The First Principle of Practical Reason
Many proponents and critics of Thomas Aquinas’ theory of natural law have understood it roughly as follows. The first principle of practical reason is a command: Do good and avoid evil. Man discovers this imperative in his conscience; it is like an inscription written there by the hand of God. Having become aware of this basic commandment, man consults his nature to see what is good and what is evil. He examines an action in comparison with his essence to see whether the action fits human nature or does not fit it. If the action fits, it is seen to be good; if it does not fit, it is seen to be bad. Once we know that a certain kind of action—for instance, stealing—is bad, we have two premises, “Avoid evil” and “Stealing is evil,” from whose conjunction is deduced: “Avoid stealing.” All specific commandments of natural law are derived in this way.1
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