Thomas Aquinas
The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself … Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.
Source details
1268 · Summa Theologica I, q. 2, a. 3 — the Second Way (from efficient cause)
Checked word-for-word against Summa Theologica · 2026-07-18