What is Natural Law in Catholic moral theology?

Natural Law is the Catholic teaching that God has inscribed a moral law in the heart of every human being, accessible through reason alone, apart from divine revelation. St. Paul describes it: 'When Gentiles who do not have the law by nature observe the prescriptions of the law, they are a law for themselves. They show that the demands of the law are written in their hearts' (Romans 2:14-15). St. Thomas Aquinas provided the most comprehensive treatment of Natural Law, teaching that it is humanity's participation in God's eternal law. Its most basic principle is 'do good and avoid evil.' From this flow precepts such as the preservation of life, the education of children, the pursuit of truth, and the requirements of justice. The Natural Law is: (1) Universal — it applies to all people in all times and cultures. (2) Immutable — it does not change with circumstances or popular opinion. (3) Knowable — it can be grasped by human reason, though sin can cloud moral perception. The Catechism teaches: 'The natural law, present in the heart of each man and established by reason, is universal in its precepts and its authority extends to all men' (CCC 1956). Natural Law provides the common moral ground for dialogue with non-Catholics and non-believers on ethical issues.