But a 'natural cause,' or physical agent, as such (res naturalis), as distinguished from a moral agent, does always act in the same way under the same circumstances. It is the circumstances that vary, not the behaviour of the natural cause. Compare Newton's second law of motion. And so natural, or physical, events come about under an hypothetical necessity. They always happen in the same way, if the antecedents, positive and negative, are the same.

In this chapter St Thomas is concerned to obviate a difficulty unlikely to occur to modern minds, -- how it is consistent with divine providence for terrestrial events, such as the weather, the growth of the crops, the healthy development of animals, not to run in regular calculable cycles, like the ordinary celestial phenomena, sunrise and sunset, equinox and solstice, the waxing and waning of the moon. From Plato and Aristotle to Newton, celestial phenomena were 'necessary,' terrestrial 'contingent.' The real difference is one of simplicity and plurality of causes. Professor Stewart, Notes on Nicomachean Ethics, vol. II, p. 9, writes: "There is no contingency in things, but there is often failure on the part of organic beings to cope with the complexity of the necessary laws which thing obey": a remark which is true, so far as things do obey necessary laws. But there is a contingency in acts of free will, and in things so far as they are consequent upon such acts. To take another point of view. Contingency, like chance, has been predicated of co-existences, or coincidences, rather than of sequences; and necessity has been made out for sequence better than for co-existence. The study of co-existences carries us far back into the dim past, even to that 'primitive collocation of materials,' which, it is argued, must have been the work of intelligence and free will. Cf. B. I, Chap. XIII: B. III, Chap. XCIV, with notes.

Of God and His Creatures: 3.72