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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, creating 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: his Five Ways, his natural law ethics, and his theory of analogy are all core topics.
Aquinas lived at a pivotal moment in European intellectual history. In the 12th and 13th centuries, the works of Aristotle — which had been largely lost to Western Europe for centuries — were rediscovered through Arabic translations and commentaries, particularly those of the Islamic philosopher Averroes (Ibn Rushd, 1126–1198). Aristotle’s philosophy posed a profound challenge to Christian theology:
Aquinas rejected the double truth theory and argued instead that faith and reason are complementary, not contradictory. Reason, properly exercised, leads to truths that are 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 divine revelation. There can be no genuine conflict between them because both originate in God, who is Truth itself.
In the Summa Theologica (Part I, Question 2, Article 3), Aquinas presented five arguments for the existence of God, known as the Five Ways. These are a posteriori arguments — they begin from observations about the world and reason backward to God as the necessary explanation. They are not five independent proofs but five complementary approaches that converge on the same conclusion.
Aquinas observed that things in the world are in motion (understood broadly as any kind of change). Whatever is in motion must be moved by something else, since nothing can move itself from potentiality to actuality. But this chain of movers cannot extend to infinity, or no motion would ever have begun. Therefore, there must be a First Mover, itself unmoved, that initiates all change. This First Mover is what we call God.
This argument draws directly on Aristotle’s Prime Mover, but Aquinas Christianises it: the First Mover is not merely an impersonal principle of attraction but the personal, creator God of Christian theology.
Aquinas observed that in the world of sensible things we find an order of efficient causes. Nothing can be the efficient cause of itself (since it would have to exist before itself, which is impossible). The chain of causes cannot go back infinitely, or there would be no first cause and hence no subsequent causes. Therefore, there must be a First Efficient Cause, which we call God.
Aquinas observed that things in the world are contingent — they come into existence and go out of existence; they might not have existed. If everything is contingent, then at some point nothing would have existed (since contingent things eventually cease to exist). But if nothing ever existed, nothing would exist now (since something cannot come from nothing). Since things do exist now, there must be a necessary being — a being that cannot not exist — that is the ground of all contingent existence. This necessary being is God.
This argument was later developed by Leibniz into the Cosmological Argument from Sufficient Reason and remains one of the most discussed arguments in the philosophy of religion.
Aquinas observed that things in the world exhibit varying degrees of goodness, truth, nobility, and other perfections. Something can be more or less good only by comparison to a maximum — just as something can be more or less hot only by comparison to something that is maximally hot. There must therefore exist something that is the maximum in being, goodness, and truth — and this maximum is the cause of all the perfections found in other things. This maximum we call God.
This argument has a distinctly Platonic flavour: it resembles Plato’s argument that the Form of the Good is the source of all goodness. Aquinas draws on both Aristotle and the Neoplatonic tradition here.
Aquinas observed that natural bodies that lack intelligence nonetheless act for an end (telos) — they behave in regular, purposeful ways that lead to beneficial outcomes. An arrow hits its target only because it is directed by an archer. Similarly, natural things achieve their ends only because they are directed by an intelligent being. This intelligent director of all natural things is God.
This is a teleological argument — an argument from purpose or design. It anticipates later design arguments by William Paley (the Watchmaker Analogy) but differs in an important respect: Aquinas’s argument is about the intrinsic purposiveness of natural things (their inherent tendencies toward ends), not about the complexity or contrivance of natural mechanisms.
The Five Ways have been subjected to extensive philosophical criticism:
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