Natural law, or the law of nature, is a set of rules for human actions, usually posited as having a divine source. As a universal moral and political code, natural law was first conceptualized by stoic philosophers, who believed that natural law was part of the fundamental structure of the universe. Some early thinkers believed that natural law applied to animals as well as humans.

Christian theorists later took up the idea of natural law as self-evident principles of human behavior that could be known only by rational beings. Thomas Aquinas (c. 1225–1274) thought that human reason could reveal God’s intentions for how we ought always to conduct ourselves so as to preserve the common good, or the good of the community. Following natural law is an important part of obedience to God. The particular laws of nations and peoples might differ, but the basic principles of natural law are universal.