The Ontological Argument: From Anselm to Plantinga

SUMMARY

The ontological argument attempts to establish God's existence through pure reason and conceptual analysis, contending that the very concept of God as a maximally perfect being entails divine existence. Spanning nearly a millennium from Anselm's medieval formulation through Ibn Sina's burhān al-ṣiddīqīn to Plantinga's contemporary modal version, the argument remains philosophy's most distinctive a priori approach to theism — and one of its most contested. Within the project framework, this argument belongs to Maslik 1 (Philosophical & Metaphysical), where it produces, at most, a probability shift rather than apodictic proof.

Anselm's Proslogion

Saint Anselm of Canterbury (1033–1109) first articulated the ontological argument in his Proslogion (1077–1078), seeking what he called a single argument (unum argumentum) that would demonstrate God's existence and attributes through reason alone. Anselm defined God as "that than which nothing greater can be conceived" (aliquid quo nihil maius cogitari possit). His reasoning proceeded as follows: even the fool who denies God's existence must have some concept of God to deny; this concept exists in the understanding; but a being that exists both in understanding and reality would be greater than one existing only in understanding; therefore God must exist in reality, for otherwise we could conceive of something greater — a contradiction.

Anselm's argument was immediately challenged by the monk Gaunilo of Marmoutiers, who constructed a parallel argument about a "perfect island" to demonstrate the reasoning's apparent absurdity. Anselm responded that his argument applied uniquely to the concept of maximal greatness, not to any contingent perfect thing — a response whose adequacy is still debated.

Recent scholarship distinguishes two arguments in Anselm's text: the Proslogion II argument (existence as a perfection) and the Proslogion III argument (necessary existence as a perfection). Norman Malcolm's influential 1960 paper "Anselm's Ontological Arguments" argued that the Proslogion III argument escapes traditional objections by trading on necessary rather than mere actual existence — a move that anticipated the modal versions of the twentieth century.

The Islamic Parallel: Ibn Sina's Burhān al-Ṣiddīqīn

Although the ontological argument is conventionally traced through Western sources, Ibn Sina (Avicenna, 980–1037) developed an a priori argument with structural similarities in al-Ishārāt wa-l-Tanbīhāt, known as the burhān al-ṣiddīqīn (proof of the truthful). Ibn Sina's argument begins from the bare concept of existence rather than from empirical observation: existence is divided into the necessary (wājib al-wujūd) and the possible (mumkin al-wujūd); if anything exists, then either it is necessary or it depends on something necessary; therefore a necessary being exists.

Whether Ibn Sina's argument is properly classified as ontological is contested. It does not move from concept to existence in the Anselmian manner, but it shares the a priori character of beginning from conceptual analysis rather than empirical premises. Mulla Sadra (1571–1640), in al-Asfār al-Arbaʿa, developed a more explicitly ontological version building on Ibn Sina's foundations, drawing on his doctrine of the primacy of existence (aṣālat al-wujūd) to argue that existence itself, as the most general and primary reality, must be necessary in its principle.

The relationship between the Avicennian and Anselmian traditions is debated: there is no clear evidence of direct influence in either direction, though both share Neoplatonic and late-antique philosophical roots.

The Classical Western Development: Descartes and Leibniz

René Descartes (1596–1650) revived the ontological argument in his Meditations on First Philosophy (1641), arguing that existence belongs to God's essence just as having three angles belongs to a triangle's essence. For Descartes, we possess an innate idea of God as a supremely perfect being, and since existence is a perfection, God must exist.

Gottfried Wilhelm Leibniz (1646–1716) refined the argument by addressing a crucial gap: previous versions assumed without proof that the concept of a maximally perfect being was coherent. Leibniz argued that perfections are simple, positive qualities that cannot contradict each other, making the concept of God logically possible. His version thus proceeds in two stages: first establishing God's possibility, then deriving that a possible necessary being must actually exist. This two-stage structure remains influential in contemporary modal formulations.

Kant's Critique

Immanuel Kant (1724–1804) delivered what many consider the decisive refutation of the ontological argument in his Critique of Pure Reason (1781/1787). Kant's primary objection held that "existence is not a real predicate" (Sein ist offenbar kein reales Prädikat) — that is, existence is not a property that adds to a concept's content. When we say something exists, we are not attributing an additional characteristic but rather positing the object itself.

Kant argued that all ontological arguments commit the fallacy of treating existence as a real predicate that could make one concept greater than another. A hundred real dollars, Kant famously observed, contain not a penny more than a hundred possible dollars. This critique fundamentally challenged the argument's logical structure rather than merely its premises.

Contemporary defenders respond in various ways: by reformulating the argument in terms of necessary existence rather than mere existence (a move arguably available in Anselm's Proslogion III); by appealing to the modal status of existence claims in possible-worlds semantics; or by arguing that even granting Kant's point about predication, his critique does not touch arguments framed in terms of essence and necessary instantiation.

Modal Logic and Contemporary Formulations

The twentieth century witnessed renewed interest through developments in modal logic. Kurt Gödel (1906–1978) constructed a rigorous modal version using symbolic logic, defining God as a being possessing all positive properties and deriving that such a being necessarily exists. Gödel's proof was published posthumously and remains technically impressive but philosophically contested.

Alvin Plantinga (b. 1932) developed what is now the standard contemporary version in The Nature of Necessity (1974). Plantinga's argument centers on the concept of "maximal greatness" — the property of having maximal excellence (including omniscience, omnipotence, and moral perfection) in every possible world. His reasoning proceeds: if maximal greatness is possibly instantiated, then it is instantiated in some possible world; but maximal greatness includes necessary existence; therefore a maximally great being exists in every possible world, including the actual world.

Plantinga himself is famously modest about the argument's persuasive force, conceding that the key premise — that maximal greatness is possibly instantiated — is contested and that the argument shows at most that theistic belief is rationally permissible if one grants this possibility. The argument is valid; the question is whether its central premise is justified.

The Reverse Ontological Argument

Critics including Graham Oppy and Peter Millican have pressed a structural objection: if Plantinga's modal argument is sound, then a parallel argument for the non-existence of God appears equally available. If "no maximally great being exists" is possibly true, then by the same modal reasoning it is necessarily true. The argument's strength depends entirely on which possibility-premise one accepts, and the argument provides no independent grounds for preferring the theistic possibility-premise to the atheistic one. This is sometimes called the "reverse ontological argument" and constitutes one of the most discussed contemporary objections.

Contemporary Assessments

Within the framework of this project, the ontological argument can be assessed soberly. It does not produce yaqīn (apodictic certainty) — the contested status of its key premise prevents that. What it can produce, on a charitable reading, is a probability shift: if the concept of a maximally great being is coherent, this gives some weight to the existence of such a being. Combined with other masalik, this shift can contribute to the cumulative case for faith, while remaining open to reasonable disagreement at every stage.

The argument's enduring significance lies less in its conclusion than in its method — the attempt to establish metaphysical truths through pure rational analysis. This methodological ambition raises fundamental questions about the scope and limits of human reason that recur throughout Maslik 1.

KEY DISTINCTIONS

• Conceptual vs. Real Existence: The crucial distinction between existing as a concept in the mind versus existing in reality, lying at the heart of most ontological arguments and their critiques.

• Analytic vs. Synthetic Judgments: Kant's distinction between statements true by definition versus those providing new information, bearing directly on whether existence claims can be established through conceptual analysis.

• Logical Possibility vs. Metaphysical Possibility: In modal versions, the difference between what is conceivable without contradiction versus what could actually obtain in reality.

• Maximal Excellence vs. Maximal Greatness: Plantinga's distinction between being perfect within a world (excellence) versus being perfect across all possible worlds (greatness).

• Validity vs. Soundness: Modal ontological arguments are generally valid; the contested question is whether their possibility-premises are true.

MAJOR PROPONENTS

• Saint Anselm of Canterbury — Originated the argument in the Proslogion; debate continues over whether Proslogion II and Proslogion III contain distinct arguments

• Ibn Sina (Avicenna) — Burhān al-ṣiddīqīn in al-Ishārāt wa-l-Tanbīhāt; arguably an a priori argument from the concept of existence to necessary existence

• Mulla Sadra — Developed an explicit ontological-style argument grounded in the doctrine of the primacy of existence (aṣālat al-wujūd)

• René Descartes — Reformulated the argument using the concept of God as a supremely perfect being whose essence includes existence

• Gottfried Wilhelm Leibniz — Added the crucial preliminary step of proving the coherence of the divine concept

• Norman Malcolm — Revived the Proslogion III interpretation in 1960, anticipating modal versions

• Kurt Gödel — Constructed a rigorous modal version using formal logic and positive properties

• Alvin Plantinga — Developed the standard contemporary modal version using possible-worlds semantics; himself modest about its conclusiveness

• Charles Hartshorne — Advanced a neoclassical version based on the modal asymmetry of necessary versus impossible existence

MAJOR CRITICS

• Gaunilo of Marmoutiers — Anselm's contemporary; the "perfect island" parallel argument

• Thomas Aquinas — Argued that the ontological argument fails because God's essence is not known to us in the manner the argument requires (Summa Theologica I, q. 2, a. 1)

• Immanuel Kant — The classic critique that existence is not a real predicate

• Bertrand Russell — Argued that the argument commits a logical fallacy by treating existence as a property rather than a quantifier

• J.L. Mackie — In The Miracle of Theism, argued that modal versions face insurmountable difficulties in justifying their possibility-premises

• Graham Oppy — Most systematic contemporary critic; in Ontological Arguments and Belief in God (1995), surveys and rejects all extant versions

• Peter Millican — Developed influential structural critiques of the argument's modal versions