Summa Theologica

This is a key underlying idea of Aquinas’ thought. He expresses it in Part 1 Q 1 Article 8, while arguing that rational arguments are necessary to theology:

“Since therefore grace does not destroy nature, but perfects it, natural reason should minister to faith” (8).

Aquinas maintains that grace—God’s freely given help and favor—does not override or destroy man’s natural capacities, but fulfills their potential. Thus, man is naturally endowed with the ability to reason and reach conclusions about the world around him. Reason is in itself a good thing. But God gives us something more, namely the gift of faith, by which we are able to know truths that we never could have figured out on our own. 

The idea that grace perfects nature implies that human nature is not totally corrupted by sin. There remains something good in it, implanted by God, which he continues to work upon and perfect. Thus, the lower is brought to a higher level by the higher power. God is constantly working through his creation to bring it to greater perfection. Eventually, all things will reach him as their final end and destiny.