The Ontological Argument

Anselmian Ontological Argument

Abstract

Anselm of Canterbury composed the Proslogium (1077–78) seeking, as the SEP's Anselm entry puts it, "a single argument that needed nothing but itself alone for proof, that would by itself be enough to show that God really exists … and whatever we believe about the divine nature" (SEP Anselm §2.3). The argument is framed as prayer — fides quaerens intellectum, faith seeking understanding — but it is aimed squarely at the fool of Ps 14:1 (bib) who "hath said in his heart, There is no God," and it is intended to convince even him (Anselm, Proslogium ch. II; SEP Anselm §2.1).

Formal statement

In Anselm's own words: "we believe that thou art a being than which nothing greater can be conceived." Even the fool, hearing this formula, "understands what he hears, and what he understands is in his understanding." But "that, than which nothing greater can be conceived, cannot exist in the understanding alone. For, suppose it exists in the understanding alone: then it can be conceived to exist in reality; which is greater." Hence "there is no doubt that there exists a being, than which nothing greater can be conceived, and it exists both in the understanding and in reality" (Anselm, Proslogium ch. II). Chapter III extends the proof to necessary existence: "it is possible to conceive of a being which cannot be conceived not to exist; and this is greater than one which can be conceived not to exist," so the being than which nothing greater can be conceived "cannot even be conceived not to exist; and this being thou art, O Lord, our God" (Anselm, Proslogium ch. III).

Key evidence / textual basis

The argument's engine is the distinction drawn in chapter II between two modes of being: "it is one thing for an object to be in the understanding, and another to understand that the object exists" — as a painter has his unexecuted work in his understanding without yet understanding it to be (Anselm, Proslogium ch. II). Chapter IV explains how the fool's denial is even possible: an object can be conceived "when the word signifying it is conceived" or "when the very entity, which the object is, is understood"; "in the former sense, then, God can be conceived not to exist; but in the latter, not at all" (Anselm, Proslogium ch. IV).

The Anselmian family has early-modern descendants. René Descartes argued in Meditation V from the idea of a supremely perfect being — "God has every perfection. Independent existence is a perfection. (So) God exists" — a form the SEP judges question-begging against informed atheists, who deny the first premise (SEP §5). Gottfried Wilhelm Leibniz sought to complete the Cartesian proof by showing the concept of a supremely perfect being coherent (SEP §Preamble).

Leading proponents

• Anselm of Canterbury — author of the original argument; also its first defender, in the Apologetic against Gaunilo (see Responses below).
• Descartes — the perfection-based variant of Meditation V (SEP §5); primary text not in corpus.
• Leibniz — the possibility-completion strategy (SEP §1); primary text not in corpus.

Strongest counter-arguments

The most historically weighty objection is an insider's. Aquinas states the Anselmian reasoning fairly as an objection and then rejects it: granted that "God" signifies something than which nothing greater can be thought, "it does not therefore follow that he understands that what the word signifies exists actually, but only that it exists mentally. Nor can it be argued that it actually exists, unless it be admitted that there actually exists something than which nothing greater can be thought; and this precisely is not admitted by those who hold that God does not exist" (Aquinas, Summa I q.2 a.1, obj. 2 and ad 2). For Aquinas the proposition "God exists" is self-evident in itself "but needs to be demonstrated by things that are more known to us … namely, by effects" (Aquinas, Summa I q.2 a.1) — which is why he builds cosmological Five Ways rather than an a priori proof.

A second cluster is interpretive-theoretical: the SEP presses that Anselm's key vocabulary — "greater," "exists in the understanding" — is so underspecified that "until we are given answers … we have no way of determining whether anyone can safely accept Anselm's theoretical framework. … If that theory has absurd consequences, then the validity of the argument is inconsequential" (SEP §6). The third and most famous cluster — parody, conceivability, and predicate objections — is treated as a view in its own right below.

Responses

Anselm answered Gaunilo directly in the Apologetic: "if a being than which a greater cannot be conceived is in any understanding, it does not exist in the understanding alone. For if it is in the understanding alone, it is a being than which a greater can be conceived, which is inconsistent with the hypothesis" (Anselm, Apologetic ch. II). On the reading defended in the SEP's Anselm entry, Anselm "insists that the original argument did not rely on any general principle to the effect that a thing is greater when it exists in reality than when it exists only in the understanding" — the very principle Gaunilo's parody exploits — and instead rests on two premises: that than which a greater cannot be thought can be thought; and if it can be thought, it exists in reality. He defends the first by an ascent "from lesser goods to greater goods," and the second on the ground that "a being that is capable of non-existence is less great than a being that exists necessarily" (SEP Anselm §2.3, quoting Anselm's Reply to Gaunilo 8). Whether this rescues the proof or merely relocates it into the modal version (View 3) remains contested.

Assessment

Assessment: Live — the Proslogion argument is among the most discussed texts in the history of philosophy, with serious recent defenses recorded in the literature (Campbell 2018, Leftow 2022, per SEP §1), but no formulation has commanded assent even among theists, and its own tradition (Aquinas) declined it.

Parody and Predicate Objections

Abstract

The critical tradition against ontological arguments spans seven centuries and three distinct strategies. Gaunilo of Marmoutiers, Anselm's own contemporary and a fellow Benedictine, argued by parody: reasoning of Anselm's form proves absurdities. David Hume argued from conceivability: no existential claim is demonstrable a priori. Immanuel Kant argued from logical grammar: existence is not a real predicate, so it cannot be a perfection or great-making property. Graham Oppy, whose taxonomy structures the contemporary debate, treats parody as the sharpest instrument: where an argument's background theory is underspecified, "parody offers the most immediate way to make it clear to proponents of the argument that there is nothing in the argument that ought to give pause to atheists and agnostics" (SEP §11).

Formal statement

The parody strategy, generalized: given an ontological argument for God, construct an argument of the same logical form whose conclusion "is recognised by all to be absurd, and yet whose premises are all no less acceptable" (SEP §11). Since the paired arguments share a form, "either they are both valid or they are both invalid. If they are both invalid, then the Proslogion II argument fails. If they are both valid, then … there must be a premise pair" the theist accepts in one and rejects in the other — and the SEP argues, premise by premise, that no such pair is available (SEP §11).

Hume's independent argument, put in Cleanthes' mouth: "Nothing is demonstrable, unless the contrary implies a contradiction. Nothing, that is distinctly conceivable, implies a contradiction. Whatever we conceive as existent, we can also conceive as non-existent. There is no being, therefore, whose non-existence implies a contradiction. Consequently there is no being, whose existence is demonstrable. I propose this argument as entirely decisive, and am willing to rest the whole controversy upon it" (Hume, Dialogues Part 9).

Key evidence / textual basis

Gaunilo's lost island, in the primary text: "it is said that somewhere in the ocean is an island, which, because of the difficulty, or rather the impossibility, of discovering what does not exist, is called the lost island," an island of "inestimable wealth" more excellent than all inhabited lands. To one who argued that because this island is understood it must therefore exist, Gaunilo replies: "either I should believe that he was jesting, or I know not which I ought to regard as the greater fool: myself, supposing that I should allow this proof; or him, if he should suppose that he had established with any certainty the existence of this island" (Gaunilo, In Behalf of the Fool §6).

The parody is not Gaunilo's only move. He also demands that "it should be proved first that this being itself really exists somewhere" before its supremacy can be inferred (Gaunilo, In Behalf of the Fool §§5–6), and he anticipates the modern conceivability objection: "I am most certainly aware of my own existence; but I know, nevertheless, that my non-existence is possible" — so inconceivability-of-non-existence cannot function as Anselm needs (Gaunilo, In Behalf of the Fool §7). Notably, Gaunilo writes as a believer: the rest of the Proslogium is "argued with such truth, such brilliancy, such grandeur" that it "should be received with great respect and honor" (Gaunilo, In Behalf of the Fool §8) — his target is the proof, not the God.

Hume adds that the concept of necessary existence is empty: "It will still be possible for us, at any time, to conceive the non-existence of what we formerly conceived to exist … The words, therefore, necessary existence, have no meaning; or, which is the same thing, none that is consistent" (Hume, Dialogues Part 9).

Kant's Critique of Pure Reason (1787) contains, per the SEP timeline, "Three objections to 'the ontological argument', including the famous objection based on the dictum that existence is not a predicate" (SEP §1); the SEP lists "existence is not a predicate (see, e.g., Kant; Smart 1955; Alston 1960)" first among the alleged keys to the failure of ontological arguments (SEP §12). The thrust: if "exists" adds no determination to a concept, then the premise that a really existing X is greater than an X in the understanding alone misfires, as does the Cartesian premise that existence is a perfection. Frege's doctrine that "existence is a second-order predicate" radicalizes the point (SEP §1). Kant's own statement is now citable directly: "Being is evidently not a real predicate, that is, a conception of something which is added to the conception of some other thing" (Kant 1787, Transcendental Dialectic, "Of the Impossibility of an Ontological Proof of the Existence of God" (Meiklejohn tr.)).

Leading proponents

• Gaunilo of Marmoutiers — In Behalf of the Fool (c. 1078), in corpus with the Proslogium.
• David Hume — Dialogues Concerning Natural Religion (1779), Part 9, "a general attack on a priori arguments (both analytic and synthetic)" (SEP §1).
• Immanuel Kant — Critique of Pure Reason (1787); via SEP only (see flag above).
• Graham Oppy — Ontological Arguments and Belief in God (1995), Arguing About Gods (2006); author of the SEP entry cited throughout this article.

Strongest counter-arguments

Against the parody, the classic reply — associated in the SEP with Alvin Plantinga — is that the island formula, unlike Anselm's, is incoherent: "since we know a priori that there is no intrinsic maximum for the greatness of islands, we can attach no sense to the words 'island than which no greater island can be conceived.' … While there is an intrinsic maximum for knowledge — knowing everything — there is no intrinsic maximum for the number of palm trees on an island" (SEP §11). Anselm himself made the uniqueness claim first, with a memorable wager: "I promise confidently that if any man shall devise anything existing either in reality or in concept alone (except that than which a greater cannot be conceived) to which he can adapt the sequence of my reasoning, I will discover that thing, and will give him his lost island, not to be lost again" (Anselm, Apologetic ch. III).

Against Hume, the standard reply (pressed also in our The Kalam Cosmological Argument article) is that conceivability is an unreliable guide to metaphysical possibility: from the fact that we can picture God's non-existence it does not follow that God's non-existence is genuinely possible. Against Kant, defenders note that on the reading of Anselm's Apologetic in the SEP's Anselm entry, the argument "did not rely on any general principle" that existence in reality is greater than existence in the understanding (SEP Anselm §2.3) — so an objection to existence-as-predicate may not touch Anselm's own proof, whatever it does to Descartes'.

Responses

The critics' rejoinders are recorded in the SEP. To the intrinsic-maximum reply: "it seems false that the addition of palm trees is guaranteed to add to the greatness of an island"; we do not know a priori that island-greatness has no intrinsic maximum; and the parody does not need islands at all — it "can, for example, be formulated in terms of scores on a test for which there is a maximum possible score" (SEP §11). To the anti-Kantian point, the critic can grant that Kant's dictum requires "far more controversial assumptions than non-theists require in order to be able to reject ontological arguments with good conscience" (SEP §12) — and simply fall back on the parody, which needs no theory of existence at all.

Assessment

Assessment: Strong — the parody strategy in particular has never been convincingly answered on behalf of the Proslogion II formulation, and the SEP records near-consensus, "even amongst theists," that no extant ontological argument is dialectically successful (SEP §12). The critique establishes the argument's persuasive failure, not the falsity of its conclusion.

Modal Ontological Argument

Abstract

The twentieth-century revival — Norman Malcolm and Charles Hartshorne reading Proslogion III, and preeminently Alvin Plantinga in The Nature of Necessity (1974) — recasts the argument in the language of possible worlds: from God's possible existence, given that God would be a necessary being, God's actual existence follows. Plantinga's version, built on maximal greatness ("a being is maximally great if and only if it exists necessarily and is essentially omnipotent, omniscient, and morally perfect"), is "the most influential MOA" (SEP §8). Corpus gap: The Nature of Necessity is copyright-locked and not in raw/; this view is reconstructed from the SEP, which quotes Plantinga's concluding verdict directly.

Formal statement

The standard S5 formulation (SEP §8):

1. Possibly, God exists. (Possibility premise)
2. Necessarily, if God exists, then it is necessary that God exists. (Necessity premise)
3. (Hence) It is necessary that God exists.
4. (Hence, by axiom M) God exists.

The inference from possible necessity to necessity is valid in S5, whose characteristic axiom is ◇p → □◇p, entailing ◇□p → □p; a weaker system such as B suffices (SEP §8).

Key evidence / textual basis

The medieval root is Proslogion III's claim that "it is possible to conceive of a being which cannot be conceived not to exist; and this is greater than one which can be conceived not to exist" (Anselm, Proslogium ch. III); Hartshorne's Man's Vision of God (1941) defended modal arguments "allegedly derived from Proslogion 3" (SEP §1). On Anselm's own gloss in the Reply to Gaunilo, "if that than which a greater cannot be thought can be thought – that is, if it is a possible being – it actually exists" (SEP Anselm §2.3). Kurt Gödel's higher-order argument from "positive properties" belongs to the same family (SEP §9).

Crucially, Plantinga himself claims less than proof: "Our verdict on these reformulated versions of St. Anselm's argument must be as follows. They cannot, perhaps, be said to prove or establish their conclusion. But since it is rational to accept their central premise, they do show that it is rational to accept that conclusion" (Plantinga 1974: 221, quoted at SEP §4).

Leading proponents

• Alvin Plantinga — The Nature of Necessity (1974), the "'victorious' modal Ontological argument" (SEP §1); not in corpus (copyright).
• Charles Hartshorne and Norman Malcolm — earlier modal formulations from Proslogion III (SEP §8); not in corpus.

Strongest counter-arguments

The decisive objection in the current literature is the symmetry problem. Construct the Reverse MOA: possibly, God does not exist; necessarily, if God exists, then it is necessary that God exists; hence it is necessary that God does not exist. "Like the MOA, the RMOA is valid in S5," and the two possibility premises seem "epistemically on par — it seems intolerably arbitrary to privilege one over the other absent further considerations. … Absent such a symmetry breaker, however, the MOA is dialectically toothless" (SEP §8). Hume's Part-9 principle underwrites the reverse premise: "Whatever we conceive as existent, we can also conceive as non-existent" (Hume, Dialogues Part 9). Even Plantinga's fallback claim — that the argument establishes theism's rational permissibility — is challenged: "at best these arguments show that certain sets of sentences (beliefs, etc.) are inconsistent … the arguments themselves say nothing about the reasonableness of accepting the premises" (SEP §12).

Responses

Defenders pursue "symmetry breakers" favouring the possibility of God over the possibility of God's non-existence; "the MOA debate has centred around symmetry breaking" (SEP §8), though such breakers often appeal to a posteriori premises or amount to distinct arguments in their own right. Challenges to S5 itself have drawn detailed defenses of its metaphysical adequacy (Pruss & Rasmussen, Williamson, Hale, Leftow, per SEP §8). The theist may also concede the dialectical point while retaining the argument as a consistency proof: necessary-being theism cannot be rejected piecemeal — one must deny the possibility premise itself. {{UNSOURCED: Plantinga's own reply to the symmetry objection in The Nature of Necessity ch. 10 — copyright-locked; add to acquisition queue.}}

Assessment

Assessment: Live — the modal argument is formally valid in S5 and its necessity premise is definitionally secure on Plantinga's characterisation; everything turns on the possibility premise, where the symmetry problem leaves the debate genuinely open, exactly as Plantinga's own modest verdict concedes (SEP §4).