Natural law, in Aquinas's account, is the rational creature's participation in the eternal law of God — the fundamental moral principles discoverable by right reason that hold universally because they are grounded in human nature as created by God. His famous definition is that natural law is "the rational creature's participation in eternal law," expressed in first principles like "do good and avoid evil" and further specified in secondary precepts about murder, property, and truth-telling. This framework allowed Aquinas to engage Aristotle's political philosophy while grounding morality in God's rational ordering of creation rather than arbitrary divine command.
The Treatise on Law — the fullest and most systematic medieval account of natural law, distinguishing eternal law (God's rational ordering of all things), natural law (reason's participation in eternal law), human law (applications to specific communities), and divine law (Scripture and revelation).
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