The Evolution of Scholastic Arguments: Aquinas to Al-Ghazali

Can reason alone prove that God exists? For medieval thinkers across both Christian and Islamic traditions, the answer was a resounding yes – and they devoted extraordinary intellectual energy to demonstrating it. Medieval scholasticism, the philosophical and theological method that dominated European and Islamic intellectual life from roughly the 12th to the 17th century, was built precisely on this conviction: that rigorous logical argument, applied to the observable world, could lead the rational mind to God. Two of scholasticism’s most important contributions to this project came from a 13th-century Italian Dominican friar and an 11th-century Persian Muslim theologian – Thomas Aquinas and Al-Ghazali. Their arguments, though rooted in different religious traditions, share a remarkable common purpose: to demonstrate, through logic and observed phenomena, that the universe demands a creator.

What is scholasticism and why does it matter? 🔗

Scholasticism was not a single philosophy but a method – one that placed dialectical reasoning and logical rigor at the center of theological inquiry. Medieval scholastic thinkers worked against a background of fixed religious dogma, yet sought to solve general philosophical problems – including the provability of God’s existence – through reason rather than faith alone. It emerged from the monastic and cathedral schools of Europe, which became centers for translating and studying medieval Judeo-Islamic philosophies, and in doing so “rediscovered” the works of Aristotle. The recovery of Aristotle’s corpus in Latin translation proved transformative. Scholastic theologians and philosophers endeavored to harmonize Aristotle’s metaphysics with Latin Catholic theology, and these monastic schools eventually became the bedrock for the development of the earliest European medieval universities.

The central ambition of scholasticism was the reconciliation of faith and reason. Medieval scholars grappled with a fundamental question: can faith and reason be compatible? Scholasticism emerged as a way of thinking that sought to examine and explain religious concepts through the power of logic. This was not a tension unique to Christian Europe. Across the Mediterranean, Islamic theologians were engaged in the same project – and their work would directly shape the arguments we examine here.

Thomas Aquinas and the Five Ways 🔗

Thomas Aquinas (c. 1225-1274) stands as the greatest figure of 13th-century European philosophy and theology, epitomizing the scholastic method of the newly founded universities. Born into a noble Italian family, he joined the Dominican order and studied under Albert the Great in Cologne, before producing a body of work that would define Catholic intellectual life for centuries. He argued that God is the source of both the light of natural reason and the light of faith, and sought to synthesize Aristotelian philosophy with the principles of Christianity.

His most concentrated contribution to the philosophy of religion is found in the Summa Theologica (1265-1274), where he presents what he called the Five Ways (Quinque Viæ) – five logical demonstrations of God’s existence drawn from features of the observable world. Aquinas developed a theological system that synthesized Western Christian theology with the philosophy of Aristotle, particularly as it had been interpreted by Aristotle’s later Islamic commentators. The Five Ways are examples of natural theology – a concerted attempt to discern divine truth in the order of the natural world.

Aquinas himself did not describe these as “proofs” in the modern mathematical sense, but as demonstrations suited to metaphysics and natural theology. They presuppose a wider Aristotelian metaphysics, including the distinction between act and potency and the notion of essentially ordered causal series. The arguments move from observed effects – motion, causation, contingency, gradation, directed behavior – to a necessary, intelligent first cause.

The argument from motion (First Way) 🔗

The First Way begins with the simple, observable fact that things in the world are in motion or undergoing change. In the world, we can see that at least some things are changing. Whatever is changing is being changed by something else. If that by which it is changing is itself changed, then it too is being changed by something else. But this chain cannot be infinitely long, so there must be something that changes others without itself changing. This unchanging changer – the Unmoved Mover – Aquinas identifies as God. Crucially, this is not an argument about who started the universe at some point in the past; it concerns what sustains motion and change right now, at every moment.

The argument from causation (Second Way) 🔗

The Second Way mirrors the First but focuses on efficient causation rather than motion. Every event has a cause, and that cause has a cause of its own. Events do not just happen – there is something that caused them to occur, and in order to have been caused, there must have been a cause before it. Since an infinite regress of causes is impossible, there must be a First Cause – uncaused and necessary – which Aquinas identifies as God. This second argument is the most widely discussed in modern philosophy and is closely related to what is now simply called the cosmological argument.

The argument from contingency (Third Way) 🔗

The third demonstration is the argument from contingency, which Aquinas advances by distinguishing between possible and necessary beings. Possible beings are those capable of existing and not existing – if every being were merely possible, there would be a time at which nothing existed, and then nothing could exist now, because no being can come into existence except through a being that already exists. Therefore, at least one necessary being – one that cannot fail to exist – must underlie all contingent reality. That necessary being is God.

The argument from gradation (Fourth Way) 🔗

The Fourth Way observes that things in the world possess qualities – goodness, truth, nobility – in varying degrees. Where there are degrees, there must be a maximum: a standard against which all lesser instances are measured. Aquinas argued that goodness or power in some finite object can only come from some other, greater source. This maximum, which causes all other instances of goodness and being, is God. While this is the least discussed of the Five Ways today, it reflects Aquinas’s deep engagement with Platonic as well as Aristotelian metaphysics.

The teleological argument (Fifth Way) 🔗

The Fifth Way is Aquinas’s teleological argument – his argument from design and purpose in nature. Aquinas’s fifth and final way is an argument from final causes, or ends, in nature. Drawing upon Aristotle, who held that each thing has its own natural purpose, Aquinas noted that some things – such as natural bodies – lack intelligence and are thus incapable of directing themselves toward their ends. An arrow does not aim itself; a bird does not consciously engineer its nest-building instinct. Yet natural things consistently act toward specific goals. This directedness toward an end implies an intelligent director – what Aquinas calls God.

It is important to distinguish this from modern Intelligent Design arguments. The Fifth Way establishes that natural bodies operate toward ends through inherent tendencies, not by chance, and the argument is distinct from Intelligent Design arguments, focusing on inherent teleology rather than biological complexity.

Al-Ghazali and the Kalam cosmological argument 🔗

While Aquinas was systematizing scholastic theology in Christian Europe, Islamic theology had already been grappling with the same fundamental questions for centuries. Among the most consequential contributions came from Abu Hamid al-Ghazali (1058-1111), a Persian Muslim theologian, jurist, and mystic widely regarded as one of the most important figures in Islamic intellectual history. Today al-Ghazali is regarded as one of the most important kalām scholars in Muslim intellectual history, making significant contributions to the cosmological argument in his works The Incoherence of the Philosophers and Moderation in Belief.

Al-Ghazali was concerned by the influence of Greek philosophy – which maintained a beginningless universe flowing necessarily out of God – on the Muslim philosophers of his day. He published a critique of this teaching in The Incoherence of the Philosophers, arguing that the universe must have had a beginning, and that if it had a beginning, there must be a transcendent creator who brought it into existence.

The structure of the Kalam argument 🔗

The Kalam cosmological argument is named after the Kalam – medieval Islamic scholasticism – from which many of its key ideas originated. Its central thesis is the metaphysical impossibility of a temporally past-infinite universe and of actual infinities existing in the real world. Al-Ghazali framed his reasoning with elegant simplicity. In his own words: “Every being which begins has a cause for its beginning; now the world is a being which begins; therefore, it possesses a cause for its beginning.” This gives us a three-premise structure:

• Premise 1: Whatever begins to exist has a cause.
• Premise 2: The universe began to exist.
• Conclusion: Therefore, the universe has a cause.

The first premise is grounded in universal causal intuition – nothing simply pops into existence uncaused. The second is the more philosophically demanding claim. Al-Ghazali argued that if the universe never began to exist, then there has been an infinite number of past events prior to today – but an actually infinite number of things cannot exist in reality. While a potentially infinite series is acceptable as an ideal limit, an actual infinity instantiated in the real world leads to logical absurdity.

Once the conclusion is granted – the universe has a cause – Al-Ghazali pressed further. Since the cause must be uncaused, and since an infinite regress of causes is impossible, there must be a First Cause that is absolutely uncaused. Conceptual analysis of what it means to cause an entire universe yields a cause that must be timeless, spaceless, and immaterial – a description that befits what most people mean by God.

What makes the Kalam distinct from Aquinas’s Second Way? 🔗

Both arguments conclude to a first cause, but their logic differs in an important way. The Kalam argument is distinguished from Aquinas’s Second Way, which rests on the impossibility of a causally ordered infinite regress, whereas the Kalam rests specifically on the impossibility of a temporally past-infinite universe. Aquinas, notably, did not believe that reason alone could prove the universe had a temporal beginning – he accepted creation as a matter of faith, not philosophical demonstration. Al-Ghazali, by contrast, made the temporal finitude of the universe a cornerstone of his argument.

The argument’s historical roots trace to early Islamic thinkers such as al-Kindi and Saadia Gaon in the 9th-10th centuries, and it was systematically formulated by al-Ghazali in the 11th century. Its transmission to medieval Christian thought occurred in the 13th century through Latin translations of Islamic philosophical works, influencing figures like Thomas Aquinas, who engaged with it in the Summa Theologica. The cross-fertilization between Islamic and Christian scholasticism was, in this sense, direct and formative.

How these arguments shaped each other – and us 🔗

The great representatives of Dominican thinking, and especially Thomas Aquinas, produced an artful synthesis of Greek rationalism and Christian doctrine that eventually came to define Catholic philosophy. But that synthesis was not built in isolation. Aquinas engaged with Averroes, Avicenna, and Maimonides, incorporating their interpretations of Aristotle into his theological synthesis. Al-Ghazali’s arguments reached Latin-speaking Europe through exactly this network of translation and commentary, entering into active dialogue with Christian scholastic thought.

The legacy of these medieval arguments is not merely historical. Philosopher and theologian William Lane Craig was principally responsible for reviving the Kalam cosmological argument for modern academic discourse through his book The Kalām Cosmological Argument (1979). The argument has since become, by one measure, the most discussed contemporary argument for God’s existence in academic philosophy journals. Meanwhile, the Five Ways of Aquinas have continued to shape neo-Scholastic revival, modern virtue ethics, and ongoing debates about causality and necessity in analytic philosophy of religion.

The teleological strand has found new life in the contemporary fine-tuning argument, which points to the precise values of physical constants necessary for a life-permitting universe – a development of Aquinas’s Fifth Way into the language of modern cosmology and physics. And the Kalam’s second premise has found unexpected empirical support: the Big Bang model places the origin of the universe at a finite point roughly 13.8 billion years ago, lending modern scientific weight to what al-Ghazali argued philosophically in the 11th century.

Criticisms and ongoing debates 🔗

Neither argument has gone unchallenged. Kant argued that the concept of a necessary being is incoherent, and that the cosmological argument presupposes its coherence. Hume argued that since we can conceive of causes and effects as separate, there is no necessary connection between them, and therefore we cannot necessarily reason from an observed effect to an inferred cause. More recently, critics have argued that the Kalam argument, even if sound, only establishes that the universe had some cause – not that this cause is a personal, omnipotent God.

Many contemporary analytic philosophers who discuss the arguments are not fully convinced by them, but the objections are not as straightforward as introductory philosophy textbooks suggest. Defenders of both Aquinas and al-Ghazali continue to develop sophisticated responses, engaging with quantum mechanics, the philosophy of time, and the metaphysics of causation. What the debate reveals, above all, is that the questions these thinkers raised – why does anything exist? what sustains the universe moment to moment? – remain genuinely open and philosophically serious.

The scholastic tradition, crossing the boundaries of Christianity and Islam, demonstrated that the deepest questions about existence need not be left to mysticism alone. By incorporating Aristotle’s logic and scientific thought into religion, Aquinas demonstrated the compatibility of reason-based philosophy and belief-based theology – and al-Ghazali had already been doing the same within Islam, a century and a half earlier. Together, they established a model of philosophical theology that continues to inform how believers and skeptics alike reason about the ultimate question: why is there something rather than nothing?

What do you think? If Aquinas’s Five Ways begin from observable features of the world and reason toward God, does the advance of modern science – which now explains many of those features differently – strengthen or weaken his case? And given that both Aquinas and al-Ghazali arrived at strikingly similar conclusions from within different religious traditions, does that convergence lend independent support to their shared conclusion, or simply reflect a shared Aristotelian inheritance?

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