“A necessary act is that, the production of which it is impossible to hinder: and therefore every act that shall be produced, shall necessarily be produced; for that it shall not be produced is impossible, because, as has already been demonstrated, every possible act shall at some time be produced. Nay, this proposition — What shall be shall be — is as necessary a proposition as this — A man is a man.
“But here, perhaps, some man will ask whether those future things which are commonly called contingents, are necessary. I say, then, that generally all contingents have their necessary causes, but are called contingents, in respect of other events on which they do not depend — as the rain which shall be to-morrow shall be necessary, that is, from necessary causes; but we think and say, it happens by chance, because we do not yet perceive the causes thereof, though they exist now. For men commonly call that casual or contingent, whereof they do not perceive the necessary cause: and in the same manner they use to speak of things past, when not knowing whether a thing be done or not, they say, It is possible it never was done.
“Wherefore all propositions concerning future things, contingent or not contingent, as this — It will rain to-morrow, or To-morrow the sun will rise — are either necessarily true or necessarily false: but we call them contingent, because we do not yet know whether they be true or false; whereas their verity depends not upon our knowledge, but upon the foregoing of their causes. But there are some, who, though they will confess this whole proposition — To-morrow it will either rain or not rain — to be true, yet they will not acknowledge the parts of it, as, To-morrow it will rain, or To-morrow it will not rain, to be either of them true by itself; because (they say) neither this nor that is true determinately. But what is this true determinately, but true upon our knowledge or evidently true? And therefore they say no more but that it is not yet known whether it be true or not; but they say it more obscurely, and darken the evidence of the truth with the same words by which they endeavour to hide their own ignorance.”
[67] The reader will find this problem admirably handled in Mr. John Stuart Mill’s System of Logic, Book iii. ch. 21, and Book vi. chs. 2 and 8; also in the volume of Professor Bain on the Emotions and the Will, Chapter on Belief.
Besides the above doctrine respecting Possible and Impossible, there is also ascribed to Diodorus a doctrine respecting Hypothetical Propositions, which, as far as I comprehend it, appears to have been a correct one.[68] He is also said to have reasoned against the reality of motion, renewing the arguments of Zeno the Eleate.
[68] Sextus Emp. Pyrrhon. Hypotyp. ii. pp. 110-115. ἀληθὲς συνημμένον. Adv. Mathemat. viii. 112. Philo maintained that an hypothetical proposition was true, if both the antecedent and consequent were true — “If it be day, I am conversing”. Diodorus denied that this proposition, as an Hypothetical proposition, was true: since the consequent might be false, though the antecedent were true. An Hypothetical proposition was true only when, assuming the antecedent to be true, the consequent must be true also.
Reasonings of Diodôrus — respecting Hypothetical Propositions — respecting Motion. His difficulties about the Now of time.
But if he reproduced the arguments of Zeno, he also employed another, peculiar to himself. He admitted the reality of past motion: but he denied the reality of present motion. You may affirm truly (he said) that a thing has been moved: but you cannot truly affirm that any thing is being moved. Since it was here before, and is there now, you may be sure that it has been moved: but actual present motion you cannot perceive or prove. Affirmation in the perfect tense may be true, when affirmation in the present tense neither is nor ever was true: thus it is true to say — Helen had three husbands (Menelaus, Paris, Deiphobus): but it was never true to say — Helen has three husbands, since they became her husbands in succession.[69] Diodorus supported this paradox by some ingenious arguments, and the opinion which he denied seems to have presented itself to him as involving the position of indivisible minima — atoms of body, points of space, instants of time. He admitted such minima of atoms, but not of space or time: and without such admission he could not make intelligible to himself the fact of present or actual motion. He could find no present Now or Minimum of Time; without which neither could any present motion be found. Plato in the Parmenidês[70] professes to have found this inexplicable moment of transition, but he describes it in terms not likely to satisfy a dialectical mind: and Aristotle denying that the Now is any portion or constituent part of time, considers it only as a boundary of the past and future.[71]
[69] Sextus Empir. adv. Mathemat. x. pp. 85-101.
[70] Plato, Parmenidês, p. 156 D-E. Πότ’ οὖν, μεταβάλλει; οὔτε γὰρ ἑστὸς ἂν οὔτε κενούμενον μετάβαλλοι, οὔτε ἐν χρόνῳ ὄν. (Here Plato adverts to the difficulties attending the supposition of actual μεταβολή, as Diodorus to those of actual κίνησις. Next we have Plato’s hypothesis for getting over the difficulties.) Ἆρ’ οὖν ἐστί τὸ ἄτοπον τοῦτο, ἐν ᾦ τότ’ ἂν εἴη ὅτε μεταβάλλει; Τὸ ποῖον δή; Τὸ ἐξαίφνης· ἡ ἐξαίφνης αὔτη φύσις ἄτοπος τις ἐγκάθηται μεταξὺ τῆς κινήσεως τε καὶ στάσεως, ἐν χρόνῳ οὐδενὶ οὖσα, καὶ εἰς ταύτην δὴ καὶ ἐκ ταύτης τό τε κινούμενον μεταβάλλει ἐπὶ τὸ ἐστάναι καὶ τὸ ἐστὸς ἐπὶ τὸ κινεῖσθαι.