To the title, one might add, “for the existence of God.” And this is exactly the type of ontology this argument deals with. (Ontology is the study of existence or being.) This particular argument is by far one of the most complicated arguments ever proposed. However, I will try to present it as simply as possible and will analyze it as well as I can.
Anselm’s First Version of the Argument
The first version of the argument was presented by Anselm of Canterbury in the 12th century, and he actually came up with two different versions of the argument. Here is one argument, found in his Proslogium, shown in six steps:
1. God is conceptually a being than which none greater can exist.
2. God exists as an idea.
3. A being that exists in the mind and in reality is greater than a being that exists only in the mind.
4. If God exists only as an idea in the mind, then a greater being can be imagined.
5. However, we cannot imagine a being greater than God, since that would be a contradiction.
6. Therefore, God exists.
To many, this argument can appear cryptic and overly complicated at first. One of the reasons for this may be that it is completely logical; that is, it does not rely on any form of empirical evidence. With that said, I will do my best to explain these six points as simply as possible:
God is by definition a being than which none greater can exist. In other words, God is the greatest possible being, otherwise God would not be God. Since we were able to define God in this way, God obviously exists at the very least as an idea in the mind. However, a being that exists in both the mind and in reality is greater than a being that exists only in the mind, so if we imagine a God that exists only as an idea, that is not the greatest possible being. Because it would be a contradiction to say that there is a greater possible being than God, who is by definition a being than which none greater can exist, God must exist both in the mind and in reality. Therefore, God exists.
As you can see, this argument may still seem “funny.” Even if every single point checks out, to some, it may still feel wrong to say that God exists because God by definition must exist, which is essentially the argument. However, if you still find it unconvincing, it might help you to know that actually there is a premise in this argument that does not hold against deeper scrutiny, and that is point number three.
Now, the reason for this is that existence is not a property. This was pointed out by Immanuel Kant. Existence is simply necessary for an object to be instantiated. A property of God might be that he is all-knowing. However, to be all-knowing is not in the same category as merely to be, and to be is what existence means.
Anselm’s Second Version of the Argument
In Anselm’s second version of the argument, he avoids this flaw. Here is his other version of the ontological argument:
1. God is a being than which none greater can exist.
2. A being that necessarily exists is greater than a being that does not necessarily exist.
3. A being which only exists in the mind is not as great a being which necessarily exists in reality.
4. However, it is not possible to imagine a being greater than God, who is defined as a being than which none greater can exist.
5. It follows then that if God exists as an idea, Got necessarily exists in reality.
6. God exists as an idea.
7. Thus, God necessarily exists in reality.
If you’re wondering how adding the word necessary here and there makes a difference, let me explain. Existence is not a property, or to put it in another way, a predicate, since we could just say that God is. As you can see, there is no predicate. However, necessary existence is a property, since we could say that God is necessary.
Plantinga’s Modal Argument
Now, the argument, in my opinion, is better expressed modally. First, let’s explain what a modal argument is. A modal argument is an argument which makes use of four modes: impossible, possible, necessary, and contingent. In the case of God, God must either necessarily exist or necessarily not exist (be impossible).
Here’s how Plantinga put it:
1. Either God necessarily exists or does not (is impossible).
2. It is possible that God exists, meaning that God’s existence is not impossible.
3. Thus, God necessarily exists.
4. Therefore, God exists.
Of course, that is not exactly how he put it, but I have tried to explain it in my own words. Now, let’s dig deeper into how this actually works. The first premise is undoubtedly the most crucial: if it is impossible that God, defined as a maximally great being, exists, then the rest of the argument fails. On the other hand, if it is not impossible that a maximally great being exists, then God necessarily exists, implying that God indeed exists in all possible worlds. (Worlds here just refers to any conceivable world, including the real world.) However, it is not impossible that God exists in some possible world. In other words, we can imagine that a maximally great being exists. Therefore, it is possible that God exists, the second premise. If it is possible that God exists, then God necessarily exists. Therefore, God does indeed exist in every possible world. If God does not exist in every possible world, God is not maximally great, which contradicts the definition of God that he is a maximally great being. Thus, if God exists in one possible world, he exists in every possible world, including the real world. Therefore God exists.
I hope that I have made this argument clear. It is still, admittedly, confusing. Furthermore, I have certainly not listed every version of the argument, nor have I listed every attempt to refute this argument. Nevertheless, the argument, as far as I can see, appears to be sound and valid.