Again, if he divides up the problem and postulates its parts (supposing eg that he had to prove that medicine is a science of what leads to health and to disease, and were to claim first the one, then the other); or if he postulates the one or the other of a pair of statements that necessarily follow one other (e.g. if he had to prove that the diagonal is incommensurable with the side, and were to postulate that the side is incommensurable with the diagonal).