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How did Thomas Aquinas connect natural law to eternal law?

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Painting by Rembrandt showing Aristotle in rich dark clothing with a gold chain, resting his hand thoughtfully on a marble bust of Homer, illuminated against a dark background. — natural law *Aristotle Contemplating the Bust of Homer*, oil on canvas by Rembrandt van Rijn, 1653; in the collection of the Metropolitan Museum of Art, New York.

St. Thomas Aquinas connected natural law to eternal law by defining the former as a specific participation of the latter within rational creatures.1

While eternal law represents the divine reason of God—which remains unknowable to humans in its absolute perfection—Aquinas argued that it is partially accessible to us through both revelation and the operations of human reason. Consequently, the law of nature comprises those precepts that humans can formulate through their rational capacity, such as the preservation of one's own good, the fulfillment of natural inclinations shared with other living beings, and the pursuit of knowledge regarding God.1

In this framework, Aquinas established that human law must function as a particular application of this natural law, ensuring that positive legal systems remain grounded in objective moral truths derived from the divine order.21