Thomas Aquinas: Conscience and Moral Values


  • A. Conscience
  • B. Responsibility and sanctions
  • C. Moral Virtues – Prudence and Justice


A. Conscience

The obligation to act in a particular way in a particular instance affects the will through the intermediary of an act of knowledge. This is evident from the data of psychology and ethics. I ought to know the moral law not only as expressed in more or less general principles by means of general judgments of the practical reason, but also as applying or not applying to the particular case before me. The act by which the reason applies a universal principle of morality to a particular case is the judgment of conscience [1]. The practical reason says: You must be honest in business and give to each his due. Conscience says: You must return to your customer the sum of a hundred dollars, above the price of the article sold to him, which he gave you by error.

A law which is not known cannot bind us, and we are never bound to act otherwise than our conscience tells us, even if its judgment happens to be erroneous. “We must say, unconditionally, that any act of will which goes astray from reason, whether that reason be correct or false, is evil” [2]. In applying his principles in this way, Aquinas shows his breadth of view, and — let us remark incidentally — demonstrates the tolerance of the thinkers of the thirteenth century in religious matters. For if anyone though in good faith that he would do wrong in becoming a Christian, he would do wrong in believing in Christ, although the Christian Faith is in itself good, and necessary for salvation [3]. For the same reason, a doubtful or ‘probable’ conscience does not bind or at any rate binds to a less degree. Obligation is a function of knowledge.

But we must add something further to this thomistic doctrine. It must not be supposed that every act of willing evil, under the impression that it is good, is morally upright, for man has a positive duty to instruct himself concerning his moral obligations, seek light on doubtful points, and weigh probabilities (XIII, B). Error, doubt, hesitation become blameworthy if they are voluntary. Still, it remains true that anything which diminishes our clear vision of what we ought to do, such as prejudices, education, heredity, organic disease or weakness, fear, anger, and other passions, defects or evil tendencies in the will, emotions, etc. (VII, E), reduces the moral character of an act, and likewise responsibility.

B. Responsibility and sanctions

Moral acts, whether obligatory or not, are imputable to the individual, in so far as they are freely performed. As Aristotle puts it, a man is the father of his acts as he is the father of his children.

Responsibility, relative to oneself or to others, involves merit and demerit. These are regarded by the Schoolmen as the natural consequences of the use of liberty. If an act freely willed, moral or immoral, had nothing to do with merit or demerit, and if ultimately we could not fall back upon a system of sanctions (i.e., rewards and punishments) which need to be completed in a future life, — not only would the good cease to be rewarded and evil punished, but liberty itself would no longer have a sufficient reason. What would be the use of liberty, if its proper or improper employment were without effect upon our final happiness?

C. Moral Virtues – Prudence and Justice

The performing of acts morally good engenders moral virtue: it impresses upon the higher part of our being a lasting bent which inclines us to act well in all the circumstances of our life. Moral virtue is the result of moral conduct in the past, and the source of similar conduct in the future. The moral virtues are prudence, justice, fortitude, temperance (VIII, C).

At the base of the moral life is prudence, the recta ratio agibilium — “right reasoning concerning things to be done” — which determines what act should be performed in particular circumstances. Certain primary and very simple judgments which are present in every mind (such as, for instance, “it is necessary to live in society”) originate a tendency or inclination to act in accordance with them (for instance, a general tendency to do all that is necessary for life in society). Then comes a series of practical judgments which, considering all the circumstances (consilium, counsel), determine our choice. This in turn the will decides to follow (imperium). A prudent man is one who by the frequency of such judgments sees and decides rapidly and without hesitation what is to be done in a particular case. Prudence therefore belongs both to knowing and to acting, and exemplifies the intimate compenetration of knowledge and will in the unity of consciousness. Situated at the threshold of the moral life, prudence impregnates all the other virtues which guide us in our actions, especially justice, fortitude and temperance.

To understand the meaning of justice we must begin by considering the notion of right (jus). Right presupposes the living together of many human beings in a community. Since I have a personal end to attain, my acts are naturally means which serve for my own perfection. If they directly benefit others, then these others owe me compensation, and right, jus, consists precisely in this requirement of equity. “Right, or that which is just, is some work related to another according to some kind of equity” [4].

Justice, the virtue par excellence of life in society, is the psychological and moral state of a man who wills “firmly and permanently to render to each one his due” [5]. It accordingly supposes a plurality of distinct persons, capable of bringing about this equity by means of their actions. “Since it belongs to justice to regulate human actions, this equity which is called for by justice must be between different persons, capable of action” [6]. This is indeed called for by the individualism which runs through the Metaphysics and Moral Philosophy of Thomas. He never loses an opportunity of stressing the value of personality.

Now, it is easy to see that the ‘other than self,’ for whose benefit justice exists, may signify an individual, or the community, and we thus obtain the division of justice into particular and social. For instance to give to a shopkeeper the price of an article purchased is to perform an act of private or particular justice [7].

In the present chapter only particular justice is in question. Since right — that which is due to others — rests upon an objective equality, it is independent of our passions and affections. The same is true of the virtue of justice. On the other hand, fortitude, which regulates boldness and fear, temperance, which bridles our appetites, and other virtues, are directly related to our passions and our inner dispositions.

We can say that Thomas Aquinas retains for the group of moral virtues the Aristotelian notion “in medio virtus” on condition that the mean here is determined by reason, and differs in the case of different virtues. For instance, not to eat when one ought to, or to eat more than we ought, is not to observe the limits of temperance dictated by the reason. Where the virtues are concerned, we must keep close to reason.

The moral philosophy of Thomas Aquinas is in close dependence upon his Metaphysics. The moral value of personality, the end of man, the notion of moral goodness, the moral richness of a human act, are all established in a way conformable with the great principles of pluralism, of universal finality, and of the goodness of being.


  1. Summa Theol., Ia IIae, q. 19, art. 5. Conscientia nihil aliud est quam applicatio scientiae ad aliquem actum.

  2. Ibid., q. 19, art. 5.

  3. Ibid., q. 19, art. 5.

  4. Jus sive justum est aliquod opus adaequatum alteri secundum aliquem modum. Ibid., q. 57, art. 1.

  5. Perpetua et constans voluntas just suum unicuique tribuendi. Ibid., q. 58, art. 1.

  6. Ibid., art. 2.

  7. In this instance there is an exchange which brings about an equality, and is called commutative justice. Besides, Aquinas considers as an act of particular justice the distribution to individuals of honors or distinctions which are at the disposal of the community, this being distributive justice. Commutative and distributive justice are the two divisions of private justice.

“The System of Thomas Aquinas” by Maurice de Wulf (1867-1947): Edited and adapted for the Web by Dr. Jonathan Dolhenty.

If you like Aquinas you may enjoy: Thomas Aquinas: Selected Writings (Penguin Classics)