Aquinas' Key Criticisms of the Ontological Argument Explained
Homework type: Essay
Added: today at 10:34
Summary:
Explore Aquinas’ key criticisms of the ontological argument to understand his challenges to existence as a predicate and the role of reason and experience in theology.
Aquinas’ Criticisms of the Ontological Argument
Debate concerning the existence of God stands as one of the enduring legacies of Western philosophy. Among the best-known attempts to settle this matter is the ontological argument, an a priori reasoning that aims to deduce God's existence from the very definition of divinity itself. Formulated in the eleventh century by St Anselm, then later refined by thinkers such as Descartes, the ontological argument suggests that once we properly grasp what is meant by ‘God’, it follows necessarily that God exists. This highly abstract form of argument has fascinated and divided thinkers for centuries.
Standing at the heart of medieval thought, Thomas Aquinas emerged as a central figure in theology and philosophy during the thirteenth century. Although he shared Anselm’s Christian faith and conviction in God’s existence, Aquinas questioned the validity of the ontological argument, instead favouring more empirical approaches. His critical engagement is not that of one opposed to faith, but of a thinker committed to rigour and clarity in the philosophical enterprise.
This essay examines Aquinas’ main criticisms of the ontological argument, drawing out his concerns about the role of definitions, existence as a predicate, and the need for observation in philosophical theology. It shall become clear that, for Aquinas, reason works in partnership with experience—rather than abstraction alone—when engaging with the question of God’s existence. Accordingly, Aquinas’ criticisms remain deeply relevant, challenging assumptions about logic and existence that continue to shape the philosophy of religion in the contemporary United Kingdom and beyond.
I. Understanding the Ontological Argument
The ontological argument starts not from the world or from personal experience, but from reflection upon a concept—the concept of God. As first expounded by St Anselm, Archbishop of Canterbury, the argument is structured as follows: God is ‘that than which nothing greater can be conceived’. It must be greater to exist in reality than only in the mind; thus, if God exists solely as an idea, God would not be the greatest conceivable being. Therefore, from the very notion of God, it is concluded that God must exist in reality.Underlying the argument are several key philosophical assumptions. It treats existence as a property that can be ascribed to the concept of a being, just as we might ascribe colour or shape. In this respect, it is an *a priori* argument, proceeding by reason alone rather than sense experience. The premise is that analytic truths—truths discoverable solely through analysing concepts—can establish existence.
Anselm’s original formulation found both advocates and critics. It was subsequently adopted and altered by philosophers such as René Descartes (albeit more influential in French and later European thought than specifically English tradition), who described God as a supremely perfect being; as perfection includes existence, non-existence becomes unthinkable. Yet, even before Aquinas, there were doubts. The English monk Gaunilo, for example, famously countered with the ‘perfect island’ parody, questioning whether perfection alone could guarantee existence. Still, it would be Aquinas who most powerfully crystallised the philosophical objections.
II. Aquinas’ General Approach to God’s Existence
Aquinas’ philosophical and theological views are rooted in a careful merging of Aristotelian reason with the Christian tradition. For Aquinas, faith and reason are not adversaries, but allies; correct use of reason can prepare the ground for faith and clarify its contents. Yet, he is precise in distinguishing what can be known through natural reason (“natural theology”) from what depends upon revelation (“revealed theology”).In exploring whether God’s existence is self-evident, Aquinas acknowledges that while to God Himself His existence is self-evident (because God’s essence is existence), this is not self-evident to us. Human intellects do not have direct access to the divine essence. Therefore, our knowledge of God must start from the things we know most immediately—the world around us.
Aquinas is thus famous for his ‘Five Ways’ (quinque viae), a series of empirical arguments beginning from motion, causation, contingency, gradation, and design, each pointing towards the existence of a first cause or necessary being: God. These are cosmological arguments, furnished from sense observation rather than pure definition. For Aquinas, it is through engagement with reality, starting from effects visible in the world, that we infer the cause who is God.
III. Key Elements of Aquinas’ Critique of the Ontological Argument
A. Existence Is Not a Predicate
A central plank of Aquinas’ critique is his contention that existence is not a ‘predicate’—that is, a real property added to the concept of a thing. While we can describe an object as blue or large, to say it exists is simply to affirm its instantiation. The idea that existence is a feature that makes something greater or a new kind of thing is, for Aquinas, a philosophical blunder.If we think of the concept of a unicorn, for instance, adding ‘exists’ to its definition does not make the unicorn real. Existence, unlike other predicates, does not give us anything new about its nature, only its being. This undermines the ontological argument’s leap from the definition of God’s essence to the claim that God exists in reality. Centuries later, Immanuel Kant would echo this point, stating, “existence is not a predicate,” often first encountered by UK A-level philosophy students as a crucial critique of Anselm’s reasoning.
B. Defining God Into Existence
Aquinas is suspicious of the idea that one can define things into existence. For example, we may have a clear concept of a ‘gold mountain’, but that does not bring such a mountain into the world. Understanding the idea of God is not the same as affirming God exists in reality. For Aquinas, there is a sharp divide between something being true “in intellectu” (in the mind or concept) and “in re” (in reality).To reinforce his argument, Aquinas sometimes draws upon analogies common in medieval education. For example, one can define a triangle as “a figure with three sides”, but such a definition does not itself create a real, existing triangle anywhere in the world. It is one thing to know what a triangle is; discovering one’s existence relies upon observation. Thus, Aquinas contends, conceptual necessity as expressed in the ontological argument is not the same as existential necessity.
C. Human Cognition and Its Limits
Another strand of Aquinas’ criticism stems from his reverence for the limits of human knowledge regarding God. For Aquinas, God is utterly transcendent—*ipsum esse subsistens*, subsistent being itself. Human minds, limited and partial, cannot fully comprehend the divine essence, nor can they grasp God’s existence through pure concepts alone. While we can know that God exists, we cannot know what God is in themselves.Aquinas reiterates the importance of indirect reasoning: we observe effects in the world—motion, causality, order—and from these infer an ultimate cause. Pure reason, unaided by experience, risks misunderstanding or misrepresenting the very nature of the divine.
D. The Necessity of Empirical Evidence
For Aquinas, knowledge of existence starts with the senses. He adopts the Aristotelian maxim, “nihil est in intellectu quod non prius fuerit in sensu”—“nothing exists in the mind that was not first in the senses.” This insists that sense experience provides all the raw material for our concepts.By seeking to prove God’s existence from mere definition, the ontological argument bypasses empirical observation, rendering it inadequate in Aquinas’ view. In keeping with the English tradition of empiricism later seen in Locke and Hume (even if those figures diverge sharply from Aquinas’ ultimate conclusions), Aquinas roots our knowledge firmly in observed reality. The ontological argument, in its abstraction, fails his test for valid knowledge: it lacks a secure connection to what actually exists.
IV. Wider Philosophical Impact
Aquinas’ criticisms were immensely influential, particularly in shaping the British philosophical tradition. Medieval scholars at Oxford and Cambridge, as well as later critics such as David Hume, would similarly emphasise the need for empirical grounding in philosophical arguments concerning God.While figures such as Descartes and Leibniz would seek to defend and refine the ontological argument, rationalism never thoroughly displaced the influence of Aquinas in the United Kingdom. The preference for arguments from experience and the focus on logical clarity can be seen not only in subsequent theology but in the empirical spirit of British philosophy as a whole. In the modern era, debates about whether ‘existence is a predicate’ loom large in the work of figures like Bertrand Russell, who also grapples with these issues in his writings.
Aquinas’ methodological legacy, therefore, is twofold: he reminds theologians and philosophers alike to distinguish between conceptual analysis and real-world existence, and he prioritises observation and reason working in harmony.
V. Addressing Misunderstandings about Aquinas
It is sometimes mistakenly claimed that Aquinas’ criticism of the ontological argument shows agnosticism or even hidden disbelief. Nothing could be further from the truth. Aquinas’ whole project is animated by deep faith, but he is scrupulous about deploying philosophical tools correctly. He does not reject God’s existence; rather, he seeks a sounder logical path to affirming it—one rooted in the world as we encounter it.“There is no reason why the same thing should not be self-evident in itself, though not to us,” writes Aquinas in his *Summa Theologica* (Part I, Question 2). He counsels humility in the enterprise of theology: our arguments are limited by our human capacities. Faith is not at odds with reason, but neither can faith be replaced by faulty logic.
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