Plantinga's Ontological Argument

5. Rational Acceptability of Plantinga's Ontological Argument

Book cover: The Miracle of Theism by J. L. Mackie

The reductio ad absurdum of Plantinga's ontological argument is that if it were sound, it would be possible to define virtually anything into existence. All a philosopher need do to grant existence to a being is simply define it in premise (29) and predicate its attributes in premises (30) and (31). Plantinga's urge to grant existence to an omniscient, omnipotent and morally perfect being, as opposed to any other kind of being, results, it seems, from the particular religious tradition in which he is embedded and not from any rational constraint. As other critics, such as Mackie [1982: 59] and Oppy [2016: §7], have noted, there is nothing special about maximal greatness.

What did Plantinga conclude for his ontological argument for the existence of God? He summed up his assessment on the final page of his God, Freedom and Evil [1975: 112]. He claimed of his argument, 'It is certainly valid; given its premise, the conclusion follows.' Furthermore, he also considered his argument sound as he accepted the only non-analytic premise (29) as being true.

In this essay, I have tried to show otherwise, arguing instead that even if we grant that his argument is valid, it remains unsound. Perhaps if Plantinga had formulated his entire argument in numbered premise form, as I have done here, his mistake may have been more obvious. Even he may have seen the difficulty faced by his final argument. Why he chose to not do this with the most critical section of his argument, I cannot say. Perhaps an unfortunate consequence of this omission was to divert critical attention away from this informally stated section of his argument and focus it instead on his three main premises, two of which are analytic and therefore beyond dispute.

Plantinga's only defence in his God, Freedom and Evil [1975: 112] of his solitary non-analytic premise (29) was to opine that 'I think it is true'. He conceded that 'not everyone who understands and reflects on its central premise—that the existence of a maximally great being is possible—will accept it'. So the argument, for him, is not a proof of God's existence. He [1975: 112] maintained, though, that the argument establishes the 'rational acceptability' of theism because 'there is nothing contrary to reason or irrational in accepting this premise.' Here, Plantinga referred us to his fuller defense of (29) in his The Nature of Necessity [1974: ch. 10, §8].

In order to tease out Plantinga's reasoning, let's grant Plantinga's view that (29) is not known to be false. Does Plantinga's ontological argument, then, give a person a rational reason to believe that God exists. The answer has to be 'No'. Plantinga [1974: 218] readily admitted that there are an infinite number of parallel arguments, all equally valid, that demonstrate the necessary non-existence of a maximally great being. It is on this basis that Plantinga [1974: 219; 1975: 112] conceded that his ontological argument is not 'a successful piece of natural theology'. But it remains 'rational' to accept (29), Plantinga asserted, for the same reason that we accept other contested premises.

Here, Plantinga advanced three analogous, equally contested propositions for which it is not irrational to accept as true. First, he appealed to the rationality of abandoning the Distributive Law in logic in the face of quantum uncertainty. Second, he used as illustration the alternative modal logic in which possible but unactualized objects exist. Third, he cited the denying of Leibniz's Law in the face of counterexamples.

... But wouldn't a God who could find a flaw in the ontological argument be even greater?

However, for each of these three examples, the advocates for each thesis advance reasons for accepting the premise; reasons that are independent of the premise being advocated. Plantinga did not do this for his premise (29). At one point, Plantinga [1974: 220] advocated accepting (29) on the grounds that treating it as true simplifies Theology. Clearly, this reason presupposes the very the existence of the being whose reality is in dispute. Given Plantinga's parenthetical request for us to take his other examples '[M]ore seriously', perhaps he intended for us to take this suggestion with a grain of salt.

On the other hand, Plantinga could push this defence for all it is worth. He could object that at least in some cases, it is allowable for a supporting reason to presuppose the thesis being supported. By way of example, Plantinga [1974: 221] did point to philosophers' acceptance of Leibniz's Law. The arguments in favour of this Law, Plantinga noted, at 'some point invoke that very principle'.

However, even if this were the case with Leibniz's Law, the situation is not analogous to Plantinga's premise (29) stating that maximal greatness is possibly instantiated. Leibniz's Law is a foundation principle in metaphysics that underpins many philosophical arguments. Premise (29), on the other hand, is not such a common principle, requiring agreement by both theists and non-theists. Secondly, unlike the advocates for Leibniz's Law, Plantinga does not advance any arguments for the truth of (29) in the face of counterexamples (i.e., the infinite number of parallel arguments mentioned above).

Plantinga [1974: 221] concluded with this analogy to his acceptance of (29):

So if we carefully ponder Leibniz's Law and the alleged objections, if we consider its connections with other propositions we accept or reject and still find it compelling, we are within our rights in accepting it—and this whether or not we can convince others.

This is precisely what Plantinga did not do with his premise (29). Sure, he could contend that he had considered its connections with his other theistic beliefs he accepts as true. However, this activity is worthless in raising the epistemic probability that God exists independently of a prior acceptance of this proposition. Seen in this light, his ontological argument is self-serving, simply bolstering confidence in a pre-existing belief. It adds nothing to the effort to convince a non-theist and, at best, lulls some theists into thinking that the argument does some actual intellectual work in raising the possibility that God exists.

Book cover: God, Freedom, and Evil by Alvin Plantinga

All of this may be conceded by Plantinga while clinging to the notion that in the absence of a reason for rejecting (29), it is not irrational to accept it. I think this is also a mistake. There are many propositions for which we have equal reason for either accepting or rejecting their truth. Using their best efforts, scientists have calculated the weight of our Earth to be 5.972 X 1024 kg. Now, it may be the case that our Earth weighs more than this amount or it may be the case that it weighs less. Either proposition may be true, but not both. We have no reason for rejecting the proposition that the Earth weighs less than 5.972 X 1024 kg. But we readily concede that that is not a reason for believing that it does, in fact, weigh less than this. It is just as probable that it weighs more. The rational thing to do in this instance is to suspend judgement.

Take another case that is more akin to the a priori nature of Plantinga's ontological argument. Mathematicians know a lot about prime numbers. However, as yet, there exists no proof for there being an infinite number of twin prime pairs (i.e., prime number pairs separated by just one other number). There is no reason for rejecting the proposition that there is an infinite number of twin prime pairs. As per the reasoning above, this fact alone does not make it reasonable to believe that this is the case. Once again, reason dictates that we neither believe nor disbelieve this proposition. The same applies in kind to the contrary proposition; that there is a finite number of twin prime pairs. Likewise, in response to Plantinga's ontological argument, a reasonable person ought to suspend judgement on the truth or otherwise of premise (29).[3] I think we can safely conclude that Plantinga was being exuberantly overconfident in announcing his final version as 'The Argument Triumphant' [1975: 111] and 'A Victorious Modal Version' [1974: 213].


  1. [3] van Inwagen [1977: 387–92] mounts a similar case against the rationality of accepting Plantinga's premise.

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