Much Catholic teaching on natural law refers to the work of St. Thomas Aquinas, and Vatican II commends him as a guide. We can therefore examine his treatment of natural law to see how the Church views the subject.
“Law” for St. Thomas primarily means a reasonable plan of action. He begins from what he calls the “eternal law”—God’s plan in creating and redeeming. Any other reasonable plan of action must somehow derive from it. People can plan their lives reasonably only because, in one way or another, they share in the universal plan present in God’s law; to the extent they try to follow some other plan, their lives are unrealistic.
Human beings, according to St. Thomas, are naturally disposed to understand some basic practical principles. These are the primary principles of natural law—what St. Paul calls the law written in one’s heart (see Rom 3.14–16). However, truths of natural law, including specific norms, are also part of revelation. As Vatican I says, this is 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). Thus natural law and divine law can be distinguished, but not separated and opposed.
Practical thinking is reasoning and judging about what might and ought to be. According to St. Thomas, its first principle is: The good is to be done and pursued; the bad is to be avoided. Scholastic natural-law theory formulated this as a moral imperative: Do good and avoid evil. But, as Thomas’ formulation shows, the first principle is not a moral norm. It expresses the intrinsic, necessary connection between human goods and actions which bear upon them; in thinking about what one might do, it is impossible to disregard entirely the goods and bads involved.
People grasp as goods all the fulfillments to which they are naturally inclined. Corresponding to each natural inclination, therefore, is a basic precept of natural law: Such and such a basic human good is to be done and/or pursued, protected, and promoted. These are general determinations of the first principle. In their light we can see why human nature and natural law morality are both stable and changing: stable because they are based on fundamental, unalterable natural inclinations; changing because these potentialities are open to continuing, expanding fulfillment.
But such principles of practical reasoning do not tell us what is morally good. Moral norms are needed to guide choices toward the fulfillment of persons in relation to human goods.
The first principle of morality might best be formulated as follows: In voluntarily acting for human goods and avoiding what is opposed to them, one ought to choose and otherwise will those and only those possibilities whose willing is compatible with a will toward integral human fulfillment. Note that this conceives human goods not simply as diverse fields for possible action, but as together comprising the totality of integral human fulfillment. This avoids subordinating moral reflection to specific objectives; instead, the upright person is to remain open to goods which go beyond his or her present capacity for realizing them in action.
There is also need for intermediate principles of morality, midway between the first principle and the completely specific norms which direct particular choices. These are the modes of responsibility. An example is the principle of fairness or impartiality expressed by the Golden Rule. These modes exclude choices involving various unreasonable (immoral) relationships of willing to the human goods. People who are thinking in a morally sound way commonly take them for granted. But for the most part they have not been systematically discussed up to now, though at one time or another almost every one of them has been mistaken for the first principle of morality.
Scripture and Christian moral teaching do not speak of modes of responsibility but of virtues. However, the virtues do not constitute moral norms distinct from the modes; rather, virtues embody modes. For virtues are aspects of a personality integrated around good commitments, and the latter are choices in accord with the first principle of morality and the modes of responsibility.