Thomism is a philosophical school of thought following the teachings of Thomas Aquinas, especially as contained in his most famous summary work, Summa Theologica, the importance of which the Roman Catholic church arguably regards as second only to the Bible.

Thomas Aquinas' philosophy


Aquinas worked to create a philosophical system which integrated Christian doctrine with elements taken from the philosophy of Aristotle. Generally, he augmented the Neo-Platonic view of philosophy which, after Augustine, had become tremendously influential amongst medieval philosophers, with insights drawn from Aristotle. In this he was greatly influenced by his reading of contemporary Arabic philosophers, especially Averroes. Aquinas, is, therefore, generally agreed to have moved the focus of Scholastic philosophy from Plato to Aristotle. The extent to which he was successful in doing this is, of course, still hotly debated.

Distinctive ideas

  • Philosophical realism: Aquinas believed that humans were composed of two parts: "prime matter" and "substantial form." The prime matter of humans is our body, and the substantial form is our soul. The soul is therefore not made of matter. Our souls are unique; there can be no two angels or humans that bear the same substantial form, or soul.
  • Moral objectivism: The nature of the universe and essences of objects do not depend on the free will of God, but on His intellect, and ultimately on His essence, which is unchanging. The natural law, springing from the mind of God, is therefore immutable. Consequently, immoral acts are immoral not simply because God forbids them, but because they are inherently immoral. (Zigliara, "Sum. phil." (3 vols., Paris, 1889), ccx, xi, II, M. 23, 24, 25)
  • Teleology: the universe is guided by principles, purpose, and design beyond the universe itself; specifically, the principles and design of God.
  • Free will: Decisions are made by the interaction of the will and the intellect; the will presents objects to the intellect, and the intellect directs the will. Acts begin with the apprehension of the good in general by the intellect. We desire happiness naturally and necessarily, and not by free will; however, we choose particular goods freely. The will is a blind faculty, always following the past determination of the intellect. (Zigliara, 51).
  • Senses: The senses are passive, in that they perceive, rather than creating, their objects. However, the will controls the exercise of the faculties, and thus determines and shapes what they perceive and how they perceive it.
  • Empiricism: Held to the Peripatetic axiom: "Nothing is in the intellect that was not first in the senses," modified by saying that the intellect can ascend to the knowledge of higher things from the basis of perception, even God, and that the soul knows of its existence by its action.
  • First principles: the basis for human knowledge is latent in the soul, not in the form of objective knowledge, but in the form of subjective inclination to believe them due to the evidentiary support: As soon as they are proposed they are known to be true. (Zigliara, op. cit., pp. 32-42).
  • Universals: Universals are the primary object of the intellect, and are formed by abstraction from sense perception. The process of abstraction is so elevated above material conditions as to prove that man is spiritual.
  • Immortality: The human soul is immortal by its very nature, because it has no principle of disintegration (Zigliara, p. 9).
  • Arguments for the existence of God are made a posteriori, rather than a priori. In other words, the existence of God can be proven through perceptions and reason, but cannot be known by any innate knowledge. Thus the ontological argument is rejected, but several other arguments are made for the existence of God; these are considered in the next section.

The five ways

Aquinas shows "five ways" to indicate the existence of God, some of which are developed in detail in his Summa Contra Gentiles.

The first way

(Prime Mover) "It is clear that there are in this world things which are moved. Now, every object which is moved receives that movement from another. If the motor is itself moved, there must be another motor moving it, and after that yet another, and so on. But it is impossible to go on indefinitely, for then there would be no first motor at all, and consequently no movement" ("Contra Gentiles," ii. 33). This proof, like much of Thomas Aquinas's thought, is taken from Aristotle, whose "unmoved mover" forms the first recorded example of the cosmological argument for God's existence.

The second way

"We discern in all sensible things a certain chain of efficient causes. We find, however, nothing which is its own efficient cause, for that cause would then be anterior to itself. On the other side, it is impossible to ascend from cause to cause indefinitely in the series of efficient causes....There must therefore exist one self-sufficient, efficient cause, and that is God" ("Contra Gent." i. 22).

The third way

"We find in nature things which may be and may not be, since there are some who are born and others who die; they consequently can exist or not exist. But it is impossible that such things should live for ever, for there is nothing which may be as well as not be at one time. Thus if all beings need not have existed, there must have been a time in which nothing existed. But, in that case, nothing would exist now; for that which does not exist can not receive life but from one who exists; . . . there must therefore be in nature a necessarily existent being."

The fourth way

Any category has its degrees, such as good and better, warm and warmer. Each also has one thing that's the ultimate of that measure, like good and "best", warm and "hottest". And whatever is the most of that category is the source of that category, as fire (or, in modern terms, energy itself) is the source of heat, and God must therefore be the source of goodness.

The fifth way

Everything, sentient or otherwise, progresses in an orderly way. Planets move in their orbits, light breaks from and combines into its spectrum, et cetera. Reality has a natural order, which could not have come from nothing, yet which precedes mere humans.

Some believe that the Fifth Way is equivalent to what is now called Intelligent Design. However, this is not an accurate presentation of Aquinas' thought, and is subject to the Cosmogonical Fallacy.

Demonstrating God's creative power

In order to demonstrate God's creative power, Thomas says: "If a being participates, to a certain degree, in an 'accident,' this accidental property must have been communicated to it by a cause which possesses it essentially. Thus iron becomes incandescent by the action of fire. Now, God is His own power which subsists by itself. The being which subsists by itself is necessarily one" ("Summa Theol." i. 44, art. 1). This idea is also expounded by Bahya ibn Pakuda in his "Duties of the Heart."

Impact of Thomism

Saint Thomas was important in shifting the influence of medieval philosophy (also known as Scholasticism) away from Plato and towards Aristotle. In this he was influenced by contemporary Arabic philosophy, especially the work of Averroes. The ensuing school of thought, through its influence on Catholicism and the ethics of the Catholic school, is by any standard one of the most influential philosophies of all time, also significant due to the sheer number of people living by its teachings. Thomism also influenced the birth of Protestantism, which arose partially as a reaction to the authoritarian, Thomic dogma of the Catholic Church.

Thomism prevailed and became the official dogma of the Catholic Church, offering a coherent, logical, and clear metaphysical picture of both the material and spiritual worlds. It prevailed as a coherent system until the discovery of Newtonian mechanics, which contradicted the Aristotelian physics, thereby discrediting supplanting much of the thomistic ontology. However, the ethical parts of Thomism, as well as a large part of its views on life, humans, and theology, transcended into the various schools of Neothomism that are the official dogma of the Roman Catholic Church today.


Both Scotists and Molinists oppose the beliefs of Thomism.

See also