In short, when two agents jointly produce an effect, each depends on the other for that effect to exist and needs the other in this respect; otherwise, they would not be acting jointly. For each either performs the action independently and alone, or does not. If one performs it independently and alone, it cannot have a partner or helper in it. If it does not perform it independently and alone, the effect is produced not by it alone, but by it together with the other. It alone is therefore insufficient to bring that effect into existence; rather, it needs and depends on the other to bring it into existence.
This entails that it is not the lord, owner, or creator of that effect, but only a partner in producing it. It also entails that it cannot dispense with its partner in producing that effect, but depends on and needs that partner. This further entails its inability and lack of power to produce the effect on its own. As we have noted, a person cannot do alone what another shared with him in doing, even if he has undergone no change that would make him fully capable of what he did jointly with the other.
For if an agent, when acting alone, possesses complete power—and we are assuming that it resolutely wills the effect, since otherwise the effect would not come into existence either when it acts alone or when it acts jointly with another—for the intended object of a will that is not resolute does not come into existence, the object brought about by