguish from its environment and treat as a separate thing whatever portion of matter appeals to their peculiar instincts and affords occasion for their characteristic modes of activity. Thus what is a separate thing for one animal is not so for another. The interests of each species are to a very large extent determined by the connate predispositions which belong to it like other specific characters. What possesses unity and distinctness of interest to an ant is nothing to the cat, and so on.

There are however also more general conditions under which a thing may detach itself from its environment and become a separate centre of interest for the animal consciousness. Thus it may be a source of peculiarly intense sensations, or it may move in an obtrusive manner. Moving objects have a peculiar power of attracting attention. This is partly because the sensory experience which they produce is more intense than that produced by things at rest. But the chief reason is that a thing which moves in an obtrusive way, challenges practical adjustment. There is need to run away from it, or at any rate keep a watch on it; for no one knows what it may do.

In general, whatever appears to the perceptual consciousness as separate is so because it is a centre of practical interest. It is capable, in its relation to the percipient, of acting independently and as a whole or of being acted on independently and as a whole. Thus Thinghood as a perceptual category is, like causality, purely and immediately practical. Like causality it is a constitutive form of cognition only because it is a constitutive form of action. And just as the perceptual consciousness is incapable of inquiring how or why a certain cause produces a certain effect, so it is incapable of inquiring how or why a thing possesses its unity and independence and its peculiar modes of behaviour. For the perceptual consciousness individuality is unanalysed and unexplained, unanalysable and unexplainable. In this respect the perceptual consciousness stands at the one extreme, and modern science at the opposite extreme. Modern science explains things till it appears almost to explain them away. It would do so altogether if it could. It obliterates the lines of demarcation drawn by common sense; it seeks to dissolve the selfsubsistent units of common sense, and exhibits them as mere modes or phases of one continuous process embracing the whole material world. It must indeed have distinct units of its own to serve as centres of action and reaction. But the ultimate units which it seeks for and more or less succeeds in finding are merely vehicles of the application of abstract and universal laws, without intrinsic character or quality of their own. It is the ideal of science to exhibit the internal unity and the distinctive behaviour of individual things as mere resultants of the complex interaction of these characterless units. For the perceptual consciousness, as we have seen, the extreme opposite is true. The unity and distinctive behaviour of the individual thing is for it unconditional and ultimate. For primitive human thinking it is also in a large measure unconditional and ultimate. The tendency of the primitive man is rather to use the unity of the individual as a principle of explanation than to regard it as something to be explained. Herein lies, as we shall see in the sequel, a most important clue to the nature and origin of primitive beliefs, magical, mythological and religious.