The Modal Ontological Argument
The most prominent modern advocate of the ontological argument is Alvin Plantinga. Plantinga is best-known for his defence of the view that religious belief is foundational, i.e. that religious belief does not stand in need of external justification, but is also known for his work on modal logic, i.e. on the logic of possibility and necessity. Plantinga applies his approach to modal logic to the ontological argument, presenting the argument in a revised form.
Plantinga favours a possible world analyis of statements about possibility and necessity. Possible worlds are ways that the world might have been. Any logically consistent description of a world is a possible world. On Plantinga’s view, to say that something is possible is to say that there is a possible world in which it is actual, and to say that something is necessary is to say that in every possible world it is actual.
The modal ontological argument, like Anselm‘s, begins with a statement about God. God, if he exists, is a necessary being. That is, if God exists at all then he exists in every possible world.
The next element in the modal ontological argument concerns the possibility that God exists. It is possible that God exists, according to the modal ontological argument. These two claims are sufficient, according to the modal ontological argument, to establish the existence of God.
For if it is possible that God exists, then there is some possible world in which God exists. If God exists in some possible world, though, then, because he is a necessary being, he exists in all possible worlds. God, then, exists in all possible worlds. If God exists in all possible worlds, though, then he certainly exists in this one. God, therefore, exists.
A more formal analysis of this argument goes like this:
(1) If God exists then he has necessary existence.
(2) Either God has necessary existence, or he doesn‘t.
(3) If God doesn‘t have necessary existence, then he necessarily doesn‘t.
(4) Either God has necessary existence, or he necessarily doesn‘t.
(5) If God necessarily doesn‘t have necessary existence, then God necessarily doesn‘t exist.
(6) Either God has necessary existence, or he necessarily doesn‘t exist.
(7) It is not the case that God necessarily doesn‘t exist.
(8) God has necessary existence.
(9) If God has necessary existence, then God exists.
(10) God exists.
The first premise is based on the idea that God is perfect, and that something is better if it has necessary existence than if it has merely contingent existence.
The second premise of the argument is simply the law of the excluded middle.
The third premise, “Becker’s Postulate”, is a widely accepted principle of modal logic. All modal properties are generally accepted to be necessary.
Four follows straightforwardly from the second and third premises.
Five is entailed by premise one.
Six follows from four and five.
Seven is plausible at first glance, but is widely thought to be the greatest point of weakness in the argument.
Eight follows from six and seven.
Nine is self-evident.
Ten follows from eight and nine.