St. Anselm’s Ontological Argument

Over the past few weeks, my son has been telling me about St. Anselm’s ontological argument for the existence of God. Last weekend I decided to play around with trying to formalise a version of this argument mathematically. This post briefly records my thoughts on the matter. I make no claim to originality, nor have I undertaken a review of what other people have done in this area; this was just a fun way to spend some of the weekend. I would also caution that I am neither a philosopher nor a logician, however I am a user of formal logic. If any philosophers or logicians happen across this blog post and strongly disagree with anything I’ve said, it would be fun to hear about it and I’d love to learn more. It is entirely possible that I’ve committed some cardinal sin. Pun intended.

The version of the argument given in the Internet Encyclopaedia of Philosophy is as follows:

  1. It is a conceptual truth (or, so to speak, true by definition) that God is a being than which none greater can be imagined (that is, the greatest possible being that can be imagined).
  2. God exists as an idea in the mind.
  3. A being that exists as an idea in the mind and in reality is, other things being equal, greater than a being that exists only as an idea in the mind.
  4. Thus, if God exists only as an idea in the mind, then we can imagine something that is greater than God (that is, a greatest possible being that does exist).
  5. But we cannot imagine something that is greater than God (for it is a contradiction to suppose that we can imagine a being greater than the greatest possible being that can be imagined.)
  6. Therefore, God exists.

Our first challenge is to understand what it means to be “an idea in the mind”. I tend to think of an idea in the mind as a description of something and, formally, as a logical formula defining a predicate. (I am not sure if this is what others think!) For the sake of simplicity, I will consider in the below “an idea in the mind” to simply mean a predicate defined on “physical things”, but keep in mind that when I refer to a predicate below, I’m really referring to the equivalence class of formulae defining this predicate.

The reading of ideas as formulae (or predicates) makes it hard to say what we mean when we compare two things, one which exists and one which doesn’t, in Line 3 of Anselm’s argument, though, an issue we will address below.

Finally, to my way of thinking, there appears to be an unnecessary implicit focus on uniqueness in the way the argument presented above is given, so let’s strip that away to simplify matters, so we just want to focus on a predicate which may be taken to describe ‘godliness’, or ‘godlike things’ – call it \Theta.


Taking the above arguments together, I will redefine the first premise as “the concept of godliness is a concept of which there is none greater.”

The other unnecessary element in this argument seems to be the use of language to draw the reader into trying to define ‘greater’ as some kind of second-order predicate denoting an order (of some kind) on first-order predicates. We will see below that this is not necessary, so we will consider ‘greater’ as simply referring here to an arbitrary 2-place second-order predicate G. Thus we obtain

1. \neg \exists P G( P, \Theta ).

We obtain Premise 2 directly by simply stating Premise 1 formally, given our conception of ‘an idea in the mind’.

Premise 3 is interesting. In our reformulation, we have a few approaches to this premise depending on what is meant by “other things being equal”. It seems that the weakest interpretation would be “for any concept describing something that doesn’t exist, there’s some greater concept”:

3a. \forall P. (\neg \exists x P x) \rightarrow \exists Q G(Q, P).

(Note that here, and in the following, I’ve used the convention that a ‘dot’ indicates an opening bracket that is closed at the end of the statement.)


The proof then follows Lines 4-6 directly:

From Premise 3a, conclude \neg \exists x. \Theta x \to \exists Q G(Q,\Theta) via universal instantiation. Then using Premise 1 and modus tollens, one obtains \exists x. \Theta x.

In English: we may conclude from Premise 3a that if a god does not exist then there exists a concept greater than godliness. But since there is no concept greater than godliness (Premise 1), a god exists.

What does this mean?

This argument is valid, but what is its semantic content? Let’s try to build some ‘unorthodox’ models to see if we can satisfy these premises and understand the rigidity these premises provide. In this section I will use \llbracket \phi \rrbracket to denote the interpretation of a formula \phi in a model, and I will use \Xi to denote the false predicate, i.e. \llbracket \Xi \rrbracket(x) is false for all x.

One simple model has (M1) \llbracket G( P, \Xi ) \rrbracket true for any predicate P with different interpretation to \Xi, (M2) \llbracket G( P, Q ) \rrbracket false for all other predicate pairs, and (M3) \llbracket \Theta \rrbracket(x) true for all x. Let’s verify the premises under this model. Since \Theta and \Xi differ in interpretation, Premise 1 is true by (M2 + M3). Equally, Premise 3a is true by (M1). What’s this model (one of several) actually saying? “Everything is godlike! A godlike thing exists!”. So quite reasonable logically, but perhaps not in line with a typical reading – perhaps it can be taken as a pantheistic reading!

Another simple model has all the same interpretations as above but with \llbracket \Theta \rrbracket(x) true for exactly one element. This model is saying “There’s a unique existent God! But the concept of God is no greater than the concept of any other existent thing.”

Both of these models do actually have G as a partial order, though there are also models where it is not. Even if we were to impose strict totality on G, then there are minimal restrictions on the order: we require only that godliness is at the top.

In general, in every model, the interpretation of G is one for which there is an epimorphism \delta from \llbracket G \rrbracket onto a specific relation on \{\top, \bot\}. Then the relation in question is \Gamma = \{ (\top,\bot), (\bot,\bot) \}. We also require \delta(\Theta) = \top and \delta(\Xi) = \bot. Another way of saying this is that any relation G will do, so long as its digraph can be partitioned into two vertex subsets: a nonempty set containing zero indegree nodes (including \Theta), and a nonempty subset containing the rest (including \Xi). Note that to achieve this, the interpretation of \Theta and \Xi must necessarily differ, i.e. a god must exist.

It appears that Anselm’s argument has basically shifted the problem ‘existence of God’ onto the problem of “what does ‘greater’ mean?” The special relation described in the preceding paragraph forms the archetype of acceptable answers to the latter question: ideas that are greatest must be those that describe an existing thing.

“Other things being equal”

Let’s go back to the line ‘other things being equal’. The best I can muster here as an alternative interpretation of Premise 3 taking this line into account is that “for every concept P describing something that doesn’t exist, there’s some greater concept describing only those things described by P that also exist”:

3b. \forall P. (\neg \exists x P x) \rightarrow \exists Q. G(Q, P) \wedge \exists x Q(x) \wedge \forall x. Q(x) \rightarrow P(x).

However, (3b) is clearly unsatisfiable: For an arbirary predicate \Pi, assume the antecedent \neg \exists x \Pi x. Then the third conjunct enforces \neg \exists x Q(x), which directly contradicts the second conjunct.


I have provided two readings of St. Anselm’s argument, based on simplification away of uniqueness of “things possessing godliness” and using formulas (second-order predicates) to discuss “ideas in the mind”. The difference of these two readings hinges on the interpretation of the phrase “other things being equal”. One reading leads to a variety of models, including a model in which every (real) thing has godliness. The other reading renders Premise 3 of St. Anselm’s argument unsound. Neither reading makes strong use of any order properties relating to the phrase ‘greater than’, however the first reading is satisfiable exactly when existence is encoded within the concept ‘greater than’.

Quite an interesting way to spend a few hours – thanks to my son for interesting me in this!

2 thoughts on “St. Anselm’s Ontological Argument

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