What’s wrong with the ontological argument? That’s something I’ve been wondering. Now of course, as is the case with every theistic argument we give a generic name, the “ontological” argument is a type of theistic argument. I only intend to focus on Plantinga’s modal ontological argument (MOA). The MOA appears like this:
1. It is possible that a maximally great being exists.
2. If it is possible that a maximally great being exists, then a maximally great being exists in some possible world.
3. If a maximally great being exists in some possible world, then it exists in every possible world.
4. If it exists in every possible world, then it exists in the actual world.
5. Therefore, a maximally great being exists.
Many people dismiss this argument, thinking it’s some kind of parlor trick. However, it is an excellent example of modal reasoning. First, let us explain the argument and how the logic works.
A maximally great being is defined to be a being possessing maximal excellence, “where maximal excellence entails such excellent-making properties as omniscience, omnipotence and moral perfection.” By (1), Plantinga means to say such an idea involves no incoherence, and is intuitively possible. That is, it really could be the case such a maximally great being exists. Most people would agree with this. The second premise just explicates the idea of possibility in a format called “possible worlds.” A possible world is a complete description of the way reality could be, down to the last detail, encompassing every proposition’s truth or falsehood in a consistent manner. Hence, if something truly is possible, then it exists in a possible world.
(3) is the premise that confuses the average layperson. Why should it be so that if a maximally great being exists in one possible world, he exists in all? Because such a being holds its greatness and excellence in a maximal way, it would do so in every possible world (else there would be a greater being displaying more excellence—namely, the one who existed in all possible worlds).
The fourth premise is just true analytically. The actual world belongs to the set of all possible worlds. This is because if the actual world were not possible, it would not be actual! But then (5) follows, and the maximally great being does in fact exist.
So it seems the crucial premise is the first one. Epistemically, one may say it is possible God does exist and possible he does not. However, we are interested in metaphysical possibility. It is on these grounds one must object. That objection can come in two forms: A. The concept of a maximally great being is incoherent, or B. We do not (and/or cannot) know whether or not the concept of a maximally great being is metaphysically possible. With respect to (B), it’s not at all clear why we cannot justifiably intuit such a being is metaphysically possible (even if it’s not epistemologically compulsory that we do). With respect to (A), it seems the maximum values of what we would call “great-making” properties are coherent, and hence it follows they are metaphysically possible.
John Feinberg, a Christian theologian, remarks on his interest in the ontological argument but seems to show agreement with its critics. He wrote, “what . . . [the ontological argument] proved is that a contingent being could not be God. Any being worthy of the title ‘God’ must be a necessary being.” The philosophical rub Feinberg seemed not to catch, however, was this: in modal logic, a being or truth that is necessarily true means it is impossible not to exist. A necessary corollary or entailment of some being or truth being necessary is that if it does turn out to be false, it is necessarily so. So the MOA demonstrating the maximally great being’s necessary existence does more than give us a curious fact. Rather, it establishes that God’s existence is either necessary or impossible. Either the maximally great being is possible or impossible. This is why it is such a great argument!
 J.P. Moreland and William Lane Craig, Philosophical Foundations for a Christian Worldview. (
: IVP Academic, 2003), 496. Downers Grove, IL
 Again, without being able to explicate every property, it is difficult to force someone to accept the metaphysical possibility of the maximally great being. However, that would not be grounds to deny it. Further, even if we could explicate every property, it is only the lack of incoherence of which we are aware that allows us to hold our intuition.
 John S. Feinberg, No One Like Him: The Doctrine of God. (
: Crossway, 2001), 190. He was commenting on Anselm’s second formulation of the argument, but the MOA relies on the same conclusion on this point. Wheaton, IL
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