Typically, second causes “go round in circles”; for example, heat comes from combustion, which needs the oxygen produced by plants through photosynthesis thanks to the sun and chlorophyll, etc. The chain of second causes must lead to a first cause which is itself its own cause. This is the Greek anankè stênai: it is necessary to stop (Aristotle, Metaphysics, L. 2, II, 994b; L. 12, III, 1070a).
The doctrine of the efficacy of second causes is crucial in metaphysics, as it demonstrates that the dependence of beings on a transcendent first cause, far from depriving them of causal efficacy, on the contrary justifies and grounds it (S. Thomas Aquinas, 13th c.). On an ontological level, the efficacy of beings is manifested through the four kinds of cause: material cause, formal cause, efficient cause and final cause. On a theological level, the creature is justified in acting by reason of what God is, first cause, pure intelligence and pure will, then cause of the very causality of the second cause.