1. According to St. Paul, even the Gentiles find the requirements of morality which conscience discerns written in their hearts. Although Gentiles do not have the law divinely revealed to the Jews, they naturally do have this given standard of conduct (see Rom 2.14–16).1
2. The Church calls these naturally known principles “natural law.” They are natural in the sense that they are not humanly enacted but are objective principles which originate in human nature (see GS 16; DH 14).2 Thus, in speaking of natural law, we are not contrasting “natural” meaning “physical” with “intellectual.” As a matter of fact, natural law does pertain to intellect. Nor is the contrast primarily that of “natural” with “supernatural,” for natural law overlaps divinely revealed law.
3. Much Catholic teaching on natural law refers to the work of St. Thomas Aquinas (see DH 3, note 3).3 Vatican II continues to commend him as a guide for theologians (see OT 16). It is therefore legitimate to examine his treatment of natural law, in order to find out how the Church views this subject.
4. St. Thomas begins from what he calls the “eternal law” (see S.t., 1–2, q. 91, a. 1; q. 93, a. 1). This simply is God’s plan, according to which he carries out his whole work of creating and redeeming. In current English, “law” refers mainly to rules and commands of people in authority; God’s plan of action is not a law in this sense.4 But Thomas thinks of law primarily as a reasonable plan of action (see S.t., 1–2, q. 90, aa. 1, 3; S.c.g., 3, 114). Since God knows well what he is doing, he must be acting according to a law. And since no one forms God’s plan of action for him, the law of his creative and redemptive work must be his own wisdom, by which he directs everything to the fulfillment he has in mind (see S.t., 1, q. 21, a. 1; q. 22, a. 1; 1–2, q. 90; q. 91, a. 1; q. 92, a. 1).
5. Since eternal law embraces the whole of creation, any other law—any other reasonable plan of action—must somehow derive from it (see S.t., 1–2, q. 19, a. 4; q. 71, a. 6; q. 91, aa. 1–2; q. 93, a. 3). People can plan their lives reasonably only because, in one way or another, they share in the universal plan perfectly present in God’s eternal law (see S.t., 1–2, q. 91, a. 2; S.c.g., 3, 113). If they try to follow a plan not somehow derived from eternal law, their lives will be unrealistic, as would be the behavior of workers on a large project who departed from the project’s master plan in order to follow some other plan.
6. According to Thomas, people are naturally disposed to understand some basic practical principles. He calls these the “primary principles of natural law.” Since everyone knows them naturally, no one can make a mistake about them. They are the law written in one’s heart of which St. Paul speaks—the law whose voice is conscience, according to Vatican II. Interpreting the text he had of Psalm 4 (“Who will show us any good? Lift thou up the light of thy countenance upon us, O Lord”), Thomas says natural law is a “light of reason which is in us, inasmuch as it can show us goods and direct our will, because it is the light of God’s countenance—that is, a light which derives from his countenance” (S.t., 1–2, q. 19, a. 4; cf. q. 91, a. 2; q. 94, a. 2; 2–2, q. 47, a. 6).
Vatican II teaches that human persons find in their conscience a law they do not impose on themselves which demands their obedience: “For man has in his heart a law written by God” (GS 16; see 3‑B). This law not only calls the person to do good and avoid evil, but it also when necessary speaks “to his heart more specifically: do this, shun that” (GS 16). The Council makes its own the explanation of St. Thomas, that this natural law is the human participation in the eternal law: “. . . the highest norm of human life is the divine law—eternal, objective, and universal—whereby God orders, directs, and governs the whole world and the ways of the human community according to the plan of his wisdom and love. God makes man a sharer in this his law, so that, by divine providence’s sweet disposing, man can recognize more and more the unchanging truth” (DH 3; translation supplied).
In the context, “unchanging truth” has a double reference. It refers back to the previous section of this document, where the Council lays the basis of its declaration on religious liberty by emphasizing the moral duty, imposed by human nature, to seek religious truth and to live by it (see DH 2). But “unchanging truth” also refers to the eternal law itself, as is clear from one of the texts of St. Thomas to which Vatican II refers (a reference omitted in the Abbott edition): “The eternal law is unchanging truth . . . and everyone somehow learns the truth, at least the general principles of the natural law, even though in other matters some people share more and some less in the knowledge of the truth” (S.t., 1–2, q. 93, a. 2).5
The Council makes it clear that this unchanging truth serves as the principle for judgments of conscience: “On his part, man perceives and acknowledges the imperatives of the divine law through the mediation of conscience. In all his activity a man is bound to follow his conscience faithfully, in order that he may come to God, for whom he was created” (DH 3). Consciences must be free of any coercive imposition by public authority; in their liberty they are supported by the Church’s teaching: “The Church is, by the will of Christ, the teacher of the truth. It is her duty to give utterance to, and authoritatively to teach, that Truth which is Christ himself, and also to declare and confirm by her authority those principles of the moral order which have their origin in human nature itself” (DH 14).
That the principles of natural law to which Vatican II refers are not only general ones is clear by its teaching on specific points, such as genocide: “Contemplating this melancholy state of humanity, the Council wishes to recall first of all the permanent binding force of universal natural law and its all-embracing principles. Man’s conscience itself gives ever more emphatic voice to these principles. Therefore, actions which deliberately conflict with these same principles, as well as orders commanding such actions, are criminal. Blind obedience cannot excuse those who yield to them. Among such must first be counted those actions designed for the methodical extermination of an entire people, nation, or ethnic minority” (GS 79).
In teaching like this, the Church carries out her divine mission of service: “In pursuit of her divine mission, the Church preaches the gospel to all men and dispenses the treasures of grace. Thus, by imparting knowledge of the divine and natural law, she everywhere contributes to strengthening peace and to placing brotherly relations between individuals and peoples on solid ground” (GS 89).
Although we are naturally disposed to know basic practical principles and can make no mistake about them, they are not by themselves sufficient for the judgment of conscience which we must make. Our ultimate end is to share in fulfillment in the Lord Jesus, and we do not judge rightly what to do unless we judge in light of this end. So we must supplement natural law with faith, by this means drawing on the eternal law in a way that goes beyond reason (see DS 3005/1786; S.t., 1–2, q. 91, a. 4; q. 98, a. 1; q. 106, aa. 1–2).
Moreover, we must bring both the basic practical principles of natural law and the way of Jesus which faith teaches to bear upon the particular possibilities we consider in our deliberation. This application of principles to possibilities under consideration is a process of reasoning, and only at the end of it do we reach our best and last judgment as to what choice we should make. So the judgment of conscience must be reached by conscientious reflection on the possibilities before us in the light of the moral standards we have from natural law and faith (see S.t., 1, q. 79, a. 13; 1–2, q. 94, a. 2; 2–2, q. 47, a. 6).
1. See C. Spicq, O.P., Théologie Morale du Nouveau Testament, 14th ed., vol. 1 (Paris: J. Gabalda, 1970), 394–406.
2. See also John XXIII, Pacem in terris, 55 AAS (1963) 258–59; The Papal Encyclicals, 270.4–9, for the contrast between natural law—an order inscribed by God in human nature and known by conscience—and the laws of lower nature. This moral order is prior to positive law and it sets the standard for all human interpersonal relationships. The encyclical goes on to develop in detail a catalogue of the rights and duties which flow directly from human nature.
3. In his encyclical, Libertas, 20 ASS (1888) 594–97; The Papal Encyclicals, 103.4–8, Leo XIII, who so highly commends the teaching of Thomas in general, closely follows his treatment of natural law and its relationship to free choice. A useful introduction to St. Thomas’ natural-law theory, with good bibliographies: Jean-Marie Aubert, Loi de Dieu, Lois des Hommes (Tournai: Desclée, 1964), 31–116.
4. St. Thomas Aquinas, Summa theologiae, 1–2, q. 96, a. 4, makes it clear that much we call “law” is not law at all for him; he considers rules and commands laws only if they conform to morality, and so to God’s law.
5. See also John XXIII, Pacem in terris, 55 AAS (1963) 266–67; The Papal Encyclicals, 270.37–38, where the universal, absolute, and immutable principles of natural law are identified with the unchanging truth which is the foundation of the moral order, and this truth is grounded in the reason of the true, personal, and transcendent God. A recent statement of moral objectivity with reference to current confusions about conscience: “Conscience et morale: Déclaration doctrinale de la Conférence épiscopale irlandaise,” Documentation catholique, 78 (1981), 31–40. A systematic theological treatment of this point: Ramón García de Haro, La conciencia moral, 2d ed. rev. (Madrid: Rialp, 1978).