  Diodorus defined the possible as what is or will be true: according to Diodorus, what actually happens is all that can happen. Diodorus, Prior later said, ‘seems to have been an ancient Greek W.V. Quine, who regarded the Aristotelian logic of possibility and necessity with some scepticism, but offered nevertheless some “harmless” senses that might be attached to modal words’ (1967a: 16). Prior found this deterministic definition of the possible uncongenial and set about locating a fallacy in the argument that Diodorus had used to support it, the so-called Master Argument: