This opinion of Aristotle is in the main consonant with that of Diodorus; who, when he denied the reality of present motion, meant probably only to deny the reality of _present motion apart from past and future motion_. Herein also we find him agreeing with Hobbes, who denies the same in clearer language.[72] Sextus Empiricus declares Diodorus to have been inconsistent in admitting past motion while he denied present motion.[73] But this seems not more inconsistent than the doctrine of Aristotle respecting the _Now_ of time. I know, when I compare a child or a young tree with what they respectively were a year ago, that they have grown: but whether they actually are growing, at every moment of the intervening time, is not ascertainable by sense, and is a matter of probable inference only.[74] Diodorus could not understand present motion, except in conjunction with past and future motion, as being the common limit of the two: but he could understand past motion, without reference to present or future. He could not state to himself a satisfactory theory respecting the beginning of motion: as we may see by his reasonings distinguishing the motion of a body all at once in its integrity, from the motion of a body considered as proceeding from the separate motion of its constituent atoms--the moving atoms preponderating over the atoms at rest, and determining them to motion,[75] until gradually the whole body came to move. The same argument re-appears in another example, when he argues--The wall does not fall while its component stones hold together, for then it is still standing: nor yet when they have come apart, for then it _has_ fallen.[76] [Footnote 72: Hobbes, First Grounds of Philosophy, ii. 8, 11. "That is said to be at rest which, during any time, is in one place; and that to be moved, or to have been moved, which whether it be now at rest or moved, was formerly in another place from that which it is now in. From which definition it may be inferred, first, that whatsoever is moved _has been_ moved: for if it still be in the same place in which it was formerly, it is at rest: but if it be in another place, it _has been_ moved, by the definition of moved. Secondly, that what _is_ moved, _will yet_ be moved: for that which is moved, leaveth the place where it is, and consequently will be moved still. Thirdly, that whatsoever is moved, is not in one place during any time, how little soever that may be: for by the definition of rest, that which is in one place during any time, is at rest. . . . From what is above demonstrated--namely, that whatsoever _is_ moved, _has also been_ moved, and _will be_ moved: this also may be collected, That there can be no conception of motion without conceiving past and future time."] [Footnote 73: Sext. Emp. adv. Mathem. x. pp. 91-97-112-116.] [Footnote 74: See this point touched by Plato in Philêbus, p. 43 B.]