This is St Thomas's way of saying that God governs according to general laws of nature and thought. -- The following argument may be added from Sum. Theol., 1a, q. 22, art. 2: "Since every agent acts for an end, the direction of effects to an end on the part of the prime agent extends wide as His causality extends. Whenever in the workings of an active cause anything occurs that is not directed to an end, it is because that effect ensues upon the working of some other cause beside the intention of the original agent. But the causality of God, the prime agent, extends to all beings: . . . hence all things, whatsoever in any way have being, are ordained by God to an end."
Of God and His Creatures: 3.64