How did Thomas Aquinas define natural law?

St. Thomas Aquinas provided a systematic treatment of natural law by defining it as the "participation of the eternal law in the rational creature." While the eternal law of divine reason exists in God’s mind and remains unknowable in its perfection, humans can access it through both revelation and the operations of their own reason.1

Aquinas maintained that natural law comprises specific precepts that humans are capable of formulating, including:1

• The preservation of one’s own good.1
• The fulfillment of inclinations shared with all animals.1
• The pursuit of the knowledge of God.1

Central to his philosophy is the thesis that valid positive law must be derived from these objective moral principles. This occurs either through immediate deduction—such as prohibiting homicide based on the moral principle that murder is wrong—or through determinatio, a process of specifying how general moral principles apply to specific circumstances to facilitate social coordination. Aquinas argued that if positive law fails to derive from these moral truths, it is "unjust" and lacks binding authority, as the ultimate purpose of law is to serve the common good.2