Once this is clear, we say:

If all existing things were contingent and dependent upon an agent other than themselves, then, if that other were itself the agent of its own agent, each would be the agent producing the other. This is circular dependence involving causal priority, which all rational people agree is impossible. If, however, that other were yet another entity, this would entail agents and things produced by them extending without end.

Or, if you prefer, you may say: this would entail an endless succession of agents exerting an effect, each exerting an effect upon another. Or you may say: this would entail an endless succession of causes, each being the effect of another.

Each of these is contingent in existence, dependent upon another, and does not exist independently.

Here two questions arise.

The first is the objection: Why could the whole not be necessary in itself, even though each of its individual members is contingent in itself?

The reply has been that this entails affirming a being whose existence is necessary in itself. Moreover, the claim is also false, because the whole consists of the assembled parts together with the configuration of their assembly. Each part is contingent in itself, and the configuration of their assembly is an accident that does not subsist independently, so it is