(CG 3/71). That the Divine Providence is not wholly inconsistent with the p... (Book 3. God, the End of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

AS divine providence does not exclude all evil from creation, neither does it exclude contingency, or impose necessity upon all things. The operation of providence does not exclude secondary causes, but is fulfilled by them, inasmuch as they act in the power of God. Now effects are called 'necessary' or 'contingent' according to their proximate causes, not according to their remote causes. Since then among proximate causes there are many that may fail, not all effects subject to providence will be necessary, but many will be contingent.

6. On the part of divine providence no hindrance will be put to the failure of the power of created things, or to an obstacle arising through the resistance of something coming in the way. But from such failure and such resistance the contingency occurs of a natural cause not always acting in the same way, but sometimes failing to do what it is naturally competent to do; and so natural effects do not come about of necessity.