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Upon this question, there has been much ink spilled in the controversies between the schools. Here, we will restrict ourselves to the scope of the passage at hand.
St. Thomas begins by distinguishing between those things that happen necessarily and those things that happen contingently. For example, that the rock fall when dropped is something that happens "necessarily," i.e., naturally. On the other hand, some things happen contingently, e.g., the will of man.
According to the first view he brought up (the view of the Averroists), this distinction is purely found in secondary causes. It is as if rain equally fell on all and some grew and some did not due to disposition of the plants to grow. This cannot be the case, for, it presupposes the Averroistic denial of Divine freedom in favor for some sort of necessary "emanation" from God.