The master principle of natural law, wrote Aquinas, was that "good is to be done and pursued and evil avoided." Aquinas stated that reason reveals particular natural laws that are good for humans such as self-preservation, marriage and family, and the desire to know God.

What does Aquinas say about evil?

From a metaphysical perspective, Aquinas asserts that the world is better for having evil within it, because evil serves a greater good. Natural evil contributes to the goodness of creation, and God sometimes inflicts evil as punishment in order to maintain the just order of the universe.

What does it mean to do good and avoid evil?

It is the idea of what should be done to insure the well ordered functioning of whatever community the ruler has care for. “The good is to be done and pursued and evil is to be avoided” is not very helpful for making actual choices.