The doctrine which I have just been canvassing is expressly cited by Aristotle as a Megaric doctrine, and was therefore probably held by his contemporary Eubulidês. From the pains which Aristotle takes (in the 'De Interpretatione' and elsewhere) to explain and vindicate his own doctrine about the Potential and the Actual, we may see that it was a theme much debated among the dialecticians of the day. And we read of another Megaric, Diodorus[60] Kronus, perhaps contemporary (yet probably a little later than Aristotle), as advancing a position substantially the same as that of Eubulidês. That alone is possible (Diodorus affirmed) which either is happening now, or will happen at some future time. As in speaking about facts of an unrecorded past, we know well that a given fact either occurred or did not occur, yet without knowing which of the two is true--and therefore we affirm only that the fact _may_ have occurred: so also about the future, either the assertion that a given fact will at some time occur, is positively true, or the assertion that it will never occur, is positively true: the assertion that it may or may not occur some time or other, represents only our ignorance, which of the two is true. That which will never at any time occur, is impossible. [Footnote 60: The dialectic ingenuity of Diodorus is powerfully attested by the verse of Ariston, applied to describe Arkesilaus (Sextus Emp. Pyrrh. Hyp. i. p. 234): [Greek: Pro/sthe Pla/tôn, o)/pithen Pu/r)r(ôn, me/ssos Dio/dôros.]] [Side-note: Sophism of Diodorus--[Greek: O( Kurieu/ôn].] The argument here recited must have been older than Diodorus, since Aristotle states and controverts it: but it seems to have been handled by him in a peculiar dialectic arrangement, which obtained the title of [Greek: O( Kurieu/ôn].[61] The Stoics (especially Chrysippus), in times somewhat later, impugned the opinion of Diodorus, though seemingly upon grounds not quite the same as Aristotle. This problem was one upon which speculative minds occupied themselves for several centuries. Aristotle and Chrysippus maintained that affirmations respecting the past were _necessary_ (one necessarily true and the other necessarily false)--affirmations respecting the future, _contingent_ (one must be true and the other false, but either might be true). Diodorus held that both varieties of affirmations were equally necessary--Kleanthes the Stoic thought that both were equally contingent.[62] [Footnote 61: Aristot. De Interpret. p. 18, a. pp. 27-38. Alexander ad Aristot. Analyt. Prior. 34, p. 163, b. 34, Schol. Brandis. See also Sir William Hamilton's Lectures on Logic, Lect. xxiii. p. 464.] [Footnote 62: Arrian ad Epiktet. ii. p. 19. Upton, in his notes on this passage of Arrian (p. 151) has embodied a very valuable and elaborate commentary by Mr. James Harris (the great English Aristotelian scholar of the 18th century), explaining the nature of this controversy, and the argument called [Greek: o( Kurieu/ôn].