§ 4. Active Movement. — Extensity, in order to become extension, must assume more or less definite order of parts. How is this acquired? The serial arrangement of colours in a qualitative scale is directly based on the intrinsic quality of the colours themselves, and it is obtained by deliberate comparison of these qualities. The arrangement into positions and distances which constitutes extension is not arrived at in this way. We cannot take local sign qualities obtained by stimulating different parts of the body, and arrange them by comparison in a qualitative scale. Indeed it is probable that there is less difference between the local signatures of corresponding parts of the two hands, than there is between sensation in the fingertips and sensation on the back of the hand. In order to account for spatial order, we must have recourse to some factor distinct altogether from extensity. This factor must show definite serial arrangement in the way of position and distance. It must also be so intimately connected with experiences of extensity that the definite arrangements which belong to it may be transferred to them: for it is not enough to have a pure experience of extensity externally conjoined with another experience showing a definite order of positions and distances. Position and distance must come to belong to the extensity itself.

Now the only factor which fulfils these conditions is movement, and, in particular, active movement of the eyes or hands. In any movement of the limbs or of the body as a whole, a series of varying sensations arises, due to the changing conditions of muscles, joints, and tendons. Following Dr. Ward, we may symbolise such a series as P1 P2 P3 P4. "P1 cannot be presented along with P2 and from P4 it is impossible to reach P1 again save through P3 and P2"* or through some other determinate motor series. These motor experiences have therefore a definite arrangement. P2 lies between P1 and P3. P2 and P3 constitute a distance separating and connecting P1 and P4. Further, if the movement is not merely made in free space, but explores the contours of some object, there is another concomitant and corresponding series having definite arrangement. Suppose the instrument of exploration is the hand. As the fingertips pass from one part of the object to another, there is a series of tactile experiences having a definite order, and varying concomitantly with the sensations arising from muscle, joint, and tendon. If the object explored is part of the cutaneous surface of the body itself, there is still another definitely ordered series. As the fingertip passes along the palm of the other hand, the contact is felt not only in the finger but in the hand explored. The successive stimulation of the parts of the hand yields a succession of local sign experiences which occur in a fixed order, and correspond to the succession of motor experiences. All these series have a definite arrangement of positions and distances: but the arrangement is not spatial. It is purely an order of timesequence. Extension can only exist when the definite order is the order of the parts of an extensive quantum simultaneously presented.

*Article "Psychology," Encyclopaedia Britannica, ninth edition, xx., p. 54.

It is essential to the possibility of this that the experience of extensity and the experience of active movement should enter as cooperative factors into a process having unity and continuity of interest. A process having unity and continuity of interest leaves behind it as a whole a total disposition,— a disposition to which each and all of its component factors in their conjoint interaction have contributed. This cumulative disposition is reexcited as a whole when the process is repeated in part. In this way the factors which enter into the process may become profoundly modified by their previous combination, so that each separately assumes a character which it has acquired from its combination with the others. It comes to mean or stand for the others. When an extensive experience has come to mean or stand for a system of active movements, the extensive experience has become a perception of extension. If, on clasping an object in the hand, you know at once how to make a systematic exploration of its parts, so that you will have nothing to learn by actually executing exploring movements, then you have an adequate perception of its shape and other spatial determinations. If, on the other hand, mere contact with the object does not fully supply precise and definite guidance to the movements of exploration, the spatial perception is pro tanto inadequate. When the perception is adequate, any two local signs or any group of local signs prompts at once the appropriate movements for passing from part to part of the object. Extensity which has thus acquired meaning is no longer mere extensity, but a continuous complex of positions and distances. Just as the passive touch acquires in this way a properly spatial significance, so the active touch which is at first a purely successive series, also acquires a spatial character. As the fingertips pass over an object, the successive tactile experiences do not present themselves as merely a timesequence. They become for consciousness the successive presentation of a whole of coexistent parts.

Extensity and active movement must, as we have said, be combined as essential factors in processes having continuity of interest: otherwise they could not modify each other in the way described. Their conjoint operation must leave behind it a total disposition which is the cumulative aftereffect of the whole process into which they enter. Each, when it occurs separately, will occur modified by its previous conjunction with the other, because it will reexcite the total disposition due to their conjoint operation. Now, if we inquire what the appetitive processes are into which extensity and active movement enter as cooperative factors, we may answer by referring to all the primitive activities by which the ends of animal life are secured. Such practical activity can only be effective in so far as active movement is delicately adjusted to the shape, size, distance, etc., of objects. The guiding clues to such motor adjustment can only be found in touch and sightexperiences. But just in so far as the touch or sightexperiences either originally possess or subsequently acquire the power of guiding active movement, they are or become perceptions of spatial order.

We have now at once to explain and to justify these general statements by an account of the special conditions by which the development of (1) the tactual, (2) the visual, perceptions of space is determined.