1. If it is possible that something exists, then it does exist in a possible world.
  2. It is possible that a maximally excellent being exists in a possible world.
  3. Therefore, a maximally excellent being exists in some possible world.
  4. A maximally great being would be a maximally excellent being that exists in all possible worlds – i.e. there is no being that has excellence superior to its.
  5. It is possible that a maximally great being exists – i.e. that a maximally excellent being exists such that it has maximal excellence in every possible world.
  6. Therefore, a maximally great being exists.
  7. God is, by definition, a maximally great being.
  8. Therefore, God exists.

Alvin Plantinga offered a modal ontological argument for the existence of God as summarized above. If God exists in all possible worlds and we inhabit a possible world, then God exists in our world. Once someone has conceded that it is possible God exists, according to the logic outlined above, they must accept that God actually does exist.

This argument is not subject to the criticism that having an idea of a maximally great being does not actually entail that such a being exists. Nor is it subject to Kant’s criticism that existence is not a predicate: God’s existence is special because it is defined as existence in all possible worlds, which makes it a different species of existence from our own. For example, while it is possible that I exist in a possible world, it is also possible that I do not exist in another possible world.

The argument hinges on the possibility of a maximally great being. This is an a priori claim. Detractors might object that such a being is not possible, therefore the argument does not succeed. But note: this criticism does not challenge the validity of the argument but its soundness. They allege that this premise is false.

The burden of proof lies with the detractors: they must give a reason for why such a being is not possible. At this point arguments are introduced that allege the divine attributes are incompatible or that God is incompatible with the possible world we inhabit. Here are a few such challenges:

  • The paradox of the stone
  • Kretzmann on omniscience and immutability
  • Omnipotence and supreme goodness
  • The Euthyphro dilemma
  • Omniscience and human free will
  • The evidential problem of evil
  • The logical problem of evil

Theists have offered defences and theodicies in response to all of these arguments. In any case, much of this is actually a distraction: the ontological argument is not meant to show that God fits our (popular) conception of Him, but that a maximally great being exists. The coherence of maximal greatness and our complete understanding of maximal greatness are not the same issue.

We treat the concept “MULTIPLY” as coherent, but do we understand everything about it? If we accept that we lack complete knowledge of such a concept, but that this does not justify throwing the concept out, then by the same reasoning we must accept that our lack of complete knowledge regarding “MAXIMAL GREATNESS” does not entitle us to throw it out.

Moreover, transcendence is part of the concept of God. Transcendence can be understood in relation to ontology (God is beyond the universe) and also in relation to epistemology (God is beyond our ultimate comprehension). This does not amount to special pleading because transcendence is part of the definition of God: God is not coherent if God lacks transcendence, whereas transcendence is not part of the definition of, say, APPLE.

We must also consider the coherence of challenging God’s existence. Now this does not show that God actually does exist, but as Quine has pointed out, it does show that we are intellectually committed to God’s existence – He is a necessary presupposition for our reasoning. This results in a curious outcome: atheism – even if true – is unjustifiable. Most epistemologies include some kind of justification as a necessary condition for knowledge: the tripartite theory; infallibilism; virtue epistemology; reliabilism; Nozick’s “tracking the truth” reliabilism. The nature of justification is disputed, but most accept that something gives validity to our true beliefs to convert them into knowledge – some principle “guides” us into truth and that principle, even if it is built on truth, is not identical to it.

The question, “Is it possible that God exists?”, is incoherent. God is defined as a necessary being, whereas possibility speaks of contingency. Possibility itself is contingent on God, not God on possibility.