Likewise (1), if it is assumed that neither of them is capable of acting independently, but that each can act only with the other’s assistance, as is the case with created beings; or if it is said that each (2) of them can act independently provided the other steps aside and leaves him free to act—in all these cases, it necessarily follows that each one’s power can arise only through the other empowering him. This is impossible, for it constitutes circular dependence in causation—among agents and efficient causes (3). Circularity is impossible with respect to that by which an agent becomes fully capable of acting, just as it is impossible with respect to the agent’s own being. Power is a condition for action, so an agent can act only through power. (* Thus, if this one’s power could arise only through that one’s power (4), while that one’s (5) power could arise only through this one’s power *) (6), this would constitute an impossible circularity.
Likewise, if the being of that one could arise only through this one, while the being of this one could arise only through the being of that one (7), this would constitute an impossible circularity, since each would be the agent that brought the other into being. This differs from a case in which one necessarily accompanies the other or is a condition of it (8), while the agent that brought them about is distinct from both; that is possible, as was mentioned regarding fatherhood and sonship.
Likewise, one who wills one of two opposites on condition (9) that he does not will the contrary—