Diodorus Cronus (Διόδωρος Κρόνος, Cronus was a nickname, the old 'crone') was a member (or perhaps a late follower) of the Megarian School, whose arguments about the truth and falsity of statements about the future may have influenced Aristotle. But they have certainly influenced modern philosophers who think that philosophical problems can be decided by logic and language games. Diodorus was known as "The Dialectician," testimony to his sophistry with words, or for his ability to create paradoxes. Epictetus wrote a diatribe "Against those who embrace philosophical opinions only in words," in Book 2, Chapter 1, of his Discourses. It is our major reference to Diodorus and his famous Master Argument (the κυριεύων or κύριος λόγος). Master Argument is a misleading translation. A better translation for κύριος λόγος might be the Main or Principal Argument. The familiar English title was probably translated by a Christian scholar, for whom κύριος translates English "Sir" and the Hebrew Ba'al, to give us "Lord" and "Master." Ba'al is husband, lord, and master in modern Israeli Hebrew. Diodorus' Master Argument is a set of propositions designed to show that the actual is the only possible and that some true statements about the future imply that the future is already determined. This follows logically from his observation that if something in the future is not going to happen, it must have been that statements in the past that it would not happen must have been true. The Master Argument was central in the Hellenistic debates about determinism, as shown by Cicero's descriptions in On Fate. It is closely related to the problem of future contingency, also discussed by Diodorus, but made famous in Aristotle's example of a Sea-Battle in De Interpretatione 9. Aristotle thought statements about the past and present must be either true or false. But statements about the future are only potentials, possibilities, so they lack any truth value until their potential becomes actual at some time in the future. Note that there are in fact some things in the past that can be changed in the future. They are the truth values of statements made in the past. Aristotle's statement "there will be a sea-battle next week," can "actually" be changed if the event does not happen, showing that the concept of a "fixed past," so important in analytic language philosophy debates about free will, has some changeability. In language philosophy, ideas from the "fixed past" are far from fixed. Diodorus was a great logician and word-juggler. Like Socrates, he wrote little or nothing and preferred verbal debates. The Dialectician was a precursor of the later language game players, Ludwig Wittgenstein, Jacques Derrida, and Daniel Dennett. Diodorus applied Democritus' great insight that much knowledge is pure convention (νόμος), but "in reality" there is only atoms and a void. For him, language definitions were conventional and quite arbitrary. His most famous example was the linguistic puzzle of how to define a "heap" (philosophers call this the Sorites paradox, from Greek σωρείτης so-ri'-tes, meaning "heaped up"). When does a number of grains become a heap? One? No. Two? No. Three? Etc. Or, given a heap of grains, as you take grains away, at which point does it stop being a heap?
Epictetus on the Master Argument
Ὁ κυριεύων λόγος ἀπὸ τοιούτων τινῶν ἀφορμῶν ἠρωτῆσθαι φαίνεται: κοινῆς γὰρ οὔσης μάχης τοῖς τρισὶ τούτοις πρὸς ἄλληλα,Epictetus continues:τῷ [τὸ] πᾶν παρεληλυθὸς ἀληθὲς ἀναγκαῖον εἶναισυνιδὼν τὴν μάχην ταύτην ὁ Διόδωρος τῇ τῶν πρώτων δυεῖν πιθανότητι συνεχρήσατο πρὸς παράστασιν
If then any man should ask me, which of these propositions do you maintain? I will answer him, that I do not know; but I have received this story, that Diodorus maintained one opinion, the followers of Panthoides, I think, and Cleanthes maintained another opinion, and those of Chrysippus a third. What then is your opinion? I was not made for this purpose, to examine the appearances that occur to me, and to compare what others say and to form an opinion of my own on the thing. Therefore I differ not at all from the grammarian.