Plantinga developed his modal version of the ontological argument for the existence of God in his two controversial books, The Nature of Necessity [1974: ch. 10] and God, Freedom and Evil [1975: part 2 c]. Here, Plantinga attempted to use the philosophical concept of possible worlds to show the necessary nature of God's existence. Since Leibniz first coined the term, 'possible world', in the seventeenth century, it has gained widespread attention. Debate about the meaning and significance of possible worlds to the discipline of modal logic remains current among philosophers. I will avoid discussion of controversial problems, such as transworld identity, because I do not think this is necessary for the purpose of this critique. I will rest content if I succeed in showing that Plantinga's attempt to appraise an existential proposition about what exists through the use of 'possible world' semantics cannot hope to succeed. For the purposes of this essay, I will focus on Plantinga's formulation of his argument in his God, Freedom and Evil. (For a substantive critique of Plantinga's more technically stated version given in his The Nature of Necessity, see Mackie .)
Plantinga progressed through a number of versions of his ontological argument. He examined each in succession, discarding them as he proceeded while repairing the weaknesses of each until he arrived at what, he claimed, is the final triumphant version. The last Achilles' heel he had designed to avoid was the argument's reliance on the concept of possible beings. He believed that there were too many inherent problems with this concept. As Plantinga [1975: 110] admitted, 'I am inclined to think the supposition that there are such things—things that are possible but don't in fact exist—is either unintelligible or necessarily false.' I will try to show below that even with the expunction of this ontologically questionable entity, Plantinga's final, revised argument fails.
Plantinga stated his final argument thus:
- There is a possible world in which maximal greatness is instantiated.
And the analogues of (27) and (28) spell out what is involved in maximal greatness:
- Necessarily, a being is maximally great only if it has maximal excellence in every world.
- Necessarily, a being has maximal excellence in every world only if it has omniscience, omnipotence, and moral perfection in every world.
. . . But if (29) is true, then there is a possible world W such that if it had been actual, then there would have existed a being that was omnipotent, omniscient, and morally perfect; this being furthermore, would have had these qualities in every possible world. So it follows that if W had been actual, it would have been impossible that there be no such being. That is, if W had been actual,
- There is no omnipotent, omniscient, and morally perfect being
would have been an impossible proposition. But if a proposition is impossible in at least one possible world, then it is impossible in every possible world; what is impossible does not vary from world to world. Accordingly (33) is impossible in the actual world, i.e., impossible simpliciter. But if it is impossible that there be no such being, then there actually exists a being that is omnipotent, omniscient, and morally perfect; this being, furthermore, has these qualities essentially and exists in every possible world.
[Plantinga 1975: 111–12]
Plantinga's argument here is very clever, displaying a high degree of intuitive plausibility. Nonetheless, it seems to have a bad smell about it. Isn't Plantinga simply defining God into existence, as Anselm did? Tooley immediately recognized this when he wrote:
The natural response to this is to ask Gaunilo fashion, what is special about maximal greatness. Let P be any property, and define the property of being maximally P as that property possessed by something if and only if it exists, and has P, in every possible world. If it is then granted that the property of being maximally P is possibly exemplified, it follows that it is exemplified. This will lead to a rather overpopulated world. It will also lead to contradictions, since one will be able to prove, for example, both that there is something that is a universal solvent, and that there is something that cannot be dissolved by anything. Or, more theologically, both that there is an omnipotent, omniscient, and perfectly good person, and that there is an omnipotent, omniscient, and completely evil person. Plantinga wisely refrains from discussing this obvious objection.
[Tooley 1977: 102]