Chapter 6

11 But though such vices may spring from natural causes, their being uprooted and utterly removed, so that the very person inclined toward them is drawn away from such great faults, depends not on natural causes but on will, effort, and training. All these are swept away if the force and nature of fate are to be established by an argument from divination.

For if divination exists, from what established principles of its art does it proceed? By “established principles” I mean what the Greeks call “theorems.” I cannot believe that other practitioners perform their work without any established principle, or that those who practice divination predict the future without one.

12 Let the astrologers' principles, then, be of this kind: “If someone was born, for example, at the rising of the Dog Star, he will not die at sea.” Be on your guard, Chrysippus, lest you abandon your position, over which you have so great a struggle with Diodorus, a formidable dialectician. For if this conditional is true, “If someone was born at the rising of the Dog Star, he will not die at sea,” then this is also true: “If Fabius was born at the rising of the Dog Star, Fabius will not die at sea.” These statements, then, conflict with one another: that Fabius was born at the rising of the Dog Star, and that Fabius will die at sea. And since it is taken as certain about Fabius that he was born at the rising of the Dog Star, these statements also conflict: that Fabius exists, and that he will die at sea. Thus this conjunction too is made up of incompatible statements: “Fabius exists, and Fabius will die at sea”—something that, on the assumption laid down, cannot even happen. Therefore “Fabius will die at sea” belongs to the class of things that cannot happen. Thus everything falsely asserted about the future is impossible.

11 Such faults can arise from natural causes. But removing them completely, roots and all, so that someone once drawn toward serious vices turns away from them, is another matter. That depends on choice, sustained effort, and education, not on natural causes. All of those disappear if arguments based on divination establish fate's nature and power.

If divination exists, what established principles does its practice start from? By “established principles” I mean the Greek “theorems.” I cannot believe other specialists do their jobs with no principles to guide them. Nor can I believe people predict future events through divination with no principles either.

12 Suppose an astrologer's principle runs like this: “Someone born as the Dog Star rises will not meet death at sea.” Be careful, Chrysippus! You are wrestling hard with Diodorus, a powerful logician. Don't surrender your case. If that conditional rule is correct, this one is correct as well: “If Fabius's birth took place as the Dog Star rose, he won't die at sea.” His being born at that rising and his later dying at sea are therefore incompatible. And we assume his birth at that rising is certain. So these are incompatible too: Fabius exists, and Fabius will meet death at sea. Combine them into “Fabius exists and will meet death at sea,” and you have combined contradictory claims. Under the stated assumption, their combination cannot happen at all. That places “Fabius will meet death at sea” among the impossible things. So every false claim about a future event describes something impossible.