How can a moral principle become a specific law according to Thomas Aquinas?

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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.

According to St. Thomas Aquinas, valid positive law must be derived from objective moral principles to serve the common good. He identified two specific ways this derivation occurs:1

Immediate Deduction: A legal rule is formulated through a direct correspondence with a moral principle. For instance, the legal prohibition of homicide is directly derived from the moral principle that murder is wrong.1
Determination (determinatio): This is an indirect process where a general moral principle is specified to facilitate human coordination in particular circumstances. While morality may not dictate a specific choice, a legitimate authority determines a rule to ensure safety or order. For example, traffic laws requiring drivers to stay on a specific side of the road are binding because they fulfill the moral requirement to avoid harming others and facilitate commerce.1

Aquinas maintained that if a law is not derived from moral principles in one of these ways, it is considered "unjust" and lacks binding authority.1