I say:

It is self-evident that one and the same thing produced cannot be the work of two agents, whether acting independently or cooperatively; nor can one and the same effect result from two causes, whether independent or joint. No rational person disputes this once its meaning is understood. For if one of them independently produces it, the entire product or effect must result from that one alone. If the other were assumed to do likewise, each would necessarily have produced the whole of it alone. Yet producing it alone excludes having a partner in producing it, let alone another independent agent.

This would therefore entail affirming two contradictories: affirming and denying the independence of one of them, and affirming and denying that it alone produced the effect. This is to affirm two contradictories together.

It is also self-evident that, in the very thing an agent produces, no other agent shares with it, any more than another independently produces that same thing. For if another shared in producing it, it would not be the first agent’s product; rather, only part of it would be its product, while the whole would be the product of both. Thus, there can be no sharing in what is the product of a single agent.

This is why the intelligible meaning of shared action is cooperation, with each doing something different from what the other does. For example, when two people cooperate in building, one carries the bricks and the other lays them. Or when they carry a timber, one bears one end and the other bears the other.

All created beings assist one another in their actions. No created being independently produces an effect all by itself; rather, it necessarily requires a cooperating partner who is independent of it. Moreover, besides needing that partner, it faces something that opposes it and obstructs its action. It therefore also needs something to prevent that obstructive opposition.