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Thomas Aquinas (1225–1274)

Aquinas achieved the most consequential philosophical synthesis of the medieval period: the integration of Aristotle’s metaphysics, epistemology, and ethics with Christian theology. The synthesis was not obvious — Aristotle’s philosophy had entered the Latin West through Arabic translations and commentaries (principally Averroes) and was regarded by many churchmen as dangerous, because Aristotle’s conclusions (the eternity of the world, the mortality of the individual soul, the sufficiency of natural reason) conflicted with Christian doctrine. Aquinas argued that faith and reason are complementary, not opposed — that philosophy can reach genuine truths about God, morality, and the natural world through reason alone, and that these truths are compatible with, though not identical to, the truths of revelation. The Summa Theologiae (1265–74, unfinished) is the systematic expression of this programme. MacIntyre identifies Aquinas’s synthesis as the paradigm of how a tradition in epistemological crisis can survive by incorporating the resources of a rival tradition — and defends the Thomistic tradition as the one best equipped to do this.

Life

Born c. 1225 at Roccasecca, near Aquino, in the Kingdom of Sicily (present-day Lazio, Italy). His family was minor nobility; they intended him for a career as a Benedictine abbot, which would have been politically and financially advantageous. Educated at the Benedictine abbey of Monte Cassino (from age five), then at the University of Naples (from 1239), where he encountered Aristotle’s philosophy for the first time through the newly available Latin translations.

Joined the Dominican order c. 1244 — a mendicant order devoted to study and preaching, not a cloistered monastic order. His family was opposed; his brothers reportedly kidnapped him and held him for a year to dissuade him. He persisted. Sent to Paris, then to Cologne to study under Albertus Magnus (“Albert the Great”), the Dominican scholar who was among the first to take Aristotle seriously as a philosophical authority within the Latin Christian tradition.

Taught at the University of Paris (1256–59, 1269–72) and at various Dominican houses in Italy. The Summa contra Gentiles (c. 1259–65) was written partly as a manual for missionaries engaging with Islamic and Jewish philosophy. The Summa Theologiae was begun in 1265 and left unfinished: in December 1273, after a mystical experience during Mass, Aquinas stopped writing, reportedly saying: “Everything I have written seems like straw compared to what has now been revealed to me.” He died on 7 March 1274 at Fossanova Abbey, on his way to the Council of Lyon.

Canonised in 1323. Declared Doctor of the Church in 1567. The Summa Theologiae became the standard text of Catholic philosophical theology; the Thomistic tradition remains the dominant intellectual tradition within Catholic philosophy.

The synthesis

Aquinas’s central achievement is the integration of two intellectual systems that appeared incompatible: Aristotelian philosophy (pagan, naturalistic, focused on the natural world) and Christian theology (revealed, supernatural, focused on salvation).

Reason and revelation. Aquinas distinguishes truths accessible to natural reason (the existence of God, the basic principles of morality, the structure of the natural world) from truths accessible only through revelation (the Trinity, the Incarnation, the specific content of salvation history). The two domains overlap but do not conflict: reason and revelation are complementary paths to truth, because both originate in God. Philosophy can demonstrate God’s existence and certain of his attributes; it cannot demonstrate the Trinity or the Incarnation. Theology begins where philosophy reaches its limit.

The five ways. Summa Theologiae I, q.2, a.3. Five arguments for the existence of God, each starting from an observable feature of the natural world:

  1. Motion — whatever is in motion is moved by something else; the chain of movers cannot extend infinitely; there must be a first unmoved mover.
  2. Efficient causation — every effect has a cause; the chain of causes cannot extend infinitely; there must be a first uncaused cause.
  3. Contingency — contingent beings (which might not exist) require a necessary being to ground their existence.
  4. Degrees of perfection — things are more or less good, true, noble; there must be a maximum that is the cause of all perfection.
  5. Governance — natural bodies act toward ends even though they lack intelligence; they must be directed by an intelligent being.

The five ways are demonstrations, not proofs in the modern formal sense. They proceed from premises Aquinas takes to be evident from experience and reach conclusions about what must exist to make the observed features of the world intelligible.

Natural law. The moral law is grounded in human nature — in the natural inclinations that human beings share (self-preservation, reproduction, social life, the pursuit of truth). These inclinations are not mere biological drives; they are expressions of the rational order God has built into creation. The first principle of natural law — “good is to be done and pursued, and evil is to be avoided” — is self-evident. The specific precepts of natural law are derived from it through practical reason. The natural-law tradition that Aquinas developed became the foundation of Catholic moral theology and influenced secular natural-law theory through Grotius, Locke, and the natural-rights tradition.

Where Aquinas stops

The five ways have each been challenged. The cosmological arguments (the first three) depend on the impossibility of infinite causal regress — a premise Hume questioned and that modern cosmology has complicated (the universe may not require a first cause in the sense Aquinas assumes). The argument from design (the fifth way) was challenged by Hume before Darwin and more decisively after: natural selection provides a non-intelligent mechanism that produces the appearance of design. The argument from degrees of perfection (the fourth way) has been the least widely defended, even within the Thomistic tradition. Whether the five ways survive as arguments for theism or only as illustrations of a metaphysical framework that must be accepted on other grounds is contested within philosophy of religion.

The natural-law framework depends on there being a determinate human nature with a determinate telos — natural inclinations that ground moral norms. The Darwinian challenge is the deepest: if human nature is the product of evolution — contingent, variable, without a designer’s purpose — then the “natural inclinations” Aquinas identifies may be evolved dispositions rather than expressions of rational order, and the inference from natural inclination to moral norm may not hold. Thomistic natural-law theorists (John Finnis, Germain Grisez) have attempted to reformulate natural law without dependence on teleological biology; whether the reformulation preserves what was distinctive about Aquinas’s account or abandons it is debated.

The synthesis itself — Aristotelian philosophy integrated with Christian theology — depends on the compatibility of two systems that pull in different directions. Aristotle’s metaphysics is naturalistic: the unmoved mover is not a personal God but an impersonal principle. Aquinas identifies the unmoved mover with the God of Christian revelation, but the identification requires adding content (personality, providence, love) that Aristotle’s metaphysics does not supply. Whether the synthesis holds — whether Aristotelian metaphysics and Christian theology are genuinely compatible or only apparently so — has been debated within Thomism since the thirteenth century. The condemnation of 1277, in which the Bishop of Paris condemned 219 propositions including several associated with Thomistic Aristotelianism, shows that the tension was recognised immediately. The tension was not resolved by the condemnation or by Aquinas’s subsequent canonisation; it is structural.

Key works

See also: Aristotle · MacIntyre · Hume