The Ontological Argument for the existence of God is in my opinion a terrible argument. Of course, by smuggling in Christian premises, God is indeed the most perfect being, but for the argument to function as an apologetic, one must not consider Christians truths as premises in an argument meant to convince unbelievers. In light of the mainstream scientific theory of evolution and the history of the universe, the ontological argument sounds even more far-fetched than it was in the time of Anselm and Aquinas.
More sophisticated versions of the ontological argument attempt to ground the ontological argument not in theories about "being" but about existence or possible worlds theory. In the possible worlds theory, the premise is stated that God exists in at least one possible world, and therefore from there it is argued that God exists. But what does it mean to say that "God exists in at least one possible world"? If it is meant that there is such a possible world that can be conceived in the mind, then we run into similar problems as Anselm's original argument. Mental conception implies nothing about whether something is possible in reality. [Of course, conversely, inability to be conceived mentally implies nothing about whether something is possible in reality — the doctrine of divine incomprehensibility and ineffability]. One could say that "God exists in at least one possible mentally conceived world" but that is not the same as saying "God exists in at least one possible real world."
The argument from existence (from Norman Malcom) has the following form:
- If God exists, his existence is necessary.
- If God does not exist, his existence is impossible.
- Either God exists or he does not exists.
- Therefore, God's existence is either necessary or impossible.
- God's existence is possible (not impossible).
- Therefore, God's existence is necessary.
[C. Stephen Evans and R. Zachary Manis, Philosophy of Religion: Thinking about Faith (2nd ed.; Contours of Christianity Philosophy; Downers Grove, IL: IVP, 1982, 2009), 65]
As the authors of this book had pointed out, premise 5 is questionable. And to clarify further, the doubting of premise 5 is only predicated of the descriptor of God as one necessarily existing. In other words, what this version of the ontological argument proves is that a God with necessary existence either exists or is impossible to exist. But as we can see, that is a tautology.
The ontological arguments I have seen thus far either suffer from ideas about "being" or ontological attributes that are disputed as to their possible perfection or existence, or they become tautologies. I do not see any way such arguments can actually function in any context, and thus we should stop using them altogether.