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Thomas Aquinas (1225–1274) is widely regarded as the greatest theologian and philosopher of the medieval period. An Italian Dominican friar, Aquinas achieved the monumental task of synthesising the philosophy of Aristotle with Christian theology, producing a comprehensive intellectual system that remains the official philosophical framework of the Roman Catholic Church. His masterwork, the Summa Theologica (Summa Theologiae), begun in 1265 and left unfinished at his death, is one of the most important texts in the history of Western thought. For AQA A-Level Religious Studies, Aquinas is indispensable: the Five Ways, natural law and the doctrine of analogy are all core topics, and his account of the soul and of faith and reason underpins much of the wider specification.
Aquinas lived at a pivotal moment. In the twelfth and thirteenth centuries the works of Aristotle — largely lost to Western Europe for centuries — were rediscovered through Arabic translations and commentaries, especially those of the Islamic philosopher Averroes (Ibn Rushd, 1126–1198). Aristotle's philosophy posed a sharp challenge to Christianity:
Aquinas rejected the double truth theory and argued instead that faith and reason are complementary, not contradictory. Reason, rightly used, reaches truths consistent with and supportive of faith. Some truths (such as God's existence) can be known by reason alone; others (such as the Trinity and the Incarnation) require revelation. There can be no genuine conflict, because both originate in God, who is Truth itself.
Key term: a posteriori argument. An argument whose premises depend on experience of the world. The Five Ways are a posteriori: they begin from observable features of reality (motion, causation, contingency, gradation, order) and reason back to God as their explanation.
In the Summa Theologica (Part I, Question 2, Article 3) Aquinas presents five arguments for the existence of God, the Five Ways. They are not five isolated proofs but five complementary approaches converging on the same conclusion: "and this everyone understands to be God."
The First Way — from motion (ex motu). Things in the world are in motion (any change from potentiality to actuality). Whatever is moved is moved by another, since nothing can move itself from potential to actual. The chain of movers cannot regress infinitely, or no motion would ever begin. Therefore there is a First Mover, itself unmoved — and this we call God. This draws directly on Aristotle's Prime Mover, but Aquinas Christianises it: the First Mover is not an impersonal object of attraction but the personal creator God.
The Second Way — from efficient causation (ex causa). In the world we find an order of efficient causes. Nothing can be the efficient cause of itself (it would have to exist before itself). The chain of causes cannot regress infinitely, or there would be no first cause and hence no later ones. Therefore there is a First Efficient Cause, which we call God.
The Third Way — from contingency (ex contingentia). Things in the world are contingent: they come to be and pass away; they might not have existed. If everything were contingent, then at some time nothing would have existed (since each contingent thing at some point does not exist). But if ever nothing existed, nothing could exist now, because nothing comes from nothing. Since things do exist, there must be a necessary being, whose existence is not derived from another, grounding all contingent existence. This is God. (The Third Way is the form of the cosmological argument the AQA specification names specifically, criticised by Hume and Russell.)
The Fourth Way — from gradation (ex gradu). Things exhibit degrees of goodness, truth, nobility and being. We judge things "more" or "less" only by comparison with a maximum. There must therefore be something that is the maximum in being and goodness, which is the cause of these perfections in everything else — and this maximum we call God. This Way has a distinctly Platonic flavour, echoing the Form of the Good as the source of all goodness.
The Fifth Way — from the governance/teleology of things (ex fine). Natural bodies that lack intelligence nonetheless act for an end (telos): they behave in regular, purposeful ways that reach beneficial outcomes. An arrow reaches its target only because directed by an archer; likewise, unintelligent things reach their ends only because directed by an intelligent being. This director of nature we call God. Note carefully that this is not Paley's later argument from complexity/contrivance: Aquinas argues from the intrinsic purposiveness (the regular goal-directedness) of natural things, not from their resemblance to designed machines.
The Five Ways have attracted extensive criticism, and AQA expects candidates to weigh them:
A fair verdict notes that the Ways are stronger as a cumulative case pointing to a necessary, intelligent ground of being than as knock-down proofs of the full Christian God, which Aquinas himself reaches only by adding revelation.
One technical point repays attention, because it answers the most common objection. When Aquinas denies an infinite regress, he is not (despite appearances) ruling out an infinite series of events stretching back in time; he actually thought reason alone could not disprove an eternal universe, and accepted creation in time only on the authority of revelation. What the First and Second Ways rule out is an infinite essentially ordered series — a series of simultaneous dependencies, where each member exercises its causal power only because something else is exercising its power now. The standard illustration is a hand moving a stick that moves a stone: remove the hand and all motion ceases at once. Such a here-and-now chain of borrowed power must terminate in something whose power is not borrowed, on pain of there being no power in the series at all. This is quite different from an accidentally ordered series (a man begets a son who begets a grandson), where earlier members need not still exist for later ones to act, and which Aquinas concedes could in principle be infinite. Critics who say "why can't the regress just be infinite?" often miss that the Ways concern the first kind of series, not the second.
Aquinas's natural law theory is one of the most influential ethical systems in Western philosophy and the foundation of Roman Catholic moral teaching. Drawing on Aristotle's teleology, Aquinas grounds morality in human nature as created by God. He distinguishes four kinds of law:
Aquinas held that there is one first principle of natural law from which all else flows: good is to be done and pursued, and evil avoided (synderesis). From this, practical reason reads off the primary precepts — the basic goods human nature is oriented towards:
These are supplemented by secondary precepts — more specific rules derived by reasoning (e.g. "do not murder" or "do not steal," derived from "preserve life" and the requirements of ordered society). A useful further distinction is between real and apparent goods: we always act for what seems good, but reason can be misled, so a person may pursue an apparent good (e.g. a drug's pleasure) that is not a real good consistent with human flourishing.
Natural law is not, for Aquinas, merely a list of rules; it is teleological, ordered to human flourishing and ultimately to God. Following Aristotle, Aquinas holds that we become good by acquiring virtues — stable dispositions to act well. He affirms the four cardinal virtues (prudence, justice, temperance and fortitude), which can be developed by habit and reason and which any human being can in principle acquire. But he adds the three theological virtues — faith, hope and charity (caritas) — which, crucially, cannot be acquired by our own effort but are infused by grace. The ultimate end of the moral life is not Aristotelian earthly flourishing alone but the beatific vision — the eternal contemplation of God — which exceeds any natural fulfilment. This is where Aquinas's "grace perfects nature" principle reappears in ethics: natural virtue is real and good, but it is completed and surpassed by the supernatural virtues that orient us to our final, God-given end.
Natural law has notable strengths and is also widely criticised, and AQA rewards balanced assessment:
In its favour, natural law is universal and rational: it grounds morality in human nature rather than in private revelation, so its conclusions are in principle available to believer and non-believer alike, giving it a basis for shared moral discourse and human rights. It also takes human flourishing seriously and offers clear, action-guiding precepts.
Against it, several objections recur. Aquinas's reasoning rests on a strong teleological assumption — that nature has built-in purposes — which modern, post-Darwinian science questions: evolution explains biological "functions" without ascribing moral ends to them, so the move from "this is what the body does" to "this is what we ought to do" can look like the naturalistic fallacy (deriving ought from is), an objection Hume's is–ought gap sharpens. The theory's conclusions on sexual ethics (drawn from the precept "reproduce") strike many as inflexible. And it arguably struggles with genuine moral dilemmas, where two precepts conflict, although the doctrine of double effect and the primary/secondary distinction give it some flexibility. A Thomist can reply that the precepts identify real goods of human nature that any honest reflection acknowledges, and that the is in question is not bare biology but the rational nature of a being made for God — but the debate remains genuinely open.
Aquinas introduced the influential principle of double effect (Summa Theologica, II-II, Q. 64, Art. 7), which holds that an action with both a good and a bad effect may be permissible provided:
This remains central to Catholic moral theology and is widely applied in medical ethics — for example, giving pain relief that may foreseeably hasten death, where death is not the intended means or end.
A major contribution of Aquinas to the philosophy of religion is his theory of analogy — his account of how human language can meaningfully refer to God despite the infinite gulf between Creator and creature. He identifies three options:
He distinguishes two forms of analogy:
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