Aquinas distinguishes four kinds of law. Eternal law is the rational governance of the entire universe in God's mind. Natural law is the human creature's rational participation in eternal law — our ability to discern, by reason alone, the basic goods that constitute human flourishing. Human law applies natural law to particular circumstances of community life. Divine law, revealed in scripture, guides humanity toward its supernatural end, correcting reason where it fails and illuminating what reason alone cannot reach.
The first principle of natural law — 'good is to be done and pursued, and evil avoided' — is self-evident, analogous to the logical principle of non-contradiction in theoretical reason. From this foundational principle, further precepts follow according to the natural inclinations common to all human beings: the preservation of life, the generation and education of offspring, the pursuit of truth, and living in society. These inclinations are not mere instincts; they are rational goods that define the content of human nature and from which all particular moral norms derive.
Natural law theory makes a distinctive claim: moral truth is not a product of will — not God's command nor human convention — but of rational insight into what it is for a human being to flourish. Because all humans share the same rational nature, the primary precepts of natural law are universally binding and in principle universally accessible. Secondary precepts, derived by reasoning from primary ones, can be obscured by passion, custom, or bad habit — which is why positive law and education are necessary supplements to natural insight.
The fullest treatment of natural law appears in the Prima Secundae, Questions 90–97 of the Summa Theologiae. Aquinas's synthesis draws on Cicero, Roman law, and Aristotle's Nicomachean Ethics alongside Augustine.
