Thomas Aquinas is one of the most influential philosophers in Western thought. He made substantial contributions to the natural law tradition in ethics through his synthesis of ancient Greco-Roman philosophy with his Christian worldview. In this article, Thomas D’Andrea sets forth the content and structure of Aquinas’s ethics.

For Aquinas, the universe is to be understood as God’s creation, directed by God’s eternal law toward perfection for God’s glory. The natural law is simply the eternal law as planted in the very design or nature of any particular creature, manifested through inclinations toward what is good. Non-rational creatures strive toward their perfection automatically. Creatures with intellect and will such as humans achieve their perfection through free, informed cooperation with God's plan for them; they therefore need knowledge of the natural law if they are to direct their actions well.

All humans with a properly functioning intellect and a minimum of experience can know the most fundamental precepts of the natural law without the aid of a special communication (“revelation”) from God. But because individuals cannot immediately grasp all of the historical details of God’s designs for them, human communities create supplemental laws to help guide their members to fulfillment. Such man-made laws are truly laws only if they agree with the precepts of the natural law.