Here's one way to state Plantinga's modal version of the ontological argument. argument. (Modal logic uses the three modes of being - possible, probable, necessary.)
- God, if he exists, is a necessary being. That is, if God exists at all then he exists in every possible world.
- It is [logically] possible that God exists.
- If it is possible that God exists, then there is some possible world in which God exists.
- If God exists in some possible world 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, then he certainly exists in this one.
- God, therefore, exists.