 The Stoics confronted a theory of modality (i.e. a theory of possibility and necessity) that claimed precisely this. Diodorus Cronus of the Dialectical school had argued that what is possible is limited to what either is or will be true at some point in the future (Boethius, 38C). So if we in fact don’t ever get around to rowing out to fight the Megarians in a sea battle, then a sea battle with the Megarians was always impossible (and of course it made no more sense to consider how we should go about it than it would be to consider how what we should do in the event that 2+2=5). The means by which Diodorus arrived at this most unwelcome account of modality was called the Master Argument. He endorsed the claim that (1) truths about the past are necessary: it is not merely that they aren’t other than they are – they can’t be other than they are, for nothing has the power to change the past (Epictetus, 38A). He also claimed that (2) nothing impossible follows from what is possible. In the so-called Master Argument, he attempted to show that these two theses were incompatible with the claim that (3) there is something which is possible, but yet does not happen. The details of the Master Argument are a matter of much dispute. We know that it was alleged to show that these three propositions formed an inconsistent triad, but exactly how it did this remains uncertain. We also know that Diodorus’ manner of resolving this inconsistency was to reject (3) and to define the possible as that which is or will be the case.