As we learn, through intercommunication with our fellows, of the existence for their perception of perceptual extension never directly presented to our own senses, the process of synthesis is extended further, so as to comprise in a single spatial system all the presentation-spaces of all the individual percipients in an order once again determined by the direction of the movements of transition from each to the others. Finally, as there is nothing in the principle of such a synthesis to impose limits upon its repetition, we think of the process as capable of indefinite continuance, and thus arrive at the concept of a space stretching out in all directions without definite bounds. This unending repetition of the synthesis of perceived spaces seems to be the foundation of what appears in theory as the Infinity of Space. Precisely similar is the synthesis by which we mentally construct a single time system for the events of the physical order. _Now_ means for me the content which occupies the centre of attentive interest. As attention is concentrated on the different stages in the realisation of an interest, this centre shifts; what was central becomes first marginal and then evanescent, what was marginal becomes central. Hence arises the conception of the events of my own inner life as forming a succession of moments, with a determinate order, each of which has been a _now_, or point of departure for directions in perceptual time, in its turn. As with space so with time, the intrasubjective intercourse of man with man makes it possible for me mentally to extend this conceptual synthesis of moments of time so as to include _nows_ belonging to the experience of others which were already past before the first _now_ of their experiences which I can synchronise with a _now_ of my own, and again _nows_ of their experiences relatively to which the last _now_ which synchronises with one of my own is past. The indefinite repetition of such a synthesis leads, as before with space, to the thought of a duration reaching out endlessly into past and future, and thus gives us the familiar concept of the Infinity of Time.[144]