The professor smiles and offers you a meandering lecture on the matter...
Ah yes, Diodorus was actually famous for more than just the Master Argument, you know. He came up with arguments like the "Veiled One" and the "Horned One". His works were so often rehearsed and debated that even the birds learnt them by heart and sang them from the rooftops. People would chalk on the walls, "Diodorus Cronus is wise", hah.
But that was not your question; let me try to focus. You want to know about the Master Argument, which was also known by the title On the Possible. It is comprised of three assertions, but only two of them can be true. For if taken together, they will result in a contradiction. Let me list them for you:
Having recognised the truth of the first two, Diodorus rejected the third. He makes these choices because he wishes to ensure that potentiality complies with ontological necessity. If something can happen, it necessarily does so. The idea of indeterminate possibilities and the idea that things can switch from true to false, that's all nonsense. I'll go through the Master Argument line by line, I hope you'll see the beauty of the puzzle our friend presents to us.
I take point one to signify that past events and facts are set in stone. For example, you walked into this room, that event cannot be eliminated. For point two, I understand a possibility to be an event that will occur in the chronology. An impossibility is an event that does not occur in the chronology. For example, I will not jump up and hit my head on the ceiling in the next sixty seconds. That event won't occur in the chronology, so it's an impossibility.
The third point should be immediately suspect to anyone who has thought at length about metaphysics. Point three introduces indeterminacy - it is possible for there to be an event which is neither true nor false until its time comes. Only when the time frame completes may we say it is true or false, categorising it as a possibility or an impossibility.
Let's skip forward a minute and review. Per point one, everything in the past is necessarily true. Therefore, taking point one and three together, it is true that in the past there is an indeterminate event where I jump up and hit my head on the ceiling within sixty seconds. That event didn't come to pass, so the event is categorised as an impossibility per point two.
Yet now we have an impossibility following from a possibility! You entering this room is the initial possibility. What follows after it? If we throw out point three, we will only posit other possibilities - events that do occur. Yet if we include point three, we will posit another thing - an indeterminate. If an indeterminate does not occur, it is an impossibility. The contradiction occurs because we put this indeterminate thing in front of a possibility and then it turned out to be an impossibility. Point one secures all the details, ensuring that the contradiction cannot be avoided.
If we wish to maintain point three, we must either break with point one, which secures the details, or point two, that an impossibility doesn't follow a possibility. We might eliminate point two and adopt a presentist model full of broken branches. In fact, we know that there are some famous people in antiquity that did maintain a different combination of the points. Yet, as I hinted earlier, there are insurmountable metaphysical objections to point three.
Sadly, we have not received the Master Argument directly from Diodorus. His works are largely lost. We have some detailed testimonia and some very brief quotes; the list of points is actually drawn from a much later Stoic author.
Anyway, forgive me if I stop our discussion there. I would highly recommend you check out the general exhibits on Change, or perhaps retire to our guesthouse. For my part, I hope to have this room set up in a day or two.
The man smiles broadly at the idea of having his exhibits ready for public viewing. You decide to...