Diodorus could not make out this φύσις ἄτοπος which Plato calls τὸ ἐξαίφνης.
[71] To illustrate this apparent paradox of Diodorus, affirming past motion, but denying present motion, we may compare what is said by Aristotle about the Now or Point of Present Time — that it is not a part, but a boundary between Past and Future.
Aristot. Physic. iv. p. 218, a. 4-10. τοῦ δὲ χρόνον τὰ μὲν γέγονε, τὰ δὲ μέλλει, ἐστι δ’ οὐδὲν, ὄντος μεριστοῦ· τὸ δὲ νῦν οὐ μέρος — τὸ δὲ νῦν πέρας ἔστι (a. 24) — p. 222, a. 10-20-223, a. 20. ὁ δὲ χρόνος καὶ ἡ κίνησις ἅμα κατά τε δύναμιν καὶ κατ’ ἐνεργείαν.
Which doctrine is thus rendered by Harris in his Hermes, ch. vii. pp. 101-103-105:—“Both Points and Nows being taken as Bounds, and not as Parts, it will follow that in the same manner as the same point may be the end of one line and the beginning of another — so the same Now may be the End of one time, and the beginning of another.… I say of these two times, that with respect to the Now, or Instant which they include, the first of them is necessarily Past time, as being previous to it: the other is necessarily Future, as being subsequent.… From the above speculations, there follow some conclusions, which may be called paradoxes, till they have been attentively considered. In the first place, there cannot (strictly speaking) be any such thing as Time Present. For if all Time be transient, as well as continuous, it cannot like a line be present altogether, but part will necessarily be gone and part be coming. If therefore any portion of its continuity were to be present at once, it would so far quit its transient nature, and be Time no longer. But if no portion of its continuity can be thus present, how can Time possibly be present, to which such continuity is essential?” — Compare Sir William Hamilton’s Discussions on Philosophy, p. 581.
Motion is always present, past, and future.
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]
[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.”
[73] Sext. Emp. adv. Mathem. x. pp. 91-97-112-116.
[74] See this point touched by Plato in Philêbus, p. 43 B.
[75] Sext. Emp. adv. Mathem. x. 113. κίνησις κατ’ εἰλικρίνειαν … κίνησις κατ’ ἐπικράτειαν. Compare Zeller, Die Philosophie der Griech. ii. p. 191, ed. 2nd.