Aquinas: Obligation and Moral Law

The Philosophical System of Thomas Aquinas, by Maurice de Wulf, edited and adapted for the web by Jonathan Dolhenty

XIII. Obligation and Moral Law

A. Nature and extension of moral obligation

The study of moral obligation is one of the chief features in which the Schoolmen advance beyond the Greek philosophers, who confined themselves to the study of the good. Among acts which are morally good some are obligatory; others are not. For instance, all men are not called upon to be heroes or martyrs, but it is required of all to respect the rights of other to life and property.

Psychologically, moral obligation manifests itself to us in the form of command, or compulsion, which pushes the will in a certain direction, and yet does not destroy liberty in those cases where there is room for freedom. For example, we are all aware that we should respect our parents, but we are all nevertheless free not to do so.

To what voluntary acts does this moral obligation belong? In the first place we are bound to will our end, i.e., our well-being, and to seek it where it is to be found — in that which answers to the deep-rooted tendencies of our rational nature — and not to look for it exclusively in those secondary goods which cease to be good when not controlled by reason. In the second place we are morally bound to will whatever is indispensable in order to reach this end, and to avoid that which must of necessity turn us away from it. Thus natural religion becomes a duty, since God is the end in which man finds his happiness, and since we are obliged to know God and to love Him, with the entire strength of our nature. With the Schoolmen, natural religion is a religion of love and inspires all human conduct. Therefore, God is not merely a frigid metaphysical skeleton, the changeless being which explains all change, but He enters into the whole moral life of man. Obligation in the case of the necessary means is a corollary from the obligation to seek the end. But obligation stops there. In order to get from Boston to New York, I must somehow cover the distance which separates the two cities, but I can get to New York by train or by airplane. So also I can freely choose between different means, when each of them leads to the end and no one is the exclusive way to reach it. This is the reason why all states of life are good, why neither marriage nor celibacy are obligatory, and why a man may choose any career which he thinks will enable him to reach his destiny. Hence moral obligation consists in the necessity of willing our supreme good, combined with the liberty of choosing the concrete objects wherein it is in fact realized.

What is the basis of moral obligation? The psychological fact of compulsion reveals moral obligation, but cannot be a sufficient reason fir it, since we may ask further: upon what does this feeling rest? For the Schoolmen, moral obligation is founded upon human nature itself and its need of well-being. Such is at any rate the proximate basis of obligation. But the ultimate foundation is a Divine decree. God alone can dictate a law which binds morally; He alone can add the necessary sanction to it. Obligation and moral law stand to man in the same relation as the natural law to all beings: they concern the application of the eternal law to a nature which is rational and free.

B. The Natural Law of Mankind

Thomas Aquinas distinguishes between two kinds of commands dictated by the natural law to man. (1) First we have the fundamental command to act according to reason, “to do good and to avoid evil,” and to follow some general precepts which flow from this fundamental obligation. For instance men are obliged “to preserve their own life and to ward off its obstacles…to know the truth about God and to live in Society” [1]. These commands are the same for all men and for all time. They may become clouded over in certain cases, but they can never be altogether effaced, for they are a corollary of our inborn tendency towards our real well-being. It follows from this that human nature is radically sound, and that the worst of criminals is capable of moral reformation.

(2) In the second place we have principles which we may describe as circumstantial, since human conduct is necessarily bound up with conditions of space and time, and physical and social surroundings. Human reason must take the circumstances into consideration in enunciating a moral law. The more closely a law is applied to particular circumstances and cases, the more numerous will be the exceptions to the law, and these exceptions will be justifiable at the bar of reason. Accordingly, Thomas says that a moral law governs only the majority of cases, “ut in pluribus.” “Consequently, in contingent matters such as natural and human things, it is enough for a thing to be true in the greater number of cases, though at times, and less frequently, it may fail” [2]. “From the principle that we must act according to reason, we can infer that we ought to return things entrusted to us, and this is true in the majority of cases. In certain instances, however, restitution would be dangerous and therefore unreasonable, as in the case where the one to whom the article was returned would make use of it to put an end to his life, or do harm to his country” [3].

III. Fixity and variability of laws

These conditions explain why in circumstantial laws — which after all are the only ones which regulate our daily life — we find both change and fixity. The historical and social circumstances may vary, and thus some elasticity in the moral laws becomes possible. But the fundamental precept, and the immediate corollaries from it, which are known by all and bind all, are fixed and invariable. They are as permanent as human nature and human reason themselves. They form a deposit in the depths of every human soul and an interior voice [4] informs us of them. They correspond to the unwritten dictates spoken by Sophocles in Antigone, Cicero, the Stoics, and the Fathers of the Church, and which the Schoolmen incorporated into their comprehensive system of metaphysics.


  1. Summa Theol., Ia IIae, q. 94, art. 2.

  2. Ibid., q. 96, art.1.

  3. Ibid., q. 94, art. 4.

  4. The mind possesses a natural facility and permanent disposition to know the first moral precepts. It is called synteresis, which Thomas defines: lex intellectus nostri inquantum est habitus continens praecepta legis naturalis quae sunt prima operum humanorum, q. 94., art. 1.

The late Dr. Jonathan Dolhenty was the Founder and President of The Center for Applied Philosophy and the Radical Academy, and is Honorary Philosophy Editor at The Moral Liberal. The Moral Liberal has adopted these projects beginning with a republishing and preserving of all of Dr. Dolhenty’s work.