I have posted a new explanation and analysis of the Second Way for proving the existence of God from Summa Theologiae Ia, q. 2, a. 3, which Saint Thomas Aquinas rather cryptically says is based on the nature (ratio) of efficient causality.
The greatest obstacle to understanding his Second Way, it has seemed to me, is determining precisely what Aquinas means by “the nature of the efficient cause” and “an order of efficient causes,” and how the Second Way is distinct from the First and Third Ways. For all three make use of efficient causes: motion, i.e., accidental change, in the case of the First, or generation and corruption, i.e., substantial change, in the Third. It seems unnecessary for Aquinas to use efficient causality in a general sense as the basis of a proof distinct from these two species of it.
In my treatment of the Five Ways in Context, I claimed that each way embodies and employs a distinct perspective on the physical world of which Aquinas pursues an ultimate explanation in God. It therefore seems unlikely that Aquinas would produce an argument for the existence of God that focuses on the genus of causality that is operative in two of his other arguments.
Nor did it seem to me that the efficient causality that he has in mind is the coming to be of substances, even though this is probably the most popular way of reading “In the world of sense, we find there is an order of efficient causes.“
For reasons I explain in the essay, I conclude that Aquinas is basing his argument from efficient causality on a third type of ‘change’ or ‘alteration,’ what he elsewhere calls immanent activity or operations of cognition and volition. Please view this exploration of an alternate reading of the Second Way here, and please share or comment.