Moreover, the statement, “One who has power (1) is one who acts if he wills and refrains if he wills,” means that, before either acting or refraining, he may will the act to occur at the second moment or may will to refrain at that moment. This freedom to choose between the two exists only when both are absent. While the act is occurring, refraining is impossible; and while one is refraining, the act is impossible. Thus, the act is necessary while it exists, not in the state in which he is (2) free to choose between acting and refraining. In the state of choice, it was not necessary; and in the state of its necessity, he was not free to choose.
It is true that, while acting, he may will to refrain after the act; but this is a subsequent act of refraining and is not (3) the refraining from that act while it exists. One who has power is never free to choose between two things while one of them exists (4 except insofar as the choice concerns the second moment; otherwise, while one of them exists, he is not 4) (4) free to choose (5) between its existence and nonexistence while it exists. At the very moment an agent is acting, it is impossible for him to be refraining. Consequently, this refraining cannot be within his power, because the impossible cannot be within one’s power. Power over two opposites is power over each of them alternatively, not power to combine them, (6) just as one is said to be capable of dyeing a garment black and bleaching it white, traveling east and west, going right and left, and marrying one sister or the other (7).