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How did Thomas Aquinas define law as an ordinance of reason?

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The Greek philosopher Plato — Philosophy of law Plato, marble portrait bust copied from an original of the 4th century bce; in the Capitoline Museums, Rome.

St. Thomas Aquinas defined law as an "ordinance of reason," meaning it is a prescription that is both created by lawmakers and understood by subjects through the exercise of the human capacity for reason.1

Rather than being arbitrary, law possesses a fundamental purpose: to serve the common good of a political community. In this framework, law functions as a promulgated plan of coordination that allows society to achieve essential goods that could not be realized through individual action alone.1

Aquinas maintained that for positive law to be valid, it must be derived from objective moral principles. This occurs either through direct deduction—such as prohibiting homicide based on the moral wrongness of murder—or through determinatio, where authorities specify how general moral principles apply to particular circumstances, such as establishing traffic regulations to ensure safety and order. If a law fails to derive from these moral foundations or is not motivated by the care of the community, Aquinas argued it is "unjust" and lacks the binding authority of true law.1