The ontological argument

Donna asks:

Could you summarize the logic of the Ontological proof?

Answer by Craig Skinner

The ontological argument has been flogged to death but just wont lie down. Anselm’s original was dismissed by Aquinas because it confuses a true semantic claim “God (necessarily) exists” (true by definition of the word “God”) with a possibly false existential claim “(Necessarily) God exists”, a simple logical fallacy (changing the scope of the modal operator from de re to de dicto, to be technical about it). Then Kant dismissed the argument on the grounds that “existence” is not a property which an entity may or may not possess, but a prerequisite for an entity to have any properties at all.

I shall deal with the modern version of the argument.
As ever we define God as a necessarily existing being, then proceed:

P1. If God exists his existence is necessary.
P2. If God doesnt exist his existence is impossible.
P3. Hence God’s existence is either necessary or impossible.
P4. God’s existence is possible (not impossible).
P5. Hence God’s existence is necessary.
Conclusion: God exists.

But note, the argument just as readily “proves” God’s nonexistence:

P1. If God is nonexistent his nonexistence is necessary.
P2. If God isnt nonexistent his nonexistence is impossible.
P3. Hence God’s nonexistence is either necessary or impossible.
P4. God’s nonexistence is possible (not impossible).
P5. Hence God’s nonexistence is necessary.
Conclusion: God is nonexistent.

The problem is P4. It begs the question. Clearly God’s existence (nonexistence) is only possible if he exists (doesnt exist). All we can really conclude is that if God exists his existence is necessary, if he doesnt his existence is impossible, but we dont know whether God exists or not.