Logical Puzzles - Aristotle's Solution

Stoics reject the idea that there is a universal form or species of giraffe. Chrysippus, like aristotle, found these consequences troubling so he proposed a novel conception of possibility and necessity for chrysippus. He said something is possible, so long as it is not in itself impossible, and furthermore, is not prevented from happening. Many things that could happen do not happen; but other things that never happen are impossible.