Front Page Titles (by Subject) Aquinas and Natural Law - Literature of Liberty, April/June 1979, vol. 2, No. 2
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Aquinas and Natural Law - Leonard P. Liggio, Literature of Liberty, April/June 1979, vol. 2, No. 2 
Literature of Liberty: A Review of Contemporary Liberal Thought was published first by the Cato Institute (1978-1979) and later by the Institute for Humane Studies (1980-1982) under the editorial direction of Leonard P. Liggio.
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Aquinas and Natural Law
“The Meaning and Nature of the Natural Law in Thomas Aquinas.” American Journal of Jurisprudence 22 (1977): 168–189.
St. Thomas Aquinas characterized the natural law as the work of human intelligence. Yet Aquinas also makes use of Ulpian's definition of natural law as that which nature has taught all animals. May argues that Aquinas's general teaching concerning the natural law can be reconciled with Ulpian's definition.
Aquinas describes law as “a rule and measure of actions whereby one is induced to act or is restrained from acting,” brought into being by reason. Consequently, law as such, both pertains to reason and consists of a set of true propositions articulated by reason. Since the universe is under the governance of God and His reason, Aquinas speaks of the eternal law as the intelligent grasp “of the governance of things that exist in God as the ruler of the universe.” The eternal law not only governs the entire created universe, but has as its object the common good of the universe.
Every created reality in the universe, says Aquinas, participates in the eternal law in accordance with its nature. Nonrational entities participate in the eternal law passively: “they have inclinations toward their proper acts and ends.” In contrast, rational beings actively participate in the eternal law through reason, and this constitutes the natural law. The natural law is thus a body of precepts or propositions articulated by the human mind. What is ‘innate’ in natural law is the capacity of creatures to discern the body of true propositions concerning the meaning of human actions.
If natural law as law is necessarily related to rational cognition, how can Aquinas make use of Ulpian's definition of natural law as that which nature has taught to all animals? May's answer is that Aquinas “accepts Ulpian's definition of the natural law only in the sense that it refers to the subject matter” of the natural law. “The tendencies that human beings share with animals are the fitting objects of the natural law taken in its proper sense as an achievement of reason.”
Why Aquinas incorporated Ulpian's definition into his writing concerns Aquinas's conception of man. For Aquinas, humans are animals, albeit very special animals. In virtue of his animality, man shares those tendencies and goods common to all animals. Man also possesses certain natural inclinations in virtue of his rationality. Through his reason, man can come to an explicit consciousness of his natural tendencies and thus can express them as law.