Natural law, according to Thomas Aquinas, is the part of eternal law (God’s rational plan for the universe) that is accessible to human reason. It provides a moral framework grounded in human nature and rationality, by which people can discern right from wrong, even without revelation or scripture.

Aquinas’s Definition of Natural Law
In his Summa Theologiae (especially I–II, q. 94), Aquinas defines natural law as:

“The rational creature’s participation in the eternal law.”

This means:

God’s eternal law governs everything.

Humans, as rational beings, can participate in that law by using their reason to understand moral truths.

Thus, natural law is written into human nature and discoverable by human intellect.

Key Features of Natural Law in Aquinas’s Thought
1. Rooted in Reason
Moral principles are not arbitrary; they are rationally knowable.

Every person, even without revelation, has access to basic moral knowledge.

Example: Knowing that murder or theft is wrong doesn't require divine instruction—it’s contrary to human flourishing and reason.

2. Grounded in Human Nature
Aquinas believes natural law is based on the inclinations of human nature:

To preserve life

To reproduce and educate offspring

To seek truth (especially about God)

To live in society

To shun ignorance and do good

These inclinations point toward moral goods and duties.

3. Universality and Immutability
Natural law applies to all humans, across all cultures and times.

Its core principles don’t change, though their application may vary depending on circumstances.

4. Hierarchy of Laws (Lex)
Aquinas outlines a hierarchy:

Eternal Law (God’s overall plan)

Divine Law (revealed law—Scripture, Church teaching)

Natural Law (moral law known by reason)

Human Law (civil law—should be based on natural law)

Just laws in human societies are valid only if they conform to natural law.

Example: Theft and Natural Law
Aquinas says stealing is naturally wrong, because it contradicts the social and rational order. However, in extreme cases, like saving a life, the application of the principle might shift, showing that natural law allows for moral reasoning, not rigid rules.

Summary
Concept Explanation
Definition Natural law is the rational participation in God's eternal law.
Source Human nature and reason
Purpose To guide moral behavior toward the good
Universality Applies to all people at all times
Connection to God Though based in reason, it reflects divine order

In short, natural law for Aquinas is how reason allows humans to discern objective moral truths rooted in their nature—serving as a bridge between divine law and human ethics.

Would you like a comparison between Aquinas’s natural law and modern secular or Enlightenment views of natural law?

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What is natural law according to Aquinas?