Ontological arguments attempt to establish the existence of God by relying on one's concept of God, or the definition of the word God, without involving truths known through experience. Such arguments have had many proponents in the history of philosophy, notably Anselm of Canterbury (1033/34–1109) and René Descartes (1596–1650), as well as many detractors, including Thomas Aquinas (c. 1225–1274) and Immanuel Kant (1724–1804). Today ontological arguments are widely considered flawed, but exactly what is wrong with them remains a matter of controversy.
The locus classicus is chapter two of Anselm's Proslogion, where he calls attention to the idea of a being greater (more excellent) than any other conceivable being, that is, the idea of a maximally great being. Anselm maintains that even those who reject the existence of a maximally great being still possess the concept of one. Now—and this is the key premise—if there were no maximally great being, one could conceive of something even greater than it by conceiving of a maximally great being that exists. But it involves a contradiction to say that one can conceive of a being greater than a maximally great being. Hence, absurdity results from the supposition that God does not exist.
A common response focuses on an assumption behind the key premise, namely that something can be greater than another thing simply by virtue of existence. What is one to make of this thesis? It appears to be false for the simple reason that a comparison of greatness requires (at least) two existing things to compare. But the proponent of the argument might reply that one can compare things without assuming their existence—for example, the strength of Achilles and Hector. It is therefore important how this is done. Perhaps it simply involves a comparison of the relevant concepts. Then the key premise means, "If nothing in existence corresponded to one's concept of God, one could generate a superior concept by representing God as existing." But this seems false; one's initial concept, which failed to correspond to anything, might well have been the concept of God-as-existing.
More plausibly, to compare the greatness of two things without assuming that they exist is to ask which of them would be greater if both were to exist. But if to compare the greatness of two things they must both be thought of as existing, existence itself cannot be considered a respect in which they differ in greatness. Thus, as Immanuel Kant argued, existence is not a "perfection"; it is not a property that can contribute to something's greatness.
There are at least two ways to avoid this objection: (1) one could claim that some objects of thought possess a mode of being distinct from existence; or (2) one could alter the argument to build on the claim that necessary existence (rather than mere existence) is a perfection.
According to the first approach, there are such things as, for example, unicorns; they just do not exist. They are abstract objects of thought that lack spatiotemporal location and causal powers. Thus, one could really consider the "greater than" relation to involve two entities even if one or both of those entities do not exist. And one can treat existence as a property that enhances the greatness of something after all.
Another general objection to the ontological argument, however, causes problems for this approach. Could one not use reasoning similar to Anselm's in order to establish the existence of all kinds of things? Consider the idea of an island greater than any other island that can be conceived. Since such an island can be the object of one's thoughts, it must (on this view) be an abstract entity, even if it lacks existence. If it does lack existence, however, one could think of a greater island, namely an island that also exists. So a maximally great island must exist. But (unfortunately) the greatest conceivable island does not exist, so the argument form cannot be sound. This parody was conceived by the monk Gaunilo, a contemporary of Anselm's.
Replying to Gaunilo's parody, Anselm insisted that the argument form can only establish the existence of that which is greatest or most perfect simpliciter, and not the most perfect island or bluebird. The argument form, he suggests, will only work if the concept one begins with is that of a being that could not have failed to exist. But all islands and other material objects are the sorts of things that could be destroyed. A rejoinder might alter the parody to involve the idea of a spiritual entity with almost every perfection (e.g., a godlike being lacking only a certain amount of knowledge but nevertheless a necessary being).
Inspired by passages in Anselm that suggest a different kind of ontological argument, some proponents avoid the above dispute by focusing on God's necessary existence rather than on God's existence. This is the approach of Charles Hartshorne, Norman Malcolm, and Alvin Plantinga. For anything to count as God, they argue, it would have to be absolutely perfect. But anything that exists and yet might not have existed is thereby deficient in some way. So if God exists, God exists necessarily; it could never be that God just happens to exist. Now, one can think of a necessary being as something that exists according to all the ways the world might have been, or "possible worlds." So either God exists in every possible world or in none. But this means that, so long as it is possible that God exists, God actually exists; after all, the way things actually are is one of the ways things can be. Thus, the argument forces a dilemma between the necessity of God's existence and its impossibility.
The key question, then, is whether the existence of God (conceived of as a necessary being) is even possible. Certain philosophers have held that possibility is something conceptual, and that unless the concept of God is somehow incoherent, the existence of God is possible. Thus Charles Hartshorne has argued that either God exists or else the term God is meaningless or self-contradictory. And on the face of it, the existence of God certainly does not appear to be incoherent, like the existence of a round square. It seems perfectly conceivable.
The trouble is that the nonexistence of God also seems conceivable. And if it were even possible—assuming that God is by definition a necessary being—it would follow that God does not exist. So it would appear that the link between conceivability and possibility in this case is tenuous. And philosophers today are widely in agreement that states of affairs may be metaphysically impossible without involving any absurdity that is accessible to a priori reflection. So it is hard to see how one can assess the possibility of God's existence unless one has reason to affirm or deny God's actual existence, which is the point at issue.
See also Cosmological Argument; God, Existence of
adams, robert merrihew. "the logical structure of anselm's arguments." the philosophical review 80 (1971): 28–54.
hartshorne, charles. man's vision of god and the logic of theism. new york: harper, 1941.
kane, robert. "the modal ontological argument." mind 93 (1984): 336–350.
lewis, david k. "anselm and actuality." noûs 4 (1970): 175–188.
oppy, graham. ontological arguments and belief in god. cambridge, uk: cambridge university press, 1995.
plantinga, alvin. the ontological argument from st. anselm to contemporary philosophers. new york: doubleday, 1965.
van inwagen, peter. "necessary being: the ontological argument." in philosophy of religion: the big questions, eds. eleonore stump and michael j. murray. oxford and malden, mass.: blackwell, 1999.
"Ontological Argument." Encyclopedia of Science and Religion. . Encyclopedia.com. (June 24, 2017). http://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/ontological-argument
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"Ontological argument." The Concise Oxford Dictionary of World Religions. . Retrieved June 24, 2017 from Encyclopedia.com: http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/ontological-argument