for when either one is capable of acting independently, the other cannot be capable of acting independently; nor can the two of them (1), at the same time, each be capable of acting independently, since that would require the same effect to exist twice at one and the same time. What is possible, however, is for one to have such power when the other is not acting (2), and vice versa. Thus, each one’s power would be conditional upon the other’s not acting alongside him; consequently, whenever either one (3) acts, the other cannot have such power.

If it were said that the two cooperating agents are not both capable of acting independently at the same time—as is possible and does occur among created beings who cooperate—this would be false [also, as will be shown] (4).

The point is that if both were capable of acting independently, it would be possible for each to bring about what lies within his power, which would entail the coexistence of two contradictories. Otherwise, the power of one of them would have to be conditional upon the other enabling him, and this is impossible, as will be shown.

Moreover, one of them (5) could will the opposite of what the other wills: one could will that a body be moved, while the other could will that it remain still. The coexistence of two opposites is impossible. If one of them could will an act only on condition that the other agreed with him, he would be powerless on his own and would become capable only through the other’s agreement.