In some ways, the language of causation is perhaps not the best for expressing what is intended. What is intended may be expressed as follows. Confining ourselves, to begin with, to the percepts of various observers, we can form groups of percepts connected approximately, though not exactly, by laws which may be called laws of "perspective." By means of these laws, together with the changes in our other percepts which are connected with the perception of bodily movement,[Pg 217] we can form the conception of a space in which percipients are situated, and we find that in this space all the percepts belonging to one group (i.e. of the same physical object, from the standpoint of common sense) can be ordered about a centre, which we take to be the place where the physical object in question is. (For us, this is a definition of the place of a physical object.) The centre is not to be conceived as a point, but as a volume, which may be as small as an electron or as large as a star. The essential assumption for what is commonly called the causal theory is, that the group of percepts can be enlarged by the addition of other events, ranged in the same space about the same centre, and connected both with each other and with the group of percepts by laws which include the laws of perspective. The essential points are (1) the arrangement about a centre, (2) the continuity between percepts and correlated events in other parts of the space derived from percepts and locomotion. The first is a matter of observation; the second is a hypothesis designed to secure simplicity and continuity in the laws of correlation suggested by the grouping of percepts. It cannot be demonstrated, but its merits are of the same kind as those of any other scientific theory, and I shall therefore henceforth assume it. FOOTNOTES: [43] On this subject, cf. chap. IV. of Dr Broad's Perception, Physics, and Reality, Cambridge, 1914. [44] Perhaps it would not really be lawless; I shall discuss this at a later stage. [45] On this subject, cf. my Knowledge of the External World. [46] See Dr Whitehead's tract on this subject (Cambridge University Press). Also Pasch, Neuere Geometrie, Leipzig, 1882. [47] The definition of an "ideal" point is as follows. Let , be any two lines in one plane, any point not in this plane. Then the planes , have a line in common, say . The class of all such lines as , when is varied while and remain fixed, is the "ideal" point determined by the two lines , . [48] The character of the "ideal" elements, also, will be less similar to that of percepts than in the above construction, or at least cannot be known to be so similar. [Pg 218] CHAPTER XXI PERCEPTION AND OBJECTIVITY