This is just as true of the so-called primary qualities of things as of any others. Thus the mass and again the kinetic energy of a conservative material system are properly names for the way in which the system will behave under determinate conditions, not of modes of behaviour which are necessarily actually exhibited throughout its existence. The laws of motion, again, are statements of the same hypothetical kind about the way in which, as we believe, particles move if certain conditions are fulfilled. The doctrine according to which all events in the physical world are actual motions, rests on no more than a metaphysical blunder of a peculiarly barbarous kind. Cf. Stallo, _Concepts and Theories of Modern Physics_, chaps. 10-12. Footnote 85: Thus, _e.g._, so fundamental a proposition in our current mechanical science as the “first law of motion” is avowedly a statement as to what _would_ be the behaviour of things under a condition which, so far as we know, is never actually realised. On the thing as the “law of its states,” see Lotze, _Metaphysic_, I. 3. 32 ff. (Eng. trans., vol. i. p. 88 ff.), and L. T. Hobhouse, _The Theory of Knowledge_, pp. 545-557. Footnote 86: Mr. Hobhouse (_op. cit._, p. 541 ff.) thinks that the solution is simply that those qualities belong to one “substance,” which are apprehended together as occupying one space. As a working criterion of what we mean by one bodily thing, this account seems satisfactory, and has probably suggested itself spontaneously to most of us. But it leaves untouched the more fundamental question how the identification of a certain sight-space with a certain touch-space is effected, and what are the motives which lead to it. Mr. Hobhouse is content to take the identification as “given in adult perception,” but it seems to me to emerge from his own good account of the matter that it is the still more primitive apprehension of my own body as a felt unity upon which the synthesis between sight and touch spaces is based. If so, the ultimate source of the “unity of substance” must be sought deeper than Mr. Hobhouse is willing to go for it. And _quaere_, whether his account, if accepted as ultimate, would not lead to the identification of substance with space? For the difficulties which arise when you say the substance is the space and its filling of qualities, see _Appearance and Reality_, chap. 2, pp. 19, 20 (1st ed.). Footnote 87: _Monadology_, §§ 8-16, 57-62. Footnote 88: