We may indeed go still further, and say that every unique moment or experience has its own unique spatial and temporal system. The method by which I weave the perceived space-time systems of different experiences within my own mental life into a single conceptual system, is in principle the same by which the spaces and times of myself and other men are made into one system for the purpose of practical intercourse. Footnote 144: For an account of the psychological processes involved in all this, see, _e.g._, Stout, _Manual of Psychology_,^3 bk. iii. pt. 2, chaps. 3-5; bk. iv. chap. 6. Footnote 145: Thus Dedekind (_Was sind und was sollen die Zahlen?_ p. xii.) maintains that none of the constructions of Euclid involve the continuity of space. Footnote 146: Of course, a physical _vacuum_ is not the same thing as _empty space_. For the purposes of any special science a _vacuum_ means a space not occupied by contents of the special kind in which that special science is interested. Thus, in the ordinary parlance of Physics, a _vacuum_ means simply a space in which there is no _mass_. Whether it is desirable, for the purposes of physical science, to assume the existence of _vacuum_, is altogether a question for Physics itself, and to decide it in the affirmative is not to maintain the existence of that unmeaning abstraction, absolutely empty space. In any case, it may be observed that the widespread notion that motion is only possible in a physical _vacuum_ is a mistake, motion being perfectly possible in a fluid _plenum_. Footnote 147: It must be carefully noted that _distance_ as thus defined is not properly a _quantitative relation_, and involves no notion of magnitude, but only of relative place in a series. It should also be observed that in assuming the existence of such a unique relation between every pair of points, it is tacitly taken for granted that the number of dimensions of the spatial order is finite. In a space of an infinite number of dimensions, such unique relation would be impossible. (See Russell, _Foundations of Geometry_, p. 161 ff.) Our justification for making this assumption, as also for taking time to be of one dimension only, seems to be that it is indispensable for all those practical purposes which depend on our ability to create a science of Geometry, and that we have no positive ground for assuming the opposite. Thus ultimately the assumption appears to be of the nature of a postulate. Footnote 148: