Aquinas Quote on Natural Law: The Eternal Law

Historical Context

Thomas Aquinas composed the Summa Theologica between 1265 and 1273 as a systematic theological synthesis designed for beginners in the study of Christian doctrine. The quote appears in the Prima Secundae (I-II), within the celebrated Treatise on Law (Questions 90-108), where Aquinas asks whether there is a natural law distinct from human law and divine law. Question 91, Article 2, addresses the specific issue of whether natural law is a habit or an act, and in answering Aquinas articulates the definition that would shape Western legal and moral thought for centuries.

Aquinas wrote within the scholastic tradition, synthesizing Christian theology with the recently recovered works of Aristotle. His theory of law draws directly on Aristotle's teleology — the idea that everything has a natural end or purpose — but elevates it by situating all law within a divine providential order. The eternal law (lex aeterna) is God's rational governance of the entire universe; the natural law (lex naturalis) is the portion of that eternal law that rational creatures can grasp through their natural faculties, without the aid of special revelation.

Meaning

The quote defines natural law as participation — not identification, not derivation, but participation. This word is carefully chosen. Human beings do not possess the eternal law in its fullness, as God does; rather, they participate in it by virtue of their rational nature. The eternal law is the comprehensive divine plan governing all creation; natural law is the reflection of that plan accessible to human reason. Because humans are rational creatures, they can discern the basic principles of good and evil through reflection on their own nature and ends. The first precept of natural law, Aquinas argues, is that "good is to be done and pursued, and evil is to be avoided." From this primary precept flow more specific obligations: preserve life, procreate and educate offspring, seek truth about God, and live in civil society. These precepts are not arbitrary divine commands imposed from without but rational insights into the natural order of things.

Philosophical Interpretation

Aquinas's definition proved foundational for the entire natural law tradition. The key philosophical insight is the harmonization of divine command with natural reason: moral obligation is neither reducible to divine fiat (voluntarism) nor independent of God (secular rationalism), but consists in rational participation in the divine order. This middle path allowed later thinkers to secularize natural law without entirely abandoning its theological roots.

The tradition runs from Aquinas through the Spanish scholastics of the sixteenth century (Francisco de Vitoria, Francisco Suárez) to John Locke, whose appeal to "the law of nature" in the Second Treatise of Government draws directly on Thomistic sources. Locke's natural rights to life, liberty, and property are secularized natural law precepts, and through Locke the lineage extends to the American Declaration of Independence and modern international human rights law. The notion that there are moral truths accessible to all rational beings, regardless of religious affiliation, remains one of the most consequential philosophical legacies of the scholastic tradition.

The interpretation is not without controversy. Philosophers debate whether natural law survives the loss of its theological foundation, whether Aquinas's precepts are genuinely discoverable by reason or are retrojections of cultural norms, and how natural law accommodates moral disagreement. Yet the enduring power of the quote lies in its elegant reconciliation of faith and reason, showing that for Aquinas the moral order is neither imposed by an arbitrary deity nor invented by human convention but discovered through the rational examination of our place in the cosmos.

Relations

• Thinkers: Thomas Aquinas, Aristotle
• Books: Summa Theologica
• Philosophies: Virtue Ethics, Epistemology
• Wisdom: Ethics, Justice, Reason, Faith, Truth
• Topics: Good Life, Philosophy of Life
• Collections: Ancient Wisdom

Sources

• Stanford Encyclopedia of Philosophy, "Aquinas" — https://plato.stanford.edu/entries/aquinas/
• Internet Encyclopedia of Philosophy, "Thomas Aquinas" — https://iep.utm.edu/aquinas/