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Key Concepts of the Philosophy of St. Thomas Aquinas

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Read in to learn key concepts in St. Thomas Aquinas’s philosophy.

Read in to learn key concepts in St. Thomas Aquinas’s philosophy.

Who Was St. Thomas Aquinas?

St. Thomas Aquinas was a 13th-century Italian priest, theologian and philosopher, whose writing shaped the basis for modern Catholic thought.

St. Aquinas was the most important philosopher of the medieval period, with influence on epistemology, metaphysics, ethics and political philosophy well into the modern period of history. While the theologians before him were heavily influenced by the works of Plato, Aquinas preferred Aristotle, and his more scientific outlook, to Plato’s “mystic” ideas about reality.

Aquinas proposed that faith and reason, and science and theology, need not be opposed to each other and could co-exist. The main pursuit of his philosophy was the balance of logic and natural sciences with the philosophical concerns of Christian doctrine.


To establish the different purviews of the physical world (science) and the spiritual world (God), Aquinas used principles based on Aristotle’s philosophy. Aquinas established the differences between primary substances and composite substances. A primary substance is the essential nature of something. For instance, all the traits that a certain human being might have, being tall, skin color, hair color, etc., are incidental to the essence of what it is. The primary substance is something beyond what it is physical, and in the human’s case, this is the essential nature of humanity.

This is an obvious attempt by Aquinas to refute Plato’s theory of the forms, and to Aquinas, this would mean a thing like a chair has an essence that is completely separate from whether it is made of wood or plastic or is round or square.

To Aquinas, things like minds and angels were primary substances, and God was a being who possessed all traits or perfections. For God, there was no separation from the primary form and the physical form. This is essentially what made God what he was and the composite forms of other beings as less than God.

Aquinas uses Aristotle’s four causes to elaborate on this concept. (see Aristotle hub) For Aquinas, the primary cause of all things is God. God is the reason for all to exist and the matter and form of things is the actualizing of the potential created by God.

Aristotle claimed that the form that created living bodies was the soul. To Aristotle, the soul was not the primary essence of being but was the “first grade of actuality” from the potential self to the actual self. So there was no reason to think of the soul as a separate entity from the body. To Aristotle, it was simply obvious that the body and soul were one entity.

Aquinas disagrees with Aristotle on the point that the body and the soul are completely one, but he also disagrees with Plato that they are completely separate. To Aquinas, he thought there was a definite difference between the idea that the soul was part of the material of the body and part of the form. The form and material were not the same things, and since the soul was what gave the material of the body its form, this suggested to Aquinas that the soul must contain some quality that the body did not have. So while the soul was part of the form of a human being, it was not part of the material body.

Another important aspect of Aquinas’s philosophy is his argument about the nature of God’s omnipotence. Aquinas thought that the idea of omnipotence had been misunderstood. While the laws of nature were created by God and reason was given to human beings as a capacity for deriving truth, Aquinas doesn’t think God can defy logic. An example of this would be if God were to make “round squares.” The concept of round squares is logically contradictory and not something that God is capable of creating, not because this limits his power of omnipotence but because the concept by itself is logically impossible.

Free Will and Ethics

Aquinas delineated the basic human drives into “the will” and “desires.” Desires are all sensual appetites that derive from the senses. The will, however, is a faculty that is always seeking the good. Aquinas believed that the good for all people was God, but the conscious mind did not need to perceive this as seeking God. All acts that human beings choose are in service of what is perceived to be good.

When a person commits an immoral act, they are still seeking the good—they are merely mistaken. This is the same as when someone moves away from God. They are still seeking the good, but they are mistaken. True happiness needs God in order to be achieved, but human beings have the freedom to choose to move away from God.

When it came to morality, Aquinas argued that we should judge goodness in how fully something exists. His example is that it is good for a blind man to exist, but his lack of sight is bad. Aquinas claimed that an action’s rightness could therefore be judged by four qualities:

  1. Existence
  2. The object it is directed toward
  3. Circumstances
  4. Goal

Aquinas thought that moral action was best defined by the object of the external activity and the goal of the action. Aristotle’s example was that a married man who steals to pay for a prostitute is more an adulterer than a thief. Aquinas agreed with this view on morality.

Aquinas believed that reason was the faculty to determine moral action. If the object of an action was agreeable to reason (such as giving to the poor), then it was good, but if it was offensive to reason (such as stealing), then it was bad. Some actions, like picking up sticks off the ground, are completely neutral and have no good or bad distinction. Ultimately, the will should act in accordance with reason, and the goal with which the will is engaged ultimately determines whether an action is moral or immoral.

Aquinas agreed with Aristotle that Virtue was the moderation between two vices, but he also was a priest who took a vow of chastity and poverty. It could be argued that both decisions are themselves extremes. Aquinas believed that the best life was a chaste life but did not think it was achievable by all individuals.

His solution to this inherent contradiction was to claim that the life of a priest was a calling that only a few had and a few were capable of fulfilling. For others, a more moderate life was the most suitable, but some are directed by their calling from God to live a life of poverty and chastity.

Natural Law Ethics

Aquinas extended his idea of virtue and goodness that had been derived from Aristotle into an ethical theory called “Natural Law” ethics. The basis of this idea was that what was good for man was what benefitted his nature. This is how Aquinas further argued that chastity was not suitable for all human beings. It was the nature of man to want to propagate the species, but it was not the obligation of every man to do so.

Aquinas thought that natural law was founded on the same elemental law that dictated the truths of the sciences. Four values were established to be key in natural law: life, procreation, knowledge and sociability. Aquinas also established “the doctrine of double effect,” which states that an act can be committed if it has two effects, one good and one bad, if it meets the following criteria:

  1. The act, considered in itself, is at least morally permissible
  2. The bad effect is unavoidable
  3. The bad effect is not the means of producing the good effect.
  4. The criterion of proportionality is satisfied. (Good effect must be at least equivalent to bad effect.)

This doctrine is still the most important and discussed part of Aquinas’s ethics and is discussed by modern ethicists, even in the schools of thought of Kantian, Utilitarian and Virtue Ethics, and has been used in many “just war” theories. Aquinas was also the most important deontological ethicist until Immanuel Kant in the late 18th century.

Biography of St. Thomas Aquinas

Sources and Further Reading


Jay C OBrien from Houston, TX USA on March 31, 2017:

From article: "Four values were established to be key in natural law: life, procreation, knowledge and sociability."

"Life" is listed first as a natural law. Life is sacred and the Ten Commandments state, "Thou shalt not kill."

Within one generation after, "Thou shalt not kill," was given, Joshua goes into Canaan and kills men, women and children. What does St. Thomas Aquinas say about the genocide of the Canaanites?

Hannah David Cini from Nottingham on February 28, 2015:

This is a really nice piece. I have not seen much written about St Thomas Aquinas and really enjoyed reading this.

Ralph Deeds from Birmingham, Michigan on October 12, 2011:

I once had a very smart boss who occasionally quoted St. Thomas Aquinas when someone brought to him a simplistic, poorly thought-out proposal or conclusion as follows:

"Seldom affirm. Never Deny. Always distinguish."