Interestingly, there is an argument used by atheists to demonstrate that God is impossible, which picks up on the ontological argument. This argument is traditionally called the reverse ontological argument. Instead of demonstrating that God a maximally great being that exists necessarily, the reverse form is used to demonstrate that God is impossible. To give a context for the atheistic argument here are the two most popular versions of the theistic ontological argument:
The Anselmian Ontological Argument (Theistic)
- God exists in the understanding
- God is a possible being
- If X exists only in the understanding and is a possible being, then X might have been greater
- Suppose God exists only in the understanding
- God might have been greater (2, 4, 3)
- God is a being than which a greater is not possible
- So, a being than which nothing greater is not possible is a being which is greater is possible
- Since 4 led to a contradiction 4 must be false
- God exists not only in the understanding alone—God exists in reality as well
- Existence in reality is a great making property
- The argument is a reductio ad absurdum. To prove X assume ~X. Show how ~X leads to a contradiction or patent falsehood.
The Plantigan [Modal] Ontological Argument (Theistic)
- The property of being maximally great is exemplified in some possible world.
- The property of being maximally great is equivalent, by definition, to the property of being maximally excellent in every possible world.
- The property of being maximally excellent entails the properties of omniscience, omnipotence, and moral perfection.
- A universal property is one that is exemplified in every possible world or none.
- Any property that is equivalent to some property that holds in every possible world is a universal property.
- Therefore, there exists a being that is essentially omniscient, omnipotent, and morally perfect (God).
The Reverse Ontological Argument (Atheistic)
- It is possible that God (MGB) does not exist.
- If it is possible that God doesn’t exist, then God doesn’t exist in some possible worlds.
- If God doesn’t exist in some possible worlds, then God doesn’t exist in all possible worlds.
- If God doesn’t exist in all possible worlds then God doesn’t exist in the actual world.
- If God doesn’t exist in the actual world the God does not exist.
The problem is in premise two–MGB must be demonstrated to be incoherent. Now, when the atheist is defending the argument he may certainly attempt to demonstrate that a MGB is incoherent but that’s the task before him. There certainly isn’t an explicit contradiction so an implicit contradiction must be explicated. This goes back to premise 1 of the theistic ontological argument while not dealing with the criticisms needed to justify premises one and two. It must be demonstrated that God is logically absurd before finishing premise one. All the theist must do is defend the case that God’s essence (and indexed properties) are logically compossible. This is the only objection the atheist can make (at least in the modal form–arguing against S5 probably isn’t the best move either…).