Exploring the Ontological Argument

One of the most intriguing arguments for God’s existence is the ontological argument. Unlike most other arguments, this one doesn’t rely on any empirical premises.

The “fine-tuning” argument, for example, depends on the empirical evidence of intelligent design, namely, that life could not have developed in the way it has unless certain nomological conditions, such as some fundamental properties of the universe, were to be different to those that are actually observed. In a similar manner, arguments relating to the cosmology of the universe, rely on particular claims regarding the explanation for the occurrence of empirical events.

In contrast, the ontological argument depends solely on the ontological definition of God. It attempts to prove God’s existence, by His very definition.

As a Christian, I am convinced on a number of levels of the existence of God, both in terms of the empirical evidence on which certain arguments rely and on the logical deductions that are made to arrive at the notion of God’s existence from them. However, I’ve struggled with the ontological argument for years and am yet to find it fully convincing.

The Alvin Plantinga’s ontological argument is worded by William Lane Craig as follows:

  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.

In an attempt to come to a conclusion about this argument, I decided to contact Dr Kirk R. MacGregor, Assistant Professor of Philosophy and Religion at McPherson College. He allowed me to share our conversation and I am grateful to Kirk for that. I hope you find our discussion interesting and informative. 

I began by asking Kirk the following:

The ontological argument starts with the assumption that it is possible that God exists and then asserts that because it is, then God exists in some possible world, and therefore that he exists in every world (and ours).

However, is there not a difference between God’s possible existence and God’s actual existence in some possible world. In other words, could it be possible that God exists in some possible world, but that he may not exist in any possible world?
Further, even if He could exist in one possible world, He would only exist actually, if that possible world in which he exists is actual. Therefore, does the ontological argument not rely on the actual existence of every possible world?


Dr MacGregor:

The ontological argument indeed starts with the assumption that it is possible that God, understood as a maximally great being, exists.  If it is possible that God exists, then by definition there is a possible world in which God exists.

You’re first wondering if God (or anything) can possibly exist in a possible world as opposed to actually existing in that possible world.  The question is impossible to answer as stated because of the way possible world semantics works.  Since a possible world is a complete description of the way reality could be, any given entity is either part or not part of that description.  Hence something either does or does not exist in a possible world.  Those are our only two choices.  Strangely but truly, an entity neither possibly exists nor actually exists in a possible world.  If the entity just possibly existed in a possible world, then we wouldn’t know if it was part of the description.  If the entity actually existed in a possible world, then we’d be committing ourselves to the accuracy of the description, which we don’t yet know.

So we have the idea that a maximally great being is part of one complete description of the way reality could be.  But if that being is maximally great—and it’s greater to be part of every complete description of the way reality could be rather than simply one description—then that being is part of every complete description of the way reality could be.  Since one of these ways is the way reality actually is (the actual world), then a maximally great being exists in the actual world.  Notice that this reasoning does not rely on the actual existence of multiple possible worlds, which is self-contradictory on possible worlds semantics; only one possible world (complete description of the way reality could be) can be the actual world (the way reality actually is).

Accordingly, once we define “possible world” as philosophers do (and philosophers, myself included, are weird), then your queries can be solved.  Please feel free to write back with any follow-up questions.


My response:

As you appear to be saying, this argument relies on it being a property of a maximally great being to necessarily be a part of every complete description of the way reality could be rather than part of simply one description.

However, how can it be shown that a property of a maximally great entity that actually exists (namely, the God the argument intends to prove actually exists) is necessarily to exist in a possible world? Surely a maximally great entity is only necessarily required to exist in an actual world. In other words, it seems paradoxical that a property of an actually existing entity is necessarily something less than actual existence – namely existence in a possible world (possible existence).


DrMacGregor:

You write, “Surely a maximally great entity is only necessarily required to exist in an actual world.”  I say, quite the contrary!  I don’t think you grasp the implications of your assertion.  For if your assertion were true, then God would not have to exist in a world that were slightly different than the actual world, which seems preposterous.  To explain why, suppose it had snowed this morning in Kansas, where I live (it didn’t).  That would be a different world than the actual world.  But if a maximally great entity is only necessarily required to exist in the actual world (there’s only one, namely, ours), then atheism would be true had it snowed this morning in Kansas!Rather, for a being to be maximally great, it would need to exist no matter what the true description of reality is.  For the law of noncontradiction to be a law of logic, it needs to hold under every description of reality (i.e., in every possible world).  For the axioms of arithmetic to be mathematical laws, they need to hold under every description of reality (i.e., in every possible world).  So it seems no less plausible that for a being to be maximally great, it needs to exist under every description of reality (i.e., in every possible world).  Any being that would exist only under some but not all descriptions of reality is a contingent being, not a necessary one.  And the very concept of God is one of a necessary being.  So in answer to your question, “how can it be shown that a property of a maximally great entity that actually exists (namely, the God the argument intends to prove actually exists) is necessarily to exist in a possible world?” I think simple reflection on other entities like the laws of logic and mathematics accomplishes the demonstration.You write, “it seems paradoxical that a property of an actually existing entity is necessarily something less than actual existence – namely existence in a possible world (possible existence).”  I don’t think this is paradoxical at all.  Take, for example, me.  One of my properties is to possibly exist.  For if I couldn’t possibly exist, I couldn’t actually exist!  So anything that actually exists must have as one of its properties possible existence.  Simply put, actual existence presupposes possible existence; actual existence includes as one of its component parts possible existence.  So for a maximally great being (unlike me), it must have unconditional possible existence, which logically implies actual existence.As a philosophy professor, I know this is a tricky argument!  You’re doing much better than most of my students.  So let me know if there are any outstanding issues I need to clear up.  And yes, feel free to quote me in a blog.


My response:

I know how cyclic these conversations can get…so one final question if I may.

Could it be argued that it is possible that God does not exist and there is therefore a possible world in which God does not exist. Therefore, even if it were the case that if it is possible for God to exist He would exist in every possible world and therefore in the actual world, that therefore it is not possible for God to exist in any world, since there is a possible world in which He does not exist?

In other words, is the logical argument reducible to whether it is more likely that it is possible that God does exist or whether it is possible that God does not exist?

Further, is an implication of the Ontological argument that it cannot both be possible for God to exist and possible for God not to exist?

On a related note, could it therefore be argued that the Ontological argument is circular in that it assumes that it is not possible that God exists, and therefore assumes that He doesn’t exist?


Dr MacGregor:

Great question! It couldn’t be argued that “it is possible that God does not exist” because God is here defined as a maximally great being.  So “it is possible that a maximally great being does not exist” is self-contradictory, since it is greater to exist than not to exist.  If it might not exist, it wouldn’t be maximally great.  Defining God as a maximally great being is what prevents the argument from becoming circular.  One doesn’t assume that a maximally great being cannot fail to exist; rather, one intuits that a maximally great being failing to exist is just as self-contradictory as a square circle or a married bachelor.So what the argument boils down to is this: is it logically possible for a maximally great being to exist?  Yes or no?  If the answer is yes, then a maximally great being must, in fact, exist.  If the answer is no, then a maximally great being does not exist.  But the “yes” answer is intuitively the right one.  To be logically possible, something must contain no self-contradiction, and there certainly doesn’t seem to be any self-contradiction in the concept of a maximally great being.  Thus for the atheist to maintain their atheism, they need to affirm that it is not merely improbable but logically impossible for a maximally great being to exist.  Then the burden of proof is on them to show the incoherence of the concept of a maximally great being, which in my estimation can’t successfully be done.Indeed, an implication of the ontological argument is that God (a maximally great being) either necessarily exists (exists under any metaphysically possible description of reality) or does not exist at all.


My response:

Thank you again for that super helpful reply! 

I’m not going to ask another question simply for the reason that I promised that I wouldn’t, but if I were going to I might ask the following before ceasing to bother you any further:

The concept of a maximally great being is not self-contradictory and is therefore possible to exist. However, am I correct in thinking that, based on your comment that ‘a maximally great being failing to exist’ is self-contradictory, this argument requires actual existence to be a necessary attribute of a maximally great being which possibly exists? 

I accept that it may be the case that actual existence is a more positive attribute of an actual being. However, I can’t immediately see a reason why it is not at least possible that, when considering the existence of possible beings, actual existence is not a necessary attribute of a being with maximal greatness. How do we know that one’s existence is necessarily a positive attribute of its essence?

That’s what I’d ask if I were allowed a final question. 


Dr MacGregor:

What I’m suggesting is that the attribute in view is not existence simpliciter, but contingent existence versus necessary existence.  Although there has been debate since Kant about whether existence simpliciter is a predicate (attribute), there is no debate in the literature as to the fact that contingent existence and necessary existence are predicates.  So the issue is: which belongs to the very concept of a maximally great being?  Contingent existence or necessary existence?  It’s got to be one or the other, logically speaking.  To me, it’s obvious that it’s greater to necessarily exist than to contingently exist. 

So it’s a moot point whether or not existence (actual existence) is a predicate (attribute) of an essence.  Similarly, it’s a moot point whether or not we’re even talking about essences here (a highly disputed matter).  All that’s relevant is which attribute belongs to a maximally great being: contingent existence or necessary existence.

I hope this proves helpful!


I’m still working through a few of the finer points of this argument and hope you report at some point on where I come to. I hope you found this conversation helpful and my thanks again to Kirk for helping us out with this one!

Leave a comment