* In connexion with this chapter, the student should recall and if necessary reread ch. ii. of bk. ii., div. ii.

§ 1. Unification of Perceptual Data. — It is the function of ideational consciousness to connect in a continuous whole the detached data of senseperception occurring in the course of individual experience. The isolated facts of senseperception are made continuous with each other by interposing between them ideally represented links. The physical object reveals itself in actual perception as existing, persisting, and changing independently of the motor activity of the percipient. Its characteristic nature as physical object essentially involves this independence of the percipient subject and his changing position in relation to it. But the percipient may not only alter his relative position in regard to it, while he is actually perceiving it; he may also turn aside from it altogether, or remove himself to such a distance that it can no longer affect his senses. As change of position on his part makes no difference to the thing as physical object, so his presence or absence can make no difference to its nature and existence. When therefore he ideally represents it, he will represent it as existing, persisting, and changing, although it is no longer perceived. He will represent it as existing, persisting, and changing in the same manner as if he were in its presence and actually observing it. Herein lies the possibility of extending knowledge of material things and processes far beyond the limits of actual perception so as to construct an ideally represented world of which only detached fragments are actually perceived.

We have now to assign the motives which prompt and guide the process of ideal construction. The first of these is that which constitutes the impulse to all theoretical as distinguished from practical thinking. It is the endeavour to clear experience from incoherence, contradiction, and ambiguity. Incoherence, contradiction, and ambiguity obstruct the onward flow of ideas. Where they rise, therefore, the course of mental activity will direct itself to their removal. Now it is obvious that conflict must continually arise between an object as actually perceived and the same object as ideally represented on the basis of previous perception. A man leaves an object at rest in one place: he returns and finds it in another place: the discrepancy can only be removed by ideally connecting the two experiences by intermediate links representing some mode in which the transference from one place to another may or must have taken place. A fire is left burning brightly; after an interval nothing is found but grey embers. Percept and remembrance must be connected by ideal representation of a fire gradually decaying. Again, the fire which is left burning brightly may after a long interval of time be found still burning as brightly as ever. Here the representation of the fire as gradually decaying collides with the actual percept. It has not gone out. The incoherence may be removed by representing some one as having interfered in the meantime to keep it alive. Apart from actual conflict between idea and perception, the mere strangeness of an object acts as a theoretical motive for ideal construction. The mere inability to fit it into the general scheme of things impels the subject to trains of thought directed to overcome the difficulty.

Merely theoretical interest however is on the whole a factor of secondary importance; and the more primitive the stage of mental development attained, the less important it is. The pursuit of knowledge for its own sake is a comparatively late outcome of mental evolution. In early stages of human development thinking is mainly subservient to practical ends, and its impelling motive lies in the pressure of practical needs. Thus the process of ideal interpretation was carried on only so far as it supplies a guide to action. Merely theoretical speculation might exist as a sort of amusement: but it was not followed out in a serious and strenuous manner.