Diodorus Cronus was a member (or perhaps a late follower) of the Megarian School, whose arguments about the truth and falsity of statements about the future may have influenced Aristotle.

The so-called "Master Argument" (the κυριεύων or κύριος λόγος) was apparently first formulated by Diodorus, although we do not have his specific argument, because he wrote nothing (or at least nothing that survives). Like Socrates, he preferred talking (the dialectic) to writing. He was known as "The Dialectician."

Epictetus reconstructs the lost Master Argument as a logical contradiction between three propositions.

1. every past truth must be necessary
2. an impossibility does not follow from a possibility
3. something is possible which neither is nor will be true

Given the first two, it is apparent that the third is false, and we can conclude that nothing is possible which is not true and never will be.

The Master Argument was central in the Hellenistic debates about determinism, as shown by Cicero's descriptions in On Fate.

For the ancients, and for modern sophists, this is the kind of logical linguistic argument used to prove that motion is impossible, Zeno's famous paradox. It is similar to Parmenides' dialectical proof that all the atoms must be in contact with one another, because there is a void (viz., nothing) between them.

Fundamentally, the Master Argument is a question abut the ontological status of the possible, the potential (δύναμις), as compared to the actual (ὁ κύριος). Epictetus' third proposition that something that neither is or ever will be is "possible" is just the common sense use of the term possible, phrased in such a way as to make it doubtful. How could something that neither is nor ever will be, be possible?

In his sophistic conclusion, that nothing is possible which is not true and never will be, Diodorus is playing language games with the meaning of "possible," just as modern compatibilist philosophers like Harry Frankfurt deny the existence of . Clearly, the Master Argument applies to the question of whether we are (or ever were) free to do otherwise.

Diodorus also argued that the actual is the only possible. He observed that if something in the future is not going to happen, it was therefore true in the past that it would not happen. From this correct observation, he mistakenly claimed that the existence of true statements about the future imply that the future is already determined.

This is the problem of future contingency, made famous in the example of Aristotle's Sea-Battle in De Interpretatione 9. Aristotle thought statements about the future lack any truth value. They are neither true nor false until the future time when they become true or false.

So a better way out might be to note that Diodorus' first proposition is dependent on the use of "truth," especially supposed "true" statements about the future, and we can reduce the Master Argument to the problem of future contingency.

The assumption that logical arguments and their "truths" are timeless is a misapplication of logic, which in any case cannot tell us much about the physical world.

Modern philosophers like C. W. Rietdijk, Hilary Putnam, J. J. C. Smart, Michael Lockwood, and Michael Levin like to think that the future is "already out there" in the relativistic space-time continuum of a "tenseless" "block universe."

Because special relativity can treat time as another dimension, philosophers mistakenly think it must have the properties of space, which is "already there." This is similar to the mistakes of both philosophers and mathematicians about chance. Since random events follow distribution laws, they must be lawful and therefore chance is not real, it is an illusion, like free will.

Modern determinists/compatibilists on free will like to argue that just as the past cannot be changed, so the future cannot be changed. "Change it from what to what?," says Daniel Dennett.

But note that the "truth value" of a statement made in the past can be changed when an event it describes does or does not happen, showing that some aspects of the "fixed past" (i.e., the truth values of past statements) "actually" have some changeability (when their potentials become actual).

Thinking about the Master Argument in terms of the growth of information provides the clearest resolution of the problem of future contingency.

Information is not a constant of nature. The cosmic creation process is constantly creating new information.

When information is created, there is always an irreducible chance element since it is quantum processes that form stable new information structures.

Some information about future events does not come into existence until the potential events become actual.

It is at that time that statements about the future acquire their truth values. Aristotle did not think in information terms, but he had the right answer to the Master Argument, one that disturbs modern logical positivists and analytic language philosophers, as shown by Richard Taylor's faux argument for Fatalism in the Psychological Review.

Note that Fatalism is a form of determinism. It is the simple idea that everything is already fated to happen, so that humans have no control over their future. But also note that Fate (personified) has arbitrary power and need not follow any causal or otherwise deterministic laws. It could include the miracles of omnipotent gods, in which case future contingency returns.

Note further that the gods' omnipotence contradicts the theological assumption that gods are omniscient. They cannot be both. That is a logical contradiction.