even if it is not an efficient cause of the other.

This is simultaneous, concomitant circular dependence, which is possible, unlike simultaneous circular dependence involving priority, which is impossible.

This is found in things generally: mutually inseparable things are each a condition of the other, so each of the two conditions is a condition of its own existence. At most, this amounts to a thing being a condition of its own existence—that is, it does not exist unless it exists. This is a valid statement, unlike saying that a thing brings itself into existence, which is impossible.

Fatherhood and sonship are each a condition of the other’s existence. Likewise, being above and being below are each a condition of the other’s existence. Occupying space and that which occupies space are each a condition of the other. Water and its distinctive nature are each a condition of the other, as are fire and its distinctive nature. There are many similar examples.

Thus, one attribute being conditional upon another, the essence being conditional upon its inseparable attributes, and the attributes being conditional upon the essence—such that none can be realized without the other, and the existence of each necessarily entails the existence of the other—is precisely what it means for each to be a condition of the other. Each is thus a condition of that upon which its own being and existence depend.

One of the fundamental sources of these people’s error is the ambiguity of the term “cause.” They frequently use it to mean anything without which a thing cannot exist, thereby including the efficient cause, the receptive cause, matter, and form. Often, however, they mean only the agent. This is what is understood by the term