Thomas Aquinas, Compendium of Theology

The contingency of effects or of causes cannot upset the certainty of divine providence. Three things seem to guarantee the certainty of providence: the infallibility of divine foreknowledge, the efficaciousness of the divine will, and the wisdom of the divine management, which discovers adequate ways of procuring an effect. None of these factors is opposed to the contingency of things.

God's infallible knowledge embraces even contingent futures, inasmuch as God beholds in His eternity future events as actually existing. But we dealt with this question above.

Moreover, God's will, since it is the universal cause of things, decides not only that something will come to pass, but that it will come about in this or that manner. The efficaciousness of the divine will demands not only that what God wishes will happen, but that it will happen in the way He wishes. But He wills that some things should happen necessarily and that other things should happen contingently; both are required for the perfection of the universe. That events may occur in both ways, He applies necessary causes to some things and contingent causes to others. In this manner, with some things happening necessarily and other things happening contingently, the divine will is efficaciously carried out.

Furthermore, it is clear that the certainty of providence is safeguarded by the wisdom of the divine dispensation, without prejudice to the contingency of things. Even the providence exercised by man can enable him so to bolster up a cause which can fail to produce an effect that, in some cases, the effect will inevitably follow. We find that a physician acts thus in exercising his healing art, as also does the vine-dresser who employs the proper remedy against barrenness in his vines. Much more, then, does the wisdom of the divine economy bring it about that, although contingent causes left to themselves can fail to produce an effect, the effect will inevitably follow when certain supplementary measures are employed; nor does this do away with the contingency of the effect. Evidently, therefore, contingency in things does not exclude the certainty of divine providence.