What Is the Ontological Argument for God’s Existence?

The ontological argument is the justification for God’s existence, and it remains a hotly debated topics within philosophy today.

Jul 29, 2023By Luke Dunne, BA Philosophy & Theology

ontological argument for gods existence god father eyck


Attempting to offer a philosophical explanation or justification for the existence of God is one of the most sustained philosophical exercises. The ontological argument is one of the most famous such attempts, as well as the most distinctive. But what is the ontological argument for God’s existence, and how good an argument is it, really? We take a closer look.


An a Priori Argument

logic painting met museum 19th century
Logic (one of a pair), possibly British (anon), late 18th-mid 19th century, via The Met


The ontological argument for God’s existence is an argument which is not based on any observation about the physical structure of the universe and God’s place in it. Indeed, it is an argument which does not rely on any kind of observation, whatsoever. Rather, it is an argument which justifies God’s existence in a purely rational way. It is an a priori rather than a posteriori argument. An a priori argument is one which uses deductive reasoning only. No new observations are required to make this argument – it can be made by virtue of what we already know, or what is latent within what we already know. Not all deductive arguments are simple. Indeed, many forms of deductive reasoning require us to understand formal systems which are used to represent deductive arguments. 


St Anselm’s Version of the Ontological Argument

Statue of St Anselm John Salmon
Statue of St Anselm, John Salmon, via Wikimedia Commons


However, the ontological argument is mercifully simple (at least to relate). The first statement of this argument is that of St Anselm. During his long career as a leading churchman (he was the Archbishop of Canterbury, which was and remains the highest post in the English Church), Anselm was also a scholar and intellectual. We can quote him directly to give a sense of how simple the argument was even in his initial version of it:


“[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… [and so] … 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.”

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Let’s try to clarify the steps of this argument. The argument resolves itself in the conclusion that there is no doubt that there exists the greatest possible being, but how defensible are the premises from which this conclusion can be drawn? 


Objections Against the Ontological Argument

God the creator and angels Pietro Perugino 1508
God the creator and angels, Pietro Perugino, 1508, via WikiArt


First, there is an assertion that God is the greatest thing which could possibly exist (“than which nothing greater can be conceived”). The plausibility of this argument to an atheist is, clearly, very limited. Indeed, this constitutes one of the first criticisms of the ontological argument – that it is an argument which can only be used to persuade those who already adhere to some theistic worldview. Of course, Anselm lived at a time when belief in God was utterly ubiquitous, and the purpose of creating an argument for God’s existence was not to show that God exists as such, but rather to give the best possible justifications for his existence.


For Christians like Anselm, as for many believers of various faiths, it is intrinsic to the nature of God that He is the greatest possible thing – the most powerful, knowledgeable, good and so forth. Second, there is the assertion that for something to be the best thing, it should also exist. Another popular line of criticism focuses on this part of the argument. Immanuel Kant famously held that ‘existence is not a predicate’. A predicate can be seen simply as a descriptive quality a thing might have. Kant’s point was that being the greatest conceivable thing is a matter of having the best version of any given predicate, but that to exist is not a quality of the same kind.

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By Luke DunneBA Philosophy & TheologyLuke is a graduate of the University of Oxford's departments of Philosophy and Theology, his main interests include the history of philosophy, the metaphysics of mind, and social theory.