The order of divine providence as carried out in things is certain. This is why Boethius could say that fate is an unchangeable disposition inherent in changeable things [De consolatione philosophiae, IV, pros. 6]. But we may not conclude from this that all things happen of necessity. For effects are said to be necessary or contingent according to the condition of proximate causes. Evidently, if the first cause is necessary and the second cause is contingent, a contingent effect will follow. Thus in the case of lower bodies, the first cause of generation is the movement of a heavenly body; although this movement takes place necessarily, generation and corruption in those lower bodies occur contingently, because the lower causes are contingent and can fail. As we demonstrated above, God carries out the order of His providence through the intermediacy of lower causes. Therefore some of the effects of divine providence will be contingent, in keeping with the condition of the lower causes.
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Contingency of Some Effects, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
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