Diodorus Cronus (late 4th–early 3rd centuries BC)

The most famous member of the Dialectical school, the Greek philosopher Diodorus Cronus maintained various paradoxical theses. He argued that any attempt to divide space, time or matter must end with little regions, periods or bodies that cannot further be divided; hence, he inferred, things cannot be in motion. Diodorus also contributed to the contemporary debate on conditionals: one proposition implies another, he held, if and only if it never has been possible, and is not now possible, to have the former proposition true and the latter proposition false. Diodorus is however most famous for inventing the master argument. The master argument relied on two assumptions: that every past truth is necessary, and that the impossible does not follow from the possible. It concluded, on these assumptions, that no proposition is possible unless it either is true or will be. The master argument was designed to support Diodorus’ definition of possibility: a proposition is possible if and only if it either is or will be true. This definition is not exactly tantamount to the fatalist doctrine that all truths are necessary, but it was felt to come too close to fatalism for comfort.