The law of God that can be known naturally (i.e., without supernatural revelation) by a reasonable person. The obligations imposed on one by God are made known either by the natural light of reason or by divine revelation. The former obligations constitute the natural law; the latter obligations the positive law. Both laws are divine (i.e., come from God) and cannot contradict each other.

This provides a premise for rejecting certain conclusions which today are often drawn--mistakenly--from the fact that natural law is naturally knowable. For instance, some theologians claim that the Church cannot authoritatively teach specific moral norms pertaining to natural law, but can only endorse norms agreed upon by people of good will, Christians and others. Indeed, some theologians hold that the authority of any specific moral norm received and handed on in the Church can be no greater than the rational arguments which can be offered to support it. Thus they conclude that while the Church can firmly teach the very general principles of morality with which no believer disagrees and may tentatively commend specific norms supported by consensus and rational arguments, the Church cannot authoritatively teach specific moral norms pertaining to natural law in such a way that the faithful ought to accept them as moral truths even if consensus and cogent rational arguments are lacking.

The teaching of Vatican I concerning the help given to fallen humankind by revelation explains why many people of good will do not see the truth of moral norms of natural law. The Church's teaching, based on divine revelation, gives us a motive to accept as true those norms of Christian morality for which we may not have completely satisfying arguments. Moreover, as part of her divine mission the Church calls attention by her sacred and certain teaching to the principles of the moral order, which we can ignore or badly articulate despite knowing them naturally (see DH 14; GS 89). (The issues concerning theological dissent from the Church's moral teaching will be examined in chapter thirty-six.)

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.

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).

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).

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). 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.

Much Catholic teaching on natural law refers to the work of St. Thomas Aquinas (see DH 3, note 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.

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. 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).

The argument that revelation contains no specific moral norms but only very general principles is at odds with the entire history of Christian moral teaching. Christians always have handed on specific norms as revealed truths and offered scriptural warrants for them. For example, the Council of Trent cites St. Paul to show that several kinds of acts other than sins against faith itself are mortal sins (see DS 1544/808). Similarly, Vatican II proposes specific norms concerning forgiving injuries and loving enemies as the teaching of Christ, citing Matthew's account of the Sermon on the Mount (see GS 28).

Moreover, both Trent and Vatican II teach that the gospel which Jesus proclaimed and commissioned the apostles to spread is the "source of all saving truth and moral teaching" (DS 1501/783; DV 7; note that the "omnis" comes before the first "et"). How the gospel contains all moral teaching will be made clear in chapter twenty-six. The key point is that God reveals in Jesus how men and women in a sinful world should respond to his love and live good human lives which necessarily will be redemptive like the life of Jesus himself.

Thus Vatican II teaches that Christ "fully reveals man to man himself and makes his supreme calling clear" (GS 22). "Whoever follows after Christ, the perfect man, becomes himself more of a man" (GS 41). By Christian holiness, "a more human way of life is promoted even in this earthly society" (LG 40).

Vatican I teaches that God, although he can be known naturally as creation's source and goal, still chooses to reveal himself and his decrees, in part so that "even in the present condition of the human race, those religious truths which are by their nature accessible to human reason can readily be known by all men with solid certitude and with no trace of error" (DS 3005/1786; translation supplied). Otherwise, our wounded nature would prevent our knowing certainly and accurately some important truths concerning human fulfillment, even though in principle these truths are naturally knowable.

In this matter Vatican I adopts the position of St. Thomas (S.t.,1, q. 1, a. 1; 1-2, q. 98, a. 5; S.c.g., 1, 4). Thomas expands on the general position with respect to the particular question: Can the natural law be wiped from the human heart? His answer is that the most common principles in themselves cannot be ignored, but their application in the concrete can be ignored due to unruly passions; and norms which must be derived from the most general principles by any sort of reasoning can be ignored due to bad customs and corrupt habits (S.t., 1-2, q. 93, a. 6; cf. q. 77, a. 2; q. 85, a. 3).

Pius XII refers to Vatican I and expands upon it, in line with the teaching of St. Thomas: ". . . though, absolutely speaking, human reason by its own natural force and light can arrive at a true and certain knowledge of the one personal God, Who by His providence watches over and governs the world, and also the natural law, which the Creator has written in our hearts, still there are not a few obstacles to prevent reason from making efficient and fruitful use of its natural ability. . . .

It is for this reason that divine revelation must be considered morally necessary so that those religious and moral truths which are not of their nature beyond the reach of reason in the present condition of the human race, may be known with a firm certainty and with freedom from all error.

Thus the Church teaches that truths of natural law are included in revelation. It follows that they belong to the proper sphere of the Church's authority to teach, and that if one does not find cogent arguments for them, they must be accepted on faith. One cannot expect people who refuse to accept such moral norms on faith to see all of them to be true.

In the place where St. Paul alludes to natural law, he also points out that Gentiles in fulfilling it were fulfilling the law--that is, the requirements of the covenant (see Rom 2.14-15). Obviously, he does not mean that the Gentiles could know and keep the precepts peculiar to Mosaic law, which are abolished in Christianity. Rather, he means that moral content common to the Old and the New Testaments--such as the Ten Commandments and their foundation in the law of love--constitutes natural law. The Decretum of Gratian, compiled in the middle of the twelfth century and authoritative as canonical law until 1917, begins with the famous definition: "Natural law is what is contained in the law and the gospel"--that is, in both the Old and New Testaments.

Hence, St. Irenaeus, St. Thomas Aquinas, and the whole Catholic tradition consider the Ten Commandments to pertain to natural law and, at the same time, to divine revelation. St. Thomas maintains that all moral precepts of the old covenant are included, in one way or another, in natural law (see S.t., 1-2, q. 100, a. 1). He also holds that the law of Jesus, in its moral aspects, is an expression of the requirements of human virtue which pertain to natural law (see S.t., 1-2, q. 108, aa. 1-3).

Vatican II's understanding of the relationship between divine and natural law is exemplified in its teaching on birth regulation. According to the Council, the Church authoritatively interprets divine law in the light of the gospel to make clear objective standards based on the nature of human persons and their acts (see GS 50-51). Again, the Council teaches that the Church contributes to international peace and friendship by imparting knowledge of divine and natural law, and that this work belongs to the Church's divine mission of preaching the gospel and dispensing the treasures of grace (see GS 89, Latin text). This teaching would be unintelligible if, as some wish, divine law and natural law were separated, with the Church's teaching authority limited to the former while all specific moral issues were consigned to independent rational reflection on experience.

It follows that natural law and divine law can be distinguished from each other and even contrasted, as they are in many documents of the Church, without being separated and opposed to each other. In the actual order of things natural law does not stand apart from the law of Christ. The dictates of natural law and the truth of divine revelation are two agreeing streams from the same divine font; the Church is the guardian of the single supernatural Christian order, in which nature and grace converge. In this Christian order, natural law is restored, completed, and elevated, so that it now serves to direct humankind to heavenly as well as earthly fulfillment. A real Christian life is a humanly good life--the only completely fulfilling life for persons who, called to share in divine life, have fallen and been redeemed.

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).

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).