Master Argument

The master argument is an argument attributed to the Megarian philosopher Diodorus Cronus, active around 300 BC, concerning the relationship between necessity, possibility and time. Diodorus held that three claims cannot all be true together: that every true statement about the past is now necessary, that an impossibility never follows from a possibility, and that something is possible even though it neither is nor ever will be the case. Using the first two claims, Diodorus argued that the third must be rejected, concluding that nothing is possible unless it either is true now or will become true at some point, collapsing the ordinary distinction between what is merely possible and what is actually necessary. The argument provoked strong responses from later philosophers, notably the Stoic Chrysippus, who denied Diodorus's second premise in order to preserve a meaningful notion of possibility for things that never in fact occur, and it remains a foundational episode in the ancient philosophical treatment of future contingents.