… and how Gaunilo, Aquinas, and Kant critiqued it, raising concerns about whether God’s existence can ever be proven through definitions alone

St. Anselm, Archbishop of Canterbury (1033–1109), was an Italian Benedictine monk, philosopher, and theologian. He is best known for his discovery of the “ontological argument” which affirms the necessity of God’s existence. There are many forms of this argument, but they all result in the same conclusion: it is fundamentally contradictory to deny the existence of the greatest possible being. This being, articulated and argued by Anselm, is the God of western theism.

Throughout history, many arguments have been advanced for the existence of God, such as the argument from intelligent design. Ontological arguments, by contrast, are entirely conceptual. Rather than appealing to external facts or causes, they derive their force from the very idea under consideration. In this case, it is the definition of God itself that is taken to demonstrate God’s existence.

For most existential claims, we rely on empirical observation. We look to nature or conduct studies in order to determine what is true or false. Ontological arguments, however, rely on concepts themselves. For instance, one does not need to search the world to determine whether a two-sided triangle exists; by definition, a triangle is a three-sided figure, and the very concept rules out any alternative. Anselm’s ontological argument for the existence of God works in a similar way: it seeks to demonstrate God’s existence purely through conceptual analysis.

In Anselm’s Proslogium (1078), he explains his argument:

[Even a] fool, when he hears of … a being than which nothing greater can be conceived … understands what he hears, and what he understands is in his understanding.… And assuredly that, than which nothing greater can be conceived, cannot exist in the understanding alone. For suppose it exists in the understanding alone: then it can be conceived to exist in reality; which is greater.… Therefore, if that, than which nothing greater can be conceived, exists in the understanding alone, the very being, than which nothing greater can be conceived, is one, than which a greater can be conceived. But obviously this is impossible. Hence, there is no doubt that there exists a being, than which nothing greater can be conceived, and it exists both in the understanding and in reality.

The Internet Encyclopedia of Philosophy breaks down Anselm’s ontological argument into six parts. Here is my rephrasing:

1. God is defined as the greatest possible being. By definition, nothing greater than God can be imagined, since God is all-great and all-powerful.
2. Because we are able to think about God, the idea of God exists in our minds.
3. Now imagine two versions: a) God as only an idea in the mind. b) God existing both as an idea in the mind and as a reality outside the mind. Clearly, the second option is greater, since it goes beyond the mind.
4. If God existed only in the mind, then we could conceive of something greater — namely, a God who exists both in the mind and in reality.
5. However, it would be impossible to conceive of something greater than God existing only in the mind, since by definition nothing can be greater than God. To think otherwise would be contradictory.
6. Therefore, God must exist not just as an idea, but in reality.

Anselm’s argument for the existence of God is still thought-provoking today. However, even at the time of his writing, the argument was not fully accepted. There are three general criticisms against the ontological argument.

First Criticism: Gaunilo of Marmoutier

The first major criticism to consider is Gaunilo of Marmoutier’s response to Anselm. A Benedictine monk and contemporary of Anselm, Gaunilo offered what has become one of the most famous objections to the ontological argument. His concern was that Anselm’s reasoning seemed to “speak” things into existence merely by definition. If that were the case, then any number of non-existent concepts could be arbitrarily declared real. To illustrate this, Gaunilo presented his example:

Now if some one should tell me that there is … an island [than which none greater can be conceived], I should easily understand his words, in which there is no difficulty. But suppose that he went on to say, as if by a logical inference: “You can no longer doubt that this island which is more excellent than all lands exists somewhere, since you have no doubt that it is in your understanding. And since it is more excellent not to be in the understanding alone, but to exist both in the understanding and in reality, for this reason it must exist. For if it does not exist, any land which really exists will be more excellent than it; and so the island understood by you to be more excellent will not be more excellent.”

Gaunilo mirrors Anselm’s own structure of reasoning in order to expose its flaw. If Anselm’s model works, then it could just as easily prove the existence of things that obviously do not exist. To make this point, Gaunilo imagines a “perfect island,” and reformulates Anselm’s logic in six parts:

1. It is true that the perfect island exists — an island that is the greatest possible island one is able to imagine.
2. Because we are able to think about the perfect island, the perfect island exists in our minds.
3. Now imagine two versions: a) the perfect island as only an idea in the mind. b) the perfect island exists both as an idea in the mind and as a reality outside of the mind. Clearly, the second option is greater, since it goes beyond the mind.
4. If the perfect island existed only in the mind, then we could conceive of something greater — namely, a perfect island that exists both in the mind and in reality.
5. However, it would be impossible to conceive of a more perfect island than a perfect island existing only in the mind, since by definition no island can be greater than the most perfect island. To think otherwise would be contradictory.
6. Therefore, the perfect island must exist not just as an idea, but in reality.

The weakness in Gaunilo’s critique lies in his first premise. Unlike God, the notion of a “perfect island” is inherently subjective and admits of endless variation. One person might imagine the perfect island as covered in coconut trees bearing the freshest coconuts, while another could improve upon this picture by making the island larger, with juicier coconuts and mango trees. No single version can be identified as the greatest conceivable island, because the perfect island has no natural maximum. God, however, is defined not by relative features but by absolute ones: omnipotence, omniscience, and moral perfection. These are intrinsic maxima, qualities that cannot be exceeded. For this reason, Anselm maintained that his argument applies uniquely to God and not to contingent or subjective ideals like Gaunilo’s island.

Second Criticism: St. Thomas Aquinas

St. Thomas Aquinas (1224–1274) was the most famous theologian of the 13th century. His Summa Theologica fused Aristotelian philosophy and Christian doctrine, offering profound arguments about the existence and nature of God. His response to Anselm’s ontological argument is particularly notable, because Aquinas raised two main criticisms.

First, Aquinas held that God’s existence is indeed self-evident in itself (because existence is God’s essence) but not self-evident to us. For this reason, he rejected the idea that one could deduce God’s existence purely from conceptual claims, as Anselm attempted. As Aquinas writes:

“… not everyone who hears this word ‘God’ understands it to signify something than which nothing greater can be thought, seeing that some have believed God to be a body.”

In this passage, Aquinas’ point is that Anselm’s argument only works if everyone accepts the same definition of God. Aquinas argues that people approach the idea of God differently, so it cannot be assumed that all minds will recognize God’s existence as self-evident. This creates a problem for Anselm’s ontological argument, which relies on the shared assumption that “God” means that which nothing greater can be conceived.

However, this criticism is problematic. Anselm’s argument does not rest on subjective definitions of God; rather, it begins from a single rational premise that aims to be universal. Let’s consider Anselm’s first premise:

God is defined as the greatest possible being. By definition, nothing greater than God can be imagined, since God is all-great and all-powerful.

Obviously, this premise could seem vulnerable to Aquinas’ criticism, since Aquinas claims not everyone shares the same definition of God. However, we can reformulate Anselm’s premise to avoid relying on any prior theological assumption. Instead of saying “God,” we can simply speak of a being than which no greater being can be conceived.

If we accept this description, then we are already committed to the idea that no greater being can exist. This definition functions independently of personal variations in how people understand “God.” Once we grant the premise, it follows that such a being must exist not merely in the mind but in reality — which is precisely what Anselm means by “God.”

Second, Aquinas argues that even if everyone has the same concept of God — per Anselm’s first premise — God still exceeds human understanding. Given this, Aquinas writes:

“…it does not therefore follow that he understands what the word signifies exists actually, but only that it exists mentally.”

Though this quote is a bit confusing, the general consensus is that Aquinas is rejecting the second premise of Anselm’s argument. When Anselm speaks of a being that is the greatest possible being of all time, Aquinas argues that it is impossible for the human mind to conceptualize this. As humans, we only have finite understanding, so we can only understand God in finite terms.

Aquinas’ second criticism still misses the mark because Anselm’s argument does not require that we fully comprehend the essence of God. All it requires is the recognition of the idea of a “greatest possible being.” The Internet Encyclopedia of Philosophy illustrates this point with the example of an infinite set of numbers. While we cannot physically see or exhaustively grasp all numbers — since for any number we can imagine, we can always add one — we nevertheless understand the general notion of infinity. Our limited understanding is enough to recognize that such a set exists in principle.

The same logic applies to Anselm’s concept of God. Even if we cannot comprehend God’s essence in its entirety, we can still understand the general idea of a being than which none greater can be conceived.

Third Criticism: Immanuel Kant

Immanuel Kant (1724–1804) was a German idealist philosopher and remains one of the most influential figures in the history of philosophy. His critique of Anselm’s ontological argument can be difficult to understand, so it is worth starting with Kant’s own formulation:

Being is evidently not a real predicate, that is, a conception of something which is added to the conception of some other thing. It is merely the positing of a thing, or of certain determinations in it. Logically, it is merely the copula of a judgement. The proposition, God is omnipotent, contains two conceptions, which have a certain object or content; the word is, is no additional predicate-it merely indicates the relation of the predicate to the subject. Now if I take the subject (God) with all its predicates (omnipotence being one), and say, God is, or There is a God, I add no new predicate to the conception of God, I merely posit or affirm the existence of the subject with all its predicates — I posit the object in relation to my conception.

Kant’s central objection is that existence is not a property (or predicate). In Anselm’s third premise, the claim is that a being that exists in reality is greater than a being that exists only in the mind. Here, existence is treated as a perfection — a quality that makes something “greater.” Kant argues this misconstrues what existence actually is. Logically speaking, existence is not a predicate you impose onto a concept. When we describe a book, for example, its concept includes properties such as having pages, a cover, being rectangular, and so on. If I say, “this book exists in the world,” I have not added a new property to the concept of the book. Existence does not change what a book is — it only changes whether the book is instantiated in reality.

Applied back to Anselm, the problem is evident: Anselm treats God’s existence as a property that elevates the concept of God into a superior reality. Kant’s point is that this misunderstands the role of existence. Existence does not make something greater — it is simply the necessary condition for that thing to have any properties at all.

Anselm’s Second Ontological Argument

Given various objections to Anselm’s ontological argument, it is fortunate that Anselm provided another ontological argument in the same text, Proslogium. He writes:

God is that, than which nothing greater can be conceived.… And [God] assuredly exists so truly, that it cannot be conceived not to exist. For, it is possible to conceive of a being which cannot be conceived not to exist; and this is greater than one which can be conceived not to exist. Hence, if that, than which nothing greater can be conceived, can be conceived not to exist, it is not that, than which nothing greater can be conceived. But this is an irreconcilable contradiction. There is, then, so truly a being than which nothing greater can be conceived to exist, that it cannot even be conceived not to exist; and this being thou art, O Lord, our God.

This revised version of the ontological argument begins with the same starting point as the first: God is the greatest conceivable being, and no greater being can be imagined. But Anselm’s second formulation introduces two important shifts.

First, rather than treating existence itself as a perfection — as the first version did — this version claims that necessary existence is a perfection. In other words, what makes a being truly greater is not merely that it exists, but that it cannot fail to exist.

Second, the argument distinguishes between two types of beings: contingent beings, which may or may not exist (for example, I might never have existed), and necessary beings, whose non-existence is impossible. On this view, contingent existence is inferior to necessary existence. Thus, a God who might not exist would be less great than a God whose existence is absolutely necessary.

Given these changes, we can now look at the formal argument (again, I’m rephrasing what the Internet Encyclopedia of Philosophy has written):

1. God is defined as the greatest possible being. By definition, nothing greater than God can be imagined, since God is all-great and all-powerful.
2. A being that necessarily exists is greater than a being that does not necessarily exist (i.e., a God who might not exist would be less great than a God whose existence is absolutely necessary).
3. If God is an idea in the mind that does not exist in reality, then it is possible to imagine something greater than God as this newfound thing would have necessary existence.
4. However, it is impossible to imagine something that is greater and more powerful than God.
5. If God exists in the mind, then God must exist in reality.
6. God is conceived of as an idea in the mind.
7. Thus, God necessarily exists.

Given the stakes of this revised argument, it effectively sidesteps Kant’s criticisms. Kant argued that existence itself is not a property and therefore cannot be treated as a perfection. But the updated ontological argument no longer relies on existence as such; instead, it appeals to necessary existence as a property. In this view, a being with necessary existence is greater than a being that exists contingently. A contingent being depends on something outside itself to exist and might not have existed at all. By contrast, a being defined as that which necessarily exists has existence as part of its essence — it cannot not exist. Thus, if God is defined as the greatest conceivable being, God must exist necessarily, since any being lacking necessary existence would be inferior.

• *Honorable Mention: Norman Malcom defended Anselm’s second ontological argument through and through. The examples he gives to demonstrate Anselm’s points are engaging and really well written. If you’re further interested in Anselm’s argument, this is definitely someone to check out.

Conclusion

Anselm’s ontological arguments continue to interest us today. For some, the arguments feel like frustrating, with many attributing the arugments to clever wordplay or semantic tricks. Yet the fact that some of the greatest philosophers in history — from Aquinas to Kant — have taken them seriously shows their philosophical force. Even now, a number of contemporary philosophers defend versions of Anselm’s reasoning.

The ultimate question remains: what premise, if any, goes awry? Which premise is mistaken — or does the logic hold after all?