Natural theology and natural law › Efficient causes and explanatory dependence

The second Way concerns efficient causes, causes that bring about effects. Aquinas denies that something can be its own efficient cause, since it would then have to be prior to itself in the relevant causal order. He argues that an ordered series requires a first cause if intermediate and ultimate effects are to occur.

He does not begin with the unrestricted premise that everything has a cause, then arbitrarily exempt God from it. The stated principle concerns the ordered efficient causes encountered in experience and their dependence. A first cause is introduced as what accounts for the derivative causal order, rather than as one more item required to have the same dependence.

Whether this succeeds remains a philosophical question. We can ask what establishes that the series has the structure assumed and why an infinite derivative order cannot explain its members. Saying that an infinite regress is forbidden without examining the kind of regress would leave the central premise undefended.

The first and second Ways overlap in their concern with dependence, but their vocabulary differs. The first develops the actualization of potentiality; the second focuses on efficient causation. Keeping those starting points distinct makes it easier to see what each inference needs.