Plantinga's Ontological Argument

4. Why Plantinga's Ontological Argument
Is Persuasive

Book cover: Critique of Pure Reason by Immanuel Kant

The interesting question is, What is it about Plantinga's modal formulation of the ontological argument that gives it its intuitive plausibility? To answer this question, first note that the only difference between the structure of my 'invincible wizard' argument in the previous section and Plantinga's ontological argument is that in my version of the argument, the meaning of 'unbeatable sorcerer' is given using only one level of recursion. (Hence, there is no proposition (30A) in my version). Plantinga, on the other hand, used two levels of recursion to define 'maximal greatness'; viz., his propositions (30) and (31). This added complexity is one reason why Plantinga's ontological argument seems so persuasive. His proposition (29) looks innocent enough to warrant our initial assent. Ordinarily, we don't think of our everyday concepts of 'maximum' and 'greatness' as leaning on the theoretical buttress of possible worlds. Not even professional philosophers.

This dependence of the meaning of 'maximal greatness' on possible world semantics is kept covert by Plantinga in his (29). The double recursion from the initial premise (29) to God's all important characteristics of omnipotence, omniscience and moral perfection in (31) provides the psychological distance needed to blur this critical reliance on possible worlds. The intermediate term 'maximal excellence' in Plantinga's (31), in effect, provides this psychological buffer zone. But once we spell out explicitly what (29) means, Plantinga's tying of ordinary-seeming language to an ontology of possible worlds become clear. Once this semantic connection is uncovered, the possibility of the instantiation of God's 'maximal greatness' amounts to:

There is a possible world [W] in which there exists a being that is omnipotent, omniscient, and morally perfect in every possible world.

It is this explicit formulation of (29) that deserves close logical scrutiny. But how does Plantinga's originally stated recursive formulation of the semantic connection lead to his logical muddle? Note how the supposed possible world [W] in the explicitly stated proposition above is uniquely identified. It is identified as the possible world in which a particular being exists in that possible world, amongst others. The self-referential nature of this world description ought to give us reason to pause. The long history of solving the semantic problems of self-reference, such as the well-known lair paradox, teach us to be very wary of thinking we know at first glance what these kinds of sentences mean. As the logical analysis in the previous section demonstrates, placing confidence in the seeming innocuousness of (29) is misplaced.

Book cover: Philosophy of Religion by John Hick

The second feature of Plantinga's ontological argument that makes it seem so compelling is his conscription of the crucial, but informally stated, modal axiom about transworld impossibility: what is impossible in at least one possible world is impossible in every possible world. Once we accept (29) as being true (i.e., that there is a possible world [W]), the leap from premise (33′) to conclusion (C1) appears intuitively as logically conclusive.

Prior to writing this essay, I considered that the inference from (33′) to (C1) was invalid as Plantinga presented (33′) as a counterfactual conditional. The state of affairs described in the antecedent of a counterfactual conditional is always false (otherwise it wouldn't be counterfactual). In this case, then, I thought that possible world [W] is never actualized. I reasoned that, ipso facto, maximal greatness is also never actualized. However, this line of reasoning is wrong. Plantinga's argument does not depend on possible world [W] in particular being actualized. It depends only on some possible world in which maximal greatness is instantiated is actualized.

A second possible line of attack is on Plantinga's assertion that the non-existence of an omnipotent, omniscient and morally perfect being is impossible simpliciter. This assertion seems mistaken because the impossibility of such a being not existing is conditional on possible world [W] being actualized. The conditional nature of (33′) seems to make this clear. Contra Plantinga, the non-existence of such a being seems to be a conditional impossibility. But herein lays the genius of Plantinga's argument. Premise (33′) is no ordinary subjunctive conditional. Its antecedent has as its object a possible world, allowing Plantinga to call into service the full armoury of possible world logic. From these considerations of the validity of Plantinga's ontological argument, it appears the only, and fatal, objection to Plantinga's argument is that his premise (29) is necessarily false.[2]


  1. [2] For a general critique of the concept of maximal greatness, see C. D. Broad [1973] and Penelhum [1971: ch. 2]. For a specific criticism of Plantinga's premise (29), see Mackie [1982: 61].

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