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In page Natural law:

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The natural law consists, for the Catholic Church, of one universal principle from which are derived all natural moral obligations or duties.[citation needed] Thomas Aquinas resumes the various ideas of Catholic moral thinkers about what this principle is: since good is what primarily falls under the apprehension of the practical reason, the supreme principle of moral action must have the good as its central idea, and therefore the supreme principle is that good is to be done and evil avoided.[2]