Chapter 7

13 But this, Chrysippus, is precisely what you do not want, and it is over this very point that your chief contest with Diodorus arises. For he says that only what is true or will be true is possible; whatever is going to happen, he says, must happen, and whatever is not going to happen, he denies can happen. You, on the other hand, say that even things that will not happen are possible: this gem, for instance, can be broken, even if it never will be. You also say it was not necessary for Cypselus to reign at Corinth, though Apollo's oracle had proclaimed it a thousand years before. But if you endorse these divine predictions, you will class false assertions about the future among impossibilities [as if one were to say that Africanus would take Carthage]; and if something is truly asserted about the future, and will happen as asserted, you will call it necessary. That is the whole doctrine of Diodorus, so hostile to your school.

14 For if this conditional is true, “If you were born at the rising of the Dog Star, you will not die at sea,” and its first clause, “You were born at the rising of the Dog Star,” is necessary—all truths about the past are necessary, according to Chrysippus, who here disagrees with his teacher Cleanthes, because they are unalterable and past events cannot change from true to false—then, if the first clause of the conditional is necessary, its consequence becomes necessary too. Chrysippus, to be sure, does not think this holds in every case; nevertheless, if there is a natural cause why Fabius will not die at sea, Fabius cannot die at sea.

13 Chrysippus, that is exactly the conclusion you reject. This very issue is your main dispute with Diodorus. According to him, a thing is possible only if it is true now or will become true. Whatever is going to occur has to occur; whatever will never occur is impossible. You say a thing may be possible without ever happening. This gemstone could break, for example, even if it never does. And you say Cypselus didn't have to become king of Corinth, although Apollo's oracle announced his reign a thousand years earlier. But approve those divine predictions, and you must put false predictions in the category of impossibilities [as when someone says Africanus will capture Carthage]. You must also call a future event necessary if it is predicted truly and will occur as predicted. That is Diodorus's whole position, the one opposed to yours.

14 Consider this true conditional: “Being born as the Dog Star rises means you won't die at sea.” Its first part is “You were born as the Dog Star rose.” That part is necessary. Chrysippus holds that all true statements concerning past events are necessary, disagreeing here with his teacher Cleanthes. The past cannot change; something true about it cannot turn false. Once the first part of a conditional is necessary, the part following it becomes necessary too. Admittedly, Chrysippus doesn't accept that rule in every case. But if nature supplies a cause that prevents Fabius's death at sea, dying at sea is impossible for him.