Aquinas and Ethics

Aquinas acknowledges that humans have only one ultimate end, which is something that satisfies one's desires. He maintains that there are objective truths about the nature of happiness and that there are nearly an infinite number of ways to manifest virtues in various professions. He also considers the expression "ultimate end" or "happiness" to be ambiguous, distinguishing between the happiness humans can possess in this life, which he calls "imperfect human happiness," and the happiness possessed by God, angels, and the blessed, which he considers perfect.

Aquinas' philosophy focuses on the nature of human happiness, which is the pursuit of good or happiness through virtuous actions. He agrees with Aristotle that the attainment of happiness involves the soul's activity expressing virtue, particularly the best virtue of contemplation, where the object of such contemplation is the best possible object, God. Aquinas believes that human beings in this life, even those who possess the infused virtues, at best attain happiness only imperfectly since their contemplation and love of God are, at best, imperfect.

Aquinas is a moral perfectionist, meaning that the means to human happiness come not by way of merely good human actions but by way of perfect or virtuous moral actions. Morally virtuous human actions are actions that perfect the human agent that performs them, leading to happiness for the agent that performs them. He rejects the view that all bodily pleasures are evil, as it is natural for human beings to experience bodily and sensitive pleasures in this life.


Mortally virtuous action is more than just morally good action; it is a combination of the kind of action, circumstances surrounding an action, and motivation for action. It arises from a good moral habit or virtue that makes it possible to act with moral excellence. However, one morally good action is not necessarily a morally virtuous act, as virtuous actions arise from a habit such that one wills to do what is virtuous with ease.

Aquinas' account of the means to happiness as moral virtues reveals two different kinds of virtue: natural virtues, which are attainable through reason and will, and supernatural virtues, which come only by grace. Aquinas believes that humans can acquire virtues that perfect human beings according to their natural end by repeatedly performing the kinds of acts a virtuous person performs, or habituation. He calls these virtues human virtues, distinguishing them from "infused" virtues, which are virtues we have only by way of a gift from God, not by habituation.

Infused virtues are perfections of our natural powers that enable us to do something well and easily. They differ from human virtues in several ways. First, unlike human virtues, infused virtues enable us to perfect our powers so that we can perform acts in this life commensurate with or as a means to eternal life in heaven. Second, infused virtues are wholly gifts from God, unlike human virtues.

Aquinas thinks that neither infused nor human virtue makes a human being impervious to committing mortal sin. Mortal sins kill supernatural life in the soul, making one fit for the supernatural reward of heaven. However, since infused virtues are not acquired through habituation but are a function of being in a state of grace as a free gift from God, just one mortal sin eliminates the infused virtues in the soul. However, such mortal sins can be forgiven by God's grace through the sacrament of penance, restoring a soul to the state of grace.

Aquinas posits that human virtues are perfections of the characteristically human powers. He distinguishes between the rational powers of intellect and will, which enable humans to think about actions in universal terms, and the apprehensive powers of the soul, such as sense and intellect, and the appetitive powers of the soul, which incline creatures towards a certain goal or end based on how objects are apprehended by the senses and intellect.

Aquinas discusses five intellectual virtues: wisdom, understanding, science, art, and prudence. Understanding involves the consideration of first principles, while science is the intellectual ability to draw correct conclusions from first principles within a particular subject domain. Art and prudence are two intellectual virtues that bring about some practical effect.

Aquinas, a virtue ethicist, posits that there are two types of human virtues: intellectual and moral. He distinguishes between perfect and imperfect human moral virtues, where imperfect virtues are dispositions that one desires to do good deeds well. In contrast, perfect virtues cannot be possessed apart from one another, as they are dispositions that one is inclined to do good deeds in the right way, at the right time, and for the proper motive.

Aquinas accepts the "unity of the virtues" thesis for two reasons. First, he distinguishes virtues according to general properties of the virtues, such as discretion belonging to prudence, rectitude to justice, moderation to temperance, and strength of mind to courage. This way, he believes that one cannot have any one of the perfect cardinal virtues without also possessing the others. Second, he argues that one cannot be perfectly prudent unless they are also perfectly temperate, just, and courageous. This is because a perfected intellect is necessary for deciding on the virtuous thing to do in any given situation. However, a perfect knowledge of the ends or principles of human action requires the possession of virtues that perfect the irascible appetite, concupiscible appetite, and will.

Aquinas also discusses moral knowledge, arguing that all humans who have reached the age of reason and received an elementary moral education have a kind of moral knowledge, namely, a knowledge of universal moral principles. There are at least three types of universal principles of the natural law: first principles (e.g., do good and avoid evil), second principles (e.g., honoring parents and gifts), and third principles (e.g., abstract formulations of the commandments of the Decalogue).

To know the primary and secondary universal precepts of the natural law is to have the human virtue of understanding with respect to the principles of moral action. Moral knowledge of other sorts is built on the back of having the virtue of understanding with respect to moral action. It is possible to have the virtue of understanding without otherwise being morally virtuous, such as prudentness and courage.

Aquinas believes that the proximate measure for the goodness and badness of human actions is human reason, or right reason. However, right reason is also a part of God's mind, making God the ultimate standard for moral goodness. He uses the language of law to discuss God as the measure of morally good acts, stating that God's infinite and perfect being is the ultimate rule or measure for all creaturely activity, including normative activity.

Aquinas believes that none of the precepts of the Decalogue are dispensable, as each of the Ten Commandments is a fundamental precept of the natural law. However, it would be a contradiction for God to will that a fundamental precept of the natural law be violated, as it would contradict God's own perfection. Therefore, God's will and perfection are the same, and a contradiction in terms would be necessary for living a good human life.