What is the Ontological Argument For the Existence of God?


The ontological argument traces back to Anselm of Canterbury (d. 1109 CE), the Father of Scholasticism who was partly a Neoplatonist and proponent seeking to square reason and faith (1). The ontological argument has also had some recent treatments, as we will acknowledge here.

St. Anselm of Canterbury and Scholasticism

Anselm and the scholastics were Christian thinkers active during the High and Late Middle Ages and within cathedral schools. Their goal was to establish an educated clergy and in the early thirteenth century, entire universities were being established in Paris and Oxford. There students would engage in the “scholastic method.” This would have students attend lectures, in particular on Aristotle and Plato, and engage in the art of oral disputations in which one defended an assigned position against objections posed by the class. The master or teacher would always resolve the matter at the end of the disputation. The purpose of the scholastic method was to familiarize students with argumentation and important authoritative texts, which together were seen as necessary skills in the development of rational and critical thinking. As the works produced by scholastic thinkers show, there were various responses to the ideas of Plato and Aristotle. Some, for example, tried to integrate Aristotle into Christianity while others tried to refute the things Aristotle said that did not seem to fit well with their religious doctrine. Many were attracted to Plato who posited such concepts as a soul and the transcendent Forms of which this world is only a poor reflection. When it comes to Anselm, although we are mostly interested in his ontological argument, it is important to acknowledge that he engaged many areas including the Trinity, logic, freewill, evil, truth, incarnational theology, and more. However, the ontological argument has persisted for almost a millennium and continues to find support in some philosophers today. Historically, we have thinkers such as Rene Descartes, G. F. W. Hegel, and Gottfried Leibniz referring to and using this argument, as have contemporary philosophers, notably Alvin Plantinga and William Lane Craig, and mathematicians, such as Kurt Godel. The ontological argument also has its critics, some of whom fault its logic yet admit difficulty in pinpointing where exactly the logic breaks down.

The Ontological Argument

The ontological argument seeks to prove the existence of God simply by reflecting on the idea of God. It thus differs from other theological arguments because it does not appeal to things or features within the world to prove God’s existence. For example, the Kalam cosmological argument appeals to the Big Bang to prove a first cause, the moral argument appeals to moral agency in humans, and the teleological argument to the design of the universe to support a designer. The ontological argument appeals to no such thing in the world and thus just to the idea of God.

The Soft Ontological Argument

There are hard and soft versions of the argument. The soft version posits God as that entity or being of which no greater can be thought. A being of which nothing greater can be thought must exist not only in the mind, but also outside of the mind. This is because a being that exists outside of the mind is greater than a being that only exists inside of the mind. Existing outside of the mind means that the being or object is more than merely an idea in one’s imagination. For example, a cheeseburger on my dinner table is greater than a cheeseburger solely existing within my mind as an idea. By the same logic, God existing outside of my mind is greater than a God who exists only in my mind. It then follows that God exists, not only as an idea but also as a being outside of the mind.

The Hard Ontological Argument

There is a harder version of the argument too. According to Anselm, believers hold that God is the greatest conceivable being, which is a being of whom nothing greater can be thought. Anselm uses the example of the “the Fool” from the Psalms who says in his heart that “there is no God.” However, although the Fool denies the existence of God he can still understand the expression “something than which nothing greater can be conceived.” This means that even the Fool, or the atheist, admits that a greatest conceivable being can exist in understanding. However, Anselm further observes how the Fool denies that there is anything more to the idea of the greatest conceivable being than its possibility. In other words, the Fool does not actually believe that there exists a greatest conceivable being. But Anselm argues that that the greatest conceivable being cannot just exist in one’s understanding, because by denying its reality the Fool becomes stuck in a contradiction: he is stuck by believing he can think of the greatest conceivable being but then also thinks he can contemplate a being even greater than this being; Anselm writes,

“If that than which nothing greater can be conceived exists in the understanding alone, that very thing than which nothing greater can be conceived is a thing than which something greater can be conceived. But this is impossible. Therefore it is beyond doubt that there exists, both in the understanding and in reality, a being that which nothing greater can be conceived” (2)

In both cases, we move from the idea of God to the existence of God. We will see a particularly helpful and modern version of the argument in Alvin Plantinga that makes the logic all the more clear.

Alvin Plantinga’s Modern Formulation

As noted, the ontological argument has had a long history of critical reflection which is partly evident in American analytic philosopher Alvin Plantinga’s version (3). Plantinga presents what he thinks is the strongest formulation of the argument:

1. It is possible that a maximally great being exists
2. If it is possible that a maximally great being exists, then a maximally great being exists in some possible world
3. If a maximally great being exists in some possible world, then it exists in every possible world
4. If a maximally great being exists in every possible world, then it exists in the actual world
5. If a maximally great being exists in the actual world, then a maximally great being exists.
6. Therefore, a maximally great being exists.

For a being to be maximally great (Plantinga’s version of the greatest conceivable being), it must be omnipotent, omniscient, all-good, and metaphysically necessary in its existence. To be necessary in its existence is for the being to exist in all possible worlds. If it exists in some possible world, then the maximally great being must exist in all possible worlds. It then follows that it must exist in the actual world, and thus it must exist. What Plantinga has helpfully done is move from the mere possibility of a maximally great being to the very existence of a maximally great being. William Craig succinctly captures the argument as follows,

“[T]he ontological argument is an argument intended to show that if God’s existence is even possible, then it follows that God exists. So, if this argument is sound it puts the atheist in a very awkward position. The atheist must deny not merely that God exists, he must maintain that it is impossible that God exists, and that is certainly a radical claim that would require great justification… So the argument is basically: if you think that God’s existence is possible then it follows that God exists” (5).

Criticisms of the Ontological Argument

What are some criticisms of the argument? The earliest criticism of the version Anselm put out came from a contemporary in the form of a Benedictine monk called Gaunilo (d. 1083). Gaunilo replies in his On Behalf of the Fool and attempts to render doubt on the idea of a greatest conceivable being. Gaunilo maintains that it is possible to populate the world with all sorts of greatest conceivable things that do not exist; he uses the example of an island and that it is possible to think of an island than which none greater can be conceived. Should one deny this, then one becomes trapped in a contradiction like Anselm’s Fool. But surely we do not have any good reason to believe in a greatest conceivable island.

A concern with Plantinga’s formulation is that it commits the fallacy of equivocation in its use of the word “possible.” There are at least two senses in which something can be said to be possible: in an epistemic sense and in an ontological sense. Plantinga uses both. Premise 1 uses the term in the epistemic sense. An example of possible in this sense would be that it is not inconceivable that in some possible world I may have been born a woman (although in the actual world I am a man). Similarly, it is not inconceivable that a maximally great being exists. However, Premise 2 uses possible in the ontological sense, namely the possibility of a maximally great being existing, thus committing the fallacy of equivocation.


1. Garvey, James., and Stangroom, Jeremy. 2012. The Story of Philosophy: A History of Western Thought. London: Hachette UK. p. 171-177.

2. Garvey, James., and Stangroom, Jeremy. 2012. Ibid. p. 176.

3. Einar Himma, Kenneth. Anselm: Ontological Argument for God’s Existence. Available.

4. YouTube. 2011. What Is the Ontological Argument? (William Lane Craig). Available.


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