Philosopher Alvin Plantinga (b. 1932) has offered a form of the ontological argument utilizing modal logic such as “possibility” or “possible worlds”.

By possible worlds, one means refers to a world that is logically possible. Such a world is possible in that it does not contain, for instance, logical contradictions like two side triangles or married bachelors. Importantly such a world is not a real world but rather a construct used by philosophers to test concepts such as propositions and properties. Plantinga also uses what he calls a “maximally great being” which refers to a being who is morally perfect, omniscient, and omnipotent in every possible world. Plantinga’s modal ontological argument is as follows (1):

1. It is possible that a being exists which is maximally great (a being that we can call God),

2. So there is a possible world in which a maximally great being exists,

3. A maximally great being is necessarily maximally excellent in every possible world (by definition),

4. Since a maximally great being is necessarily maximally excellent in every possible world, that being is necessarily maximally excellent in the actual world.

5. Therefore, a maximally great being (i.e. God) exists in the actual world.

The argument is valid which means if its premises are true, its conclusion must also be true. This argument according to Plantinga does not provide conclusive proof that God exists, although there is nothing irrational or unreasonable about it. 

Premise 1 states that it is possible that a maximally great being (“God”) exists. Premise 2 refers to possible worlds in which this being exists. Premise 3 says that the definition of a maximally great being is necessarily morally perfect, omniscient, and omnipotent in every possible world. Anything less than the being possessing moral perfection, omniscience, and omnipotence means that such a being is not maximally great in all possible worlds. Premise 4 affirms that one of the possible worlds is the actual world. Therefore, if a maximally great being is necessarily maximally excellent in every possible world, that being exists in the actual world. The conclusion to the argument then follows that a maximally great being exists in the actual world.

Objections

There are several objections to the modal ontological argument (2). 

First, one can dispute whether or not a maximally great being exists. Some philosophers believe not. They refer to logical challenges to belief in God such as the problem of evil and suffering that counts strongly against the existence of a morally perfect, omniscient, and omnipotent being (3). Some critics argue that the traditional attributes ascribed to God are logically contradictory (e.g. that divine omniscience contradicts divine perfection), which means such a being cannot exist. If these objections are successful, then there is no possible world in which God exists.

A second objection concerns the cogency of the notion of possible worlds (4). Although almost all of us agree that the actual world exists, there is no universal agreement concerning the ontological or functional role possible worlds should play in metaphysical discussions,

“Consider this example. Jane Austen could have written a book about slavery in England in the eighteenth century. Or she could have written a book about the Trojan War. But does the fact that she could have written these books entail that they really exist in a possible world? What would it mean to say that they do? You cannot touch these books; you cannot read these books; you cannot even see these books. There is nothing you can do with these books because they are not real; they do not exist. So it seems odd to say that they exist in a possible world. If one of the reasons that novels by Jane Austen on slavery and the Trojan War do not exist is because nothing exists in a possible world, then it would be false to assert that God (i.e. a maximally great being) exists in a possible world. And if God does not exist in a possible world, then premise 2 of Plantinga’s argument is false, and the argument is unsound” (5).

Third, conceivability does not entail possibility. Plantinga’s modal ontological argument appeals to the conceivability of the existence of God: if God conceivably exists, then it is possible that God exists. This is controversial according to many philosophers and a critic might respond by rejecting the claim that conceivability entails possibility. He rejects the idea that if something is conceivable then it is possible for that thing to exist.

Another objection arises from atheist philosopher Michael Martin (b. 1932). Martin parodies the ontological argument by arguing that by using its logic one can also affirm the existence of fairies (6). Here a fairy is defined as a tiny woodland creature with magical powers that exists in every possible world. Martin’s argument runs as follows,

1. It is possible that a special fairy exists.
2. So there is a possible world in which a special fairy exists.
3. A special fairy is necessarily a tiny woodland creature with magical powers in every possible world (by definition)
4. Since a special fairy is necessarily a tiny woodland creature with magical powers in every possible world, that fairy is necessarily a tiny woodland creature with magical powers in the actual world.
5. Therefore, a special fairy exists in the actual world.

The success of Martin’s parody would depend on whether or not a “special fairy” can be considered a maximally great being. If it is not such a being, then it does not exist in every possible world and the parody fails.