Didorus Cronus, Megarian logician active in the 4th century BC, is now known primarily as the author of what has come to be called the Master Argument - of which only the conclusion is known (as transmitted by Epictetus in his discussion of same in Dissertationes II).

The Master Argument led to the conclusion that the following three claims are inconsistent:

  1. Everything about the past is now necessary. The past is fixed.
  2. The impossible does not follow from the possible.
  3. There is something that is possible, and yet neither is nor will be true.

Unfortunately, Epictetus gives no hint as to the content of the actual argument leading to the above conclusion. Many scholars have attempted to reconstruct the argument from the conclusion (see Arthur Prior, Past, Present, and Future), which continues to foster debate.

In any case, Diodorus made his argument as to the incompatibility of the three statements, and ultimately rejected the third. The resulting thesis posited in its place was:

Whatever is possible either is or is going to be true.

The above claim is the distillation of what has come to be called Diodorean Modality, one of the ways of defining the modal notions of possibility and necessity in terms of time. Its primary consequence is that all true conditionals are necessary.

Thoughts, Words and Things: An Introduction to Late Mediaeval Logic and Semantic Theory, by Paul Spade