AQUINAS — SUMMA THEOLOGICA
Medieval
Lineage
Author
Thomas Aquinas (1225–1274) entered the Dominican order as a young man against the wishes of his aristocratic family — his brothers reportedly kidnapped him to prevent him from taking the habit. He studied under Albertus Magnus in Cologne, through whom he encountered the newly translated works of Aristotle, and spent his career demonstrating that Aristotelian philosophy and Christian revelation were not merely compatible but mutually illuminating. He lectured in Paris and Rome, wrote prodigiously, and reportedly dictated to multiple secretaries simultaneously. He left the Summa Theologica unfinished in December 1273, after an experience at Mass that caused him to cease writing entirely; he died three months later. His synthesis — Thomism — was declared the official philosophy of the Roman Catholic Church in Leo XIII's encyclical Aeterni Patris (1879), making him the only philosopher in history to have his system endorsed by a major religious institution as authoritative. The Summa, written as an introduction for beginners in theology, is nevertheless the most comprehensive systematic theology ever produced in the Western tradition.
Historical Impact
The Summa Theologica reorganized Christian theology as a deductive philosophical system, establishing the scholastic method — question, objection, reply to objections, response — that structured European intellectual life for centuries. The five arguments for the existence of God (quinque viae) became the standard proofs that every subsequent natural theology had to engage: Hume's Dialogues Concerning Natural Religion and Kant's Critique of Pure Reason are both organized partly as responses to the cosmological and ontological versions of these arguments. The Summa's natural law theory — articulated in the Treatise on Law (I-II, qq. 90–108) — grounded the tradition of natural law jurisprudence that runs through Suárez, Grotius, Locke's theory of rights, and the twentieth century's Universal Declaration of Human Rights. Alasdair MacIntyre's After Virtue (1981), the most influential work of moral philosophy of the late twentieth century, argues that Thomistic virtue ethics — Aristotle corrected and completed by Aquinas — is the only coherent response to the crisis of modern moral philosophy.
Reception History
Aquinas was condemned twice during and immediately after his lifetime: the Bishop of Paris condemned several propositions associated with Thomistic Aristotelianism in 1277, three years after his death, reflecting conservative Augustinian anxiety about the degree to which he had allowed pagan philosophy to reshape Christian doctrine. The fourteenth century saw the rise of competing scholastic systems — Scotism (Duns Scotus) and Nominalism (Ockham) — and Thomism was one school among several. The Protestant Reformation largely rejected scholastic method; Luther's contempt for Aristotle's Ethics was contempt for the Thomistic enterprise. The nineteenth-century Thomistic revival (Neo-Scholasticism) was a Catholic response to Kantian idealism and positivism, attempting to provide a philosophical foundation for natural law against the rising tide of legal positivism and moral relativism. Contemporary Thomism is notably diverse: analytic Thomism (Eleonore Stump, Brian Davies, John Finnis) engages with philosophy of mind, action theory, and metaethics using the tools of analytic philosophy; political Thomism (MacIntyre, Finnis) develops natural law theory in direct engagement with Rawlsian liberalism and international human rights law.
Key Passages
The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not-to-be at some time is not. Therefore, if everything is possible not-to-be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence — which is absurd.
Summa Theologica, I, Q. 2, A. 3 (The Third Way). Trans. Fathers of the English Dominican Province (1920).
The Third Way (the contingency argument) is the most philosophically rigorous of the five proofs. Where the First and Second Ways argue from motion and causation, the Third argues from the modal distinction between contingent beings (those that can fail to exist) and necessary being. If every existing thing were contingent, there would have been a time when nothing existed — from which nothing could come. Since something exists now, there must be at least one necessary being. This argument is the ancestor of Leibniz's principle of sufficient reason and the contemporary modal cosmological argument.
Law is nothing else than an ordinance of reason for the common good, made by whoever has care of the community, and promulgated.
Summa Theologica, I-II, Q. 90, A. 4. Trans. Fathers of the English Dominican Province (1920).
The formal definition of law that opens the Treatise on Law — the section of the Summa most consequential for political philosophy and jurisprudence. Four elements are specified: law must be an ordinance of reason (not mere command); for the common good (not the ruler's private benefit); made by legitimate authority; and promulgated (publicly known). Each element has generated extensive commentary. The natural law tradition derives from Aquinas's argument that human law participates in eternal law through natural law — the rational creature's participation in the divine ordering of things.
I answer that it is necessary to admit a first efficient cause, to which everyone gives the name of God. The existence of God can be proved in five ways. The first and more manifest way is the argument from motion.
Summa Theologica, I, Q. 2, A. 3 (opening). Trans. Fathers of the English Dominican Province (1920).
The framing of the quinque viae. Aquinas claims not to be offering a priori proofs from the concept of God (that is Anselm's ontological argument, which he rejects) but a posteriori demonstrations from features of the experienced world — motion, causation, contingency, degrees of perfection, and teleology. The structure of each argument moves from an observable feature of the world to the requirement for a first unmoved mover, uncaused cause, or necessary being that common usage identifies with God.
Scholarly Works
The Christian Philosophy of St. Thomas Aquinas — Étienne Gilson
The major twentieth-century philosophical study of Aquinas. Gilson argues that Thomism is not merely Christianity baptizing Aristotelianism but a genuinely new synthesis, and his account of the esse/essentia distinction (being as act) remains the most philosophically rigorous treatment of Aquinas's metaphysics. University of Notre Dame Press.
The Thought of Thomas Aquinas — Brian Davies
The best accessible scholarly introduction to the Summa for readers without a scholastic background. Davies covers the Five Ways, natural law, the attributes of God, and the Thomistic account of human nature in clear philosophical prose. Oxford University Press, 1992.
After Virtue — Alasdair MacIntyre
Not a commentary on Aquinas but the most influential twentieth-century argument for Thomistic virtue ethics as the answer to modern moral philosophy's crisis. MacIntyre argues that only the Aristotelian-Thomistic tradition, with its account of human nature as teleologically ordered toward a determinate good, can provide an adequate foundation for ethics. University of Notre Dame Press, 1981.
The Cambridge Companion to Aquinas — Norman Kretzmann and Eleonore Stump (eds.)
A collection of essays by leading scholars covering Aquinas's metaphysics, philosophy of mind, ethics, and theology. The best map of contemporary scholarship across the Summa's major topics. Cambridge University Press, 1993.
Lectures
Aquinas & the Cosmological Arguments — Crash Course Philosophy #10 — Hank Green, Crash Course / PBS Digital Studios
Accessible introduction to the Five Ways and Aquinas's arguments for the existence of God for general audiences. A useful first orientation before engaging with the primary text.
Gifford Lectures — Natural Theology — Various, University of Edinburgh / Glasgow
The Gifford Lectures, established in 1887, have produced the major works of natural theology in the English-speaking tradition, many of which engage directly with Aquinas's arguments. Lectures are freely available on the Gifford Lectures website with full texts and audio where available.
Further Reading
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