  Diodorus’ definition of possibility rules out future contingents and implies the counterintuitive thesis that only the actual is possible. Diodorus tried to prove this claim with his famous Master Argument, which sets out to show the incompatibility of (i) ‘every past truth is necessary’, (ii) ‘the impossible does not follow from the possible’, and (iii) ‘something is possible which neither is nor will be true’ (Epict. Diss. II.19). The argument has not survived, but various reconstructions have been suggested. Some affinity with the arguments for logical determinism in Aristotle’s De Interpretatione 9 is likely.