§ 3. _The Construction of the Conceptual Space and Time Order of Science._ For the purposes of practical life, no less than for the subsequent object of scientific description of the physical order, it is indispensably necessary to establish equations or correspondences between the individual space and time systems of different percipients. Apart from such correspondences, it would be impossible for one subject to translate the spatial and temporal system of any other into terms of his own experience, and thus all practical intercourse for the purpose of communicating directions for action would come to an end. For the communication of such practical directions it is imperative that we should be able mentally to reconstruct the spatial and temporal aspects of our experience in a form independent of reference to the special _here_ and _now_ of this or that individual moment of experience. Thus, like the rest of our scientific constructions, the establishment of a single conceptual space and time system for the whole of the physical order is ultimately a postulate required by our practical needs, and we must therefore be prepared to face the possibility that, like other postulates of the same kind, it involves assumptions which are not logically defensible. The construction is valuable, so far as it does its work of rendering intercommunication between individuals possible; that it should correspond to the ultimate structure of Reality any further than the requirements of practical life demand is superfluous. The main processes involved in the construction of the conceptual space and time of descriptive science are three,—synthesis, analysis, abstraction. (_a_) _Synthesis._ Psychologically speaking, it is ultimately by the active movements of individual percipients that the synthesis of the individual’s various perceptual spaces into one is effected. As attention is successively directed, even while the body as a whole remains stationary, to different parts of the whole expanse before the eye, the visual space which was originally “focal” in presentation becomes “marginal,” and the “marginal” focal by a sensibly gradual transition. When to the movements of head and eyes which accompany such changes in attention there are added movements of locomotion of the whole body, this process is carried further, and we have the gradual disappearance of originally presented spaces from presentation, accompanied by the gradual emergence of spaces previously not presented at all. This leads to the mental construction of a wider space containing _all_ the individual’s different presentation-spaces, the order in which it contains them being determined by the felt direction of the movements required for the transition from one to another.