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An ontological argument is a philosophical argument for the existence of God that uses ontology. Many arguments fall under the category of the ontological, and they tend to involve arguments about the state of being or existing. More specifically, ontological arguments tend to start with a priori theory about the organization of the universe. If that organizational structure is true, the argument will provide reasons why God must exist.
The first ontological argument in the Western Christian tradition was proposed by Anselm of Canterbury in his 1078 work Proslogion. Anselm defined God as "a being than which no greater can be conceived", and argued that this being must exist in the mind, even in the mind of the person who denies the existence of God. He suggested that, if the greatest possible being exists in the mind, it must also exist in reality. If it exists only in the mind, then an even greater being must be possible—one which exists both in the mind and in reality. Therefore, this greatest possible being must exist in reality. Seventeenth-century French philosopher René Descartes employed a similar argument. Descartes published several variations of his argument, each of which centred on the idea that God's existence is immediately inferable from a "clear and distinct" idea of a supremely perfect being. In the early eighteenth century, Gottfried Leibniz augmented Descartes' ideas in an attempt to prove that a "supremely perfect" being is a coherent concept. A more recent ontological argument came from Kurt Gödel, who proposed a formal argument for God's existence. Norman Malcolm revived the ontological argument in 1960 when he located a second, stronger ontological argument in Anselm's work; Alvin Plantinga challenged this argument and proposed an alternative, based on modal logic. Attempts have also been made to validate Anselm's proof using an automated theorem prover. Other arguments have been categorised as ontological, including those made by Islamic philosophers Mulla Sadra and Allama Tabatabai.
Since its proposal, few philosophical ideas have generated as much interest and discussion as the ontological argument. Nearly all of the great minds of Western philosophy have found the argument worthy of their attention, and a number of criticisms and objections have been mounted. The first critic of the ontological argument was Anselm's contemporary, Gaunilo of Marmoutiers. He used the analogy of a perfect island, suggesting that the ontological argument could be used to prove the existence of anything. This was the first of many parodies, all of which attempted to show that the argument has absurd consequences. Later, Thomas Aquinas rejected the argument on the basis that humans cannot know God's nature. Also, David Hume offered an empirical objection, criticising its lack of evidential reasoning and rejecting the idea that anything can exist necessarily. Immanuel Kant's critique was based on what he saw as the false premise that existence is a predicate. He argued that "existing" adds nothing (including perfection) to the essence of a being, and thus a "supremely perfect" being can be conceived not to exist. Finally, philosophers including C. D. Broad dismissed the coherence of a maximally great being, proposing that some attributes of greatness are incompatible with others, rendering "maximally great being" incoherent.
- 1 Classification
- 2 Development
- 3 Criticisms and objections
- 4 Notes
- 5 Bibliography
- 6 External links
The traditional definition of an ontological argument was given by Immanuel Kant. He contrasted the ontological argument (literally any argument "concerned with being") with the cosmological and physio-theoretical arguments. According to the Kantian view, ontological arguments are those founded on a priori reasoning.
Graham Oppy, who elsewhere expressed the view that he "see[s] no urgent reason" to depart from the traditional definition, defined ontological arguments as those that begin with "nothing but analytic, a priori and necessary premises" and conclude that God exists. Oppy admitted, however, that not all of the "traditional characteristics" of an ontological argument (analyticity, necessity, and a priority) are found in all ontological arguments and, in his 2007 work Ontological Arguments and Belief in God, suggested that a better definition of an ontological argument would employ only considerations "entirely internal to the theistic worldview".
Oppy subclassified ontological arguments into definitional, conceptual (or hyperintensional), modal, Meinongian, experiential, mereological, higher-order, or Hegelian categories, based on the qualities of their premises. He defined these qualities as follows: definitional arguments invoke definitions; conceptual arguments invoke "the possession of certain kinds of ideas or concepts"; modal arguments consider possibilities; Meinongian arguments assert "a distinction between different categories of existence"; experiential arguments employ the idea that God exists solely to those who have had experience of him; and Hegelian arguments are from Hegel. He later categorised mereological as arguments that "draw on... the theory of the whole-part relation".
William Lane Craig criticised Oppy's study as too vague for useful classification. Craig argued that an argument can be classified as ontological if it attempts to deduce the existence of God, along with other necessary truths, from his definition. He suggested that proponents of ontological arguments would claim that, if one fully understood the concept of God, one must accept his existence. William L. Rowe defined ontological arguments as those that start from the definition of God and, using only a priori principles, conclude with God's existence.
Although a version of the ontological argument appears explicitly in the writings of the ancient Greek philosopher Xenophanes and variations appear in writings by Parmenides, Plato, and the Neoplatonists, the mainstream view is that the ontological argument was first clearly stated and developed by Anselm of Canterbury. Some scholars argued that the Islamic philosopher Avicenna (Ibn Sina) developed a special kind of ontological argument before Anselm, but other scholars have doubted this position. Daniel Dombrowski marked three major stages in the development of the argument: Anselm's initial explicit formulation; the eighteenth-century criticisms of Kant and Hume; and the identification of a second ontological argument in Anselm's Proslogion by twentieth-century philosophers.
Theologian and philosopher Anselm of Canterbury (1033–1109) proposed an ontological argument in the second and third chapters of his Proslogion. Anselm's argument was not presented in order to prove God's existence; rather, Proslogion was a work of meditation in which he documented how the idea of God became self-evident to him.
In Chapter 2 of the Proslogion, Anselm defined God as a "being than which no greater can be conceived". He suggested that even "the fool" can understand this concept, and this understanding itself means that the being must exist in the mind. The concept must exist either only in our mind, or in both our mind and in reality. If such a being exists only in our mind, then a greater being—that which exists in the mind and in reality—can be conceived (this argument is generally regarded as a reductio ad absurdum because the view of the fool is proven to be inconsistent). Therefore, if we can conceive of a being than which nothing greater can be conceived, it must exist in reality. Thus, a being than which nothing greater could be conceived, which Anselm defined as God, must exist in reality.
Anselm's argument in Chapter 2 can be summarized as follows:
- It is a conceptual truth (or, so to speak, true by definition) that God is a being than which none greater can be imagined (that is, the greatest possible being that can be imagined).
- God exists as an idea in the mind.
- A being that exists as an idea in the mind and in reality is, other things being equal, greater than a being that exists only as an idea in the mind.
- Thus, if God exists only as an idea in the mind, then we can imagine something that is greater than God (that is, a greatest possible being that does exist).
- But we cannot imagine something that is greater than God (for it is a contradiction to suppose that we can imagine a being greater than the greatest possible being that can be imagined.)
- Therefore, God exists.
In Chapter 3, Anselm presented a further argument in the same vein:
- By definition, God is a being than which none greater can be imagined.
- A being that necessarily exists in reality is greater than a being that does not necessarily exist.
- Thus, by definition, if God exists as an idea in the mind but does not necessarily exist in reality, then we can imagine something that is greater than God.
- But we cannot imagine something that is greater than God.
- Thus, if God exists in the mind as an idea, then God necessarily exists in reality.
- God exists in the mind as an idea.
- Therefore, God necessarily exists in reality.
This contains the notion of a being that cannot be conceived not to exist. He argued that if something can be conceived not to exist, then something greater can be conceived. Consequently, a thing than which nothing greater can be conceived cannot be conceived not to exist and so it must exist. This can be read as a restatement of the argument in Chapter 2, although Norman Malcolm believed it to be a different, stronger argument.
René Descartes (1596–1650) proposed a number of ontological arguments, which differed from Anselm's formulation. Generally speaking, they are less formal arguments than natural intuition.
Descartes wrote in the Fifth Meditation:
But, if the mere fact that I can produce from my thought the idea of something entails that everything that I clearly and distinctly perceive to belong to that thing really does belong to it, is not this a possible basis for another argument to prove the existence of God? Certainly, the idea of God, or a supremely perfect being, is one that I find within me just as surely as the idea of any shape or number. And my understanding that it belongs to his nature that he always exists is no less clear and distinct than is the case when I prove of any shape or number that some property belongs to its nature.— Descartes, (AT 7:65; CSM 2:45)
Descartes argued that God's existence can be deduced from his nature, just as geometric ideas can be deduced from the nature of shapes—he used the deduction of the sizes of angles in a triangle as an example. He suggested that the concept of God is that of a supremely perfect being, holding all perfections. He seems to have assumed that existence is a predicate of a perfection. Thus, if the notion of God did not include existence, it would not be supremely perfect, as it would be lacking a perfection. Consequently, the notion of a supremely perfect God who does not exist, Descartes argues, is unintelligible. Therefore, according to his nature, God must exist.
In Spinoza's Ethics, he wrote a section titled "Treating of God and What Pertains to Him", in which he discusses God's existence and what God is. He starts off by saying: "whether there is a God, this, we say, can be proved". His proof for God follows a similar structure as Descartes' ontological argument. Descartes attempts to prove God's existence by arguing that there "must be some one thing that is supremely good, through which all good things have their goodness". Spinoza's argument differs in that he does not move straight from the conceivability of the greatest being to the existence of God, but rather uses a deductive argument from the idea of God. Spinoza says that man's ideas do not come from himself, but from some sort of external cause. Thus the things whose characteristics a man knows must have come from some prior source. So, if man has the idea of God, then God must exist before this thought, because man cannot create an idea of his own imagination.
Gottfried Wilhelm Leibniz saw a problem with Descartes' ontological argument: that Descartes had not asserted the coherence of a "supremely perfect" being. He proposed that, unless the coherence of a supremely perfect being could be demonstrated, the ontological argument fails. Leibniz saw perfection as impossible to analyse; therefore, it would be impossible to demonstrate that all perfections are incompatible. He reasoned that all perfections can exist together in a single entity, and that Descartes' argument is still valid.
Mulla Sadra (c. 1571/2 – 1640) was an Iranian Shia Islamic philosopher who was influenced by earlier Muslim philosophers such as Avicenna and Suhrawardi, as well as the Sufi metaphysician Ibn 'Arabi. Sadra discussed Avicenna's arguments for the existence of God, claiming that they were not a priori. He rejected the argument on the basis that existence precedes essence, or that the existence of human beings is more fundamental than their essence.
Sadra put forward a new argument, known as Seddiqin Argument or Argument of the Righteous. The argument attempts to prove the existence of God through the reality of existence, and to conclude with God's pre-eternal necessity. In this argument, a thing is demonstrated through itself, and a path is identical with the goal. In other arguments, the truth is attained from an external source, such as from the possible to the necessary, from the originated to the eternal origin, or from motion to the unmoved mover. In the argument of the righteous, there is no middle term other than the truth. His version of the ontological argument can be summarized as follows:
- There is existence
- Existence is a perfection above which no perfection may be conceived
- God is perfection and perfection in existence
- Existence is a singular and simple reality; there is no metaphysical pluralism
- That singular reality is graded in intensity in a scale of perfection (that is, a denial of a pure monism).
- That scale must have a limit point, a point of greatest intensity and of greatest existence.
- Hence God exists.
Mulla Sadra describes this argument in his main work al-asfar al-arba‘a [four journeys] as follows:
Existence is a single, objective and simple reality, and there is no difference between its parts, unless in terms of perfection and imperfection, strength, and weakness... And the culmination of its perfection, where there is nothing more perfect, is its independence from any other thing. Nothing more perfect should be conceivable, as every imperfect thing belongs to another thing and needs this other to become perfect. And, as it has already been explicated, perfection is prior to imperfection, actuality to potency, and existence to non-existence. Also, it has been explained that the perfection of a thing is the thing itself, and not a thing in addition to it. Thus, either existence is independent of others or it is in need of others. The former is the Necessary, which is pure existence. Nothing is more perfect than Him. And in Him there is no room for non-existence or imperfection. The latter is other than Him, and is regarded as His acts and effects, and for other than Him there is no subsistence, unless through Him. For there is no imperfection in the reality of existence, and imperfection is added to existence only because of the quality of being caused, as it is impossible for an effect to be identical with its cause in terms of existence.
Although Kant was critical of Descartes' formulation of the ontological argument, he did believe that the argument was persuasive when created correctly.
Kant's argument rested on the belief that everything that it is possible may exist must have a grounds for this possibility: in other words, nothing is possible merely in virtue of its nature. He thus concludes that every possibility must be based upon a single necessity, which he identified as being God. Kant attempted to show in his works that this being possessed many of the common attributes of God, such as omnipotence, omniscience and omnipresence.
Mathematician Kurt Gödel provided a formal argument for God's existence. The argument was constructed by Gödel but not published until long after his death. He provided an argument based on modal logic; he uses the conception of properties, ultimately concluding with God's existence.
Definition 1: x is God-like if and only if x has as essential properties those and only those properties which are positive
Definition 2: A is an essence of x if and only if for every property B, x has B necessarily if and only if A entails B
Definition 3: x necessarily exists if and only if every essence of x is necessarily exemplified
Axiom 1: If a property is positive, then its negation is not positive
Axiom 2: Any property entailed by—i.e., strictly implied by—a positive property is positive
Axiom 3: The property of being God-like is positive
Axiom 4: If a property is positive, then it is necessarily positive
Axiom 5: Necessary existence is positive
Axiom 6: For any property P, if P is positive, then being necessarily P is positive
Theorem 1: If a property is positive, then it is consistent, i.e., possibly exemplified
Corollary 1: The property of being God-like is consistent
Theorem 2: If something is God-like, then the property of being God-like is an essence of that thing
Theorem 3: Necessarily, the property of being God-like is exemplified
Gödel defined being "god-like" as having every positive property. He left the term "positive" undefined. Gödel proposed that it is understood in an aesthetic and moral sense, or alternatively as the opposite of privation (the absence of necessary qualities in the universe). He warned against interpreting "positive" as being morally or aesthetically "good" (the greatest advantage and least disadvantage), as this includes negative characteristics. Instead, he suggested that "positive" should be interpreted as being perfect, or "purely good", without negative characteristics.
Gödel's listed theorems follow from the axioms, so most criticisms of the theory focus on those axioms or the assumptions made. Oppy argued that Gödel gives no definition of "positive properties". He suggested that if these positive properties form a set, there is no reason to believe that any such set exists which is theologically interesting, or that there is only one set of positive properties which is theologically interesting.
Modal versions of the ontological argument
Modal logic deals with the logic of possibility as well as necessity. Paul Oppenheimer and Edward N. Zalta note that, for Anselm's Proslogion chapter 2, "Many recent authors have interpreted this argument as a modal one." In the phrase 'that than which none greater can be conceived', the word 'can' could be construed as referring to a possibility. Nevertheless, the authors write that "the logic of the ontological argument itself doesn't include inferences based on this modality." However, there have been newer, avowedly modal logic versions of the ontological argument, and on the application of this type of logic to the argument, James Franklin Harris writes:
[D]ifferent versions of the ontological argument, the so-called "modal" versions of the argument, which arguably avoid the part of Anselm's argument that "treats existence as a predicate," began to emerge. The [modal logic version] of these forms of defense of the ontological argument has been the most significant development.
Hartshorne and Malcolm
Charles Hartshorne and Norman Malcolm are primarily responsible for introducing modal versions of the argument into the contemporary debate. Both claimed that Anselm had two versions of the ontological argument, the second of which was a modal logic version. According to James Harris, this version is represented by Malcolm thus:
If it [that than which nothing greater can be conceived] can be conceived at all it must exist. For no one who denies or doubts the existence of a being a greater than which is inconceivable, denies or doubts that if it did exist its nonexistence, either in reality or in the understanding, would be impossible. For otherwise it would not be a being a greater than which cannot be conceived. But as to whatever can be conceived but does not exist: if it were to exist its nonexistence either in reality or in the understanding would be possible. Therefore, if a being a greater than which cannot be conceived, can even be conceived, it must exist.
Hartshorne says that, for Anselm, "necessary existence is a superior manner of existence to ordinary, contingent existence and that ordinary, contingent existence is a defect." For Hartshorne, both Hume and Kant focused only upon whether what exists is greater than what does not exist. However, "Anselm's point is that what exists and cannot not exist is greater than that which exists and can not exist." This avoids the question of whether or not existence is a predicate.
Referring to the two ontological arguments proposed by Anselm in Chapters 2 and 3 of his Proslogion, Malcolm supported Kant's criticism of Anselm's argument in Chapter 2: that existence cannot be a perfection of something. However, he identified what he sees as the second ontological argument in Chapter 3 which is not susceptible to such criticism.
In Anselm's second argument, Malcolm identified two key points: first, that a being whose non-existence is logically impossible is greater than a being whose non-existence is logically possible, and second, that God is a being "than which a greater cannot be conceived". Malcolm supported that definition of God and suggested that it makes the proposition of God's existence a logically necessarily true statement (in the same way that "a square has four sides" is logically necessarily true). Thus, while rejecting the idea of existence itself being a perfection, Malcolm argued that necessary existence is a perfection. This, he argued, proved the existence of an unsurpassably great necessary being.
Jordon Sobel writes that Malcolm is incorrect in assuming that the argument he is expounding is to be found entirely in Proslogion chapter 3. "Anselm intended in Proslogion III not an independent argument for the existence of God, but a continuation of the argument of Proslogion II."
Christian Analytic philosopher Alvin Plantinga criticized Malcolm's and Hartshorne's arguments, and offered an alternative. He argued that, if Malcolm does prove the necessary existence of the greatest possible being, it follows that there is a being which exists in all worlds whose greatness in some worlds is not surpassed. It does not, he argued, demonstrate that such a being has unsurpassed greatness in this world.
In an attempt to resolve this problem, Plantinga differentiated between "greatness" and "excellence". A being's excellence in a particular world depends only on its properties in that world; a being's greatness depends on its properties in all worlds. Therefore, the greatest possible being must have maximal excellence in every possible world. Plantinga then restated Malcolm's argument, using the concept of "maximal greatness". He argued that it is possible for a being with maximal greatness to exist, so a being with maximal greatness exists in a possible world. If this is the case, then a being with maximal greatness exists in every world, and therefore in this world.
The conclusion relies on a form of modal axiom S5, which states that if something is possibly true, then its possibility is necessary (it is possibly true in all worlds). Plantinga's version of S5 suggests that "To say that p is possibly necessarily true is to say that, with regard to one world, it is true at all worlds; but in that case it is true at all worlds, and so it is simply necessary." A version of his argument is as follows:
- A being has maximal excellence in a given possible world W if and only if it is omnipotent, omniscient and wholly good in W; and
- A being has maximal greatness if it has maximal excellence in every possible world.
- It is possible that there is a being that has maximal greatness. (Premise)
- Therefore, possibly, it is necessarily true that an omniscient, omnipotent, and perfectly good being exists.
- Therefore, (by axiom S5) it is necessarily true that an omniscient, omnipotent and perfectly good being exists.
- Therefore, an omniscient, omnipotent and perfectly good being exists.
Plantinga argued that, although the first premise is not rationally established, it is not contrary to reason. Michael Martin argued that, if certain components of perfection are contradictory, such as omnipotence and omniscience, then the first premise is contrary to reason. Martin also proposed parodies of the argument, suggesting that the existence of anything can be demonstrated with Plantinga's argument, provided it is defined as perfect or special in every possible world.
Another Christian apologist, William Lane Craig, characterizes Plantinga's argument in a slightly different way:
- It is possible that a maximally great being exists.
- If it is possible that a maximally great being exists, then a maximally great being exists in some possible world.
- If a maximally great being exists in some possible world, then it exists in every possible world.
- If a maximally great being exists in every possible world, then it exists in the actual world.
- If a maximally great being exists in the actual world, then a maximally great being exists.
- Therefore, a maximally great being exists.
According to Craig, premises (2)–(5) are relatively uncontroversial among philosophers, but "the epistemic entertainability of premise (1) (or its denial) does not guarantee its metaphysical possibility." Furthermore, Richard M. Gale argued that premise three, the "possibility premise", begs the question. He stated that one only has the epistemic right to accept the premise if one understands the nested modal operators, and that if one understands them within the system S5—without which the argument fails—then one understands that "possibly necessarily" is in essence the same as "necessarily". Thus the premise begs the question because the conclusion is embedded within it. On S5 systems in general, James Garson writes that "the words ‘necessarily’ and ‘possibly’, have many different uses. So the acceptability of axioms for modal logic depends on which of these uses we have in mind."
An approach to supporting the possibility premise in Plantinga's version of the argument was attempted by Alexander Pruss. He started with the 8th–9th-century AD Indian philosopher Sankara's dictum that if something is impossible, we cannot have a perception (even a non-veridical one) that it is the case. It follows that if we have a perception that p, then even though it might not be the case that p, it is at least the case that possibly p. If mystics in fact perceive the existence of a maximally great being, it follows that the existence of a maximally great being is at least possible.
Paul Oppenheimer and Edward N. Zalta used an automated theorem prover—Prover9—to validate Anselm's ontological thesis. Prover9 subsequently discovered a simpler, formally valid (if not necessarily sound) ontological argument from a single non-logical premise.
Christoph Benzmuller and Bruno Woltzenlogel Paleo used an automated theorem prover to validate Scott's version of Gödel's ontological argument. It has been shown by the same researchers that Gödel's ontological argument is inconsistent. However, Scott's version of Gödel's ontological argument is consistent and thus valid.
The novelist and philosopher Iris Murdoch formulated a version of the ontological argument in her book Metaphysics as a Guide to Morals. Though she believed her version of the argument to be superior, she did reserve praise for Descartes' formulation. Her argument was phrased by her in the following way:
There is no plausible 'proof' of the existence of God except some form of the ontological proof, a 'proof' incidentally which must now take on an increased importance in theology as a result of the recent 'de-mythologising'. If considered carefully, however, the ontological proof is seen to be not exactly a proof but rather a clear assertion of faith (it is often admitted to be appropriate only for those already convinced), which could only be confidently be made on a certain amount of experience. This assertion could be put in various ways. The desire for God is certain to receive a response. My conception of God contains the certainty of its own reality. God is an object of love which uniquely excludes doubt and relativism. Such obscure statements would of course receive little sympathy from analytical philosophers, who would divide their content between psychological fact and metaphysical nonsense.
In other words, atheists may feel objections to such an argument purely on the basis that they rely on a priori methodology. Her formulations rely upon the human connections of God and man, and what such a faith does to people.
Criticisms and objections
One of the earliest recorded objections to Anselm's argument was raised by one of Anselm's contemporaries, Gaunilo of Marmoutiers. He invited his reader to conceive an island "more excellent" than any other island. He suggested that, according to Anselm's proof, this island must necessarily exist, as an island that exists would be more excellent. Gaunilo's criticism does not explicitly demonstrate a flaw in Anselm's argument; rather, it argues that if Anselm's argument is sound, so are many other arguments of the same logical form, which cannot be accepted. He offered a further criticism of Anselm's ontological argument, suggesting that the notion of God cannot be conceived, as Anselm had asserted. He argued that many theists would accept that God, by nature, cannot be fully comprehended. Therefore, if humans cannot fully conceive of God, the ontological argument cannot work.
Anselm responded to Gaunilo's criticism by arguing that the argument applied only to concepts with necessary existence. He suggested that only a being with necessary existence can fulfill the remit of "that than which nothing greater can be conceived". Furthermore, a contingent object, such as an island, could always be improved and thus could never reach a state of perfection. For that reason, Anselm dismissed any argument that did not relate to a being with necessary existence.
Other parodies have been presented, including the devil corollary, the no devil corollary and the extreme no devil corollary. The devil corollary proposes that a being than which nothing worse can be conceived exists in the understanding (sometimes the term lesser is used in place of worse). Using Anselm's logical form, the parody argues that if it exists in the understanding, a worse being would be one that exists in reality; thus, such a being exists. The no devil corollary is similar, but argues that a worse being would be one that does not exist in reality, so does not exist. The extreme no devil corollary advances on this, proposing that a worse being would be that which does not exist in the understanding, so such a being exists neither in reality nor in the understanding. Timothy Chambers argued that the devil corollary is more powerful than Gaunilo's challenge because it withstands the challenges that may defeat Gaunilo's parody. He also claimed that the no devil corollary is a strong challenge, as it "underwrites" the no devil corollary, which "threatens Anselm's argument at its very foundations".
Thomas Aquinas, while proposing five proofs of God's existence in his Summa Theologica, objected to Anselm's argument. He suggested that people cannot know the nature of God and, therefore, cannot conceive of God in the way Anselm proposed. The ontological argument would be meaningful only to someone who understands the essence of God completely. Aquinas reasoned that, as only God can completely know His essence, only He could use the argument. His rejection of the ontological argument caused other Catholic theologians to also reject the argument.
Scottish philosopher and empiricist David Hume argued that nothing can be proven to exist using only a priori reasoning. In his Dialogues Concerning Natural Religion, the character Cleanthes proposes a criticism:
...there is an evident absurdity in pretending to demonstrate a matter of fact, or to prove it by any arguments a priori. 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.
Hume also suggested that, as we have no abstract idea of existence (apart from as part of our ideas of other objects), we cannot claim that the idea of God implies his existence. He suggested that any conception of God we may have, we can conceive either of existing or of not existing. He believed that existence is not a quality (or perfection), so a completely perfect being need not exist. Thus, he claimed that it is not a contradiction to deny God's existence. Although this criticism is directed against a cosmological argument, similar to that of Samuel Clarke in his first Boyle Lecture, it has been applied to ontological arguments as well.
Immanuel Kant put forward an influential criticism of the ontological argument in his Critique of Pure Reason. His criticism is primarily directed at Descartes, but also attacks Leibniz. It is shaped by his central distinction between analytic and synthetic propositions. In an analytic proposition, the predicate concept is contained in its subject concept; in a synthetic proposition, the predicate concept is not contained in its subject concept.
Kant questions the intelligibility of the concept of a necessary being. He considers examples of necessary propositions, such as "a triangle has three angles", and rejects the transfer of this logic to the existence of God. First, he argues that such necessary propositions are necessarily true only if such a being exists: If a triangle exists, it must have three angles. The necessary proposition, he argues, does not make the existence of a triangle necessary. Thus he argues that, if the proposition "X exists" is posited, it would follow that, if X exists, it exists necessarily; this does not mean that X exists in reality. Second, he argues that contradictions arise only when the subject and predicate are maintained and, therefore, a judgement of non-existence cannot be a contradiction, as it denies the predicate.
Kant then proposes that the statement "God exists" must be analytic or synthetic—the predicate must be inside or outside of the subject, respectively. If the proposition is analytic, as the ontological argument takes it to be, then the statement would be true only because of the meaning given to the words. Kant claims that this is merely a tautology and cannot say anything about reality. However, if the statement is synthetic, the ontological argument does not work, as the existence of God is not contained within the definition of God (and, as such, evidence for God would need to be found).
Kant goes on to write, "'being' is evidently not a real predicate" and cannot be part of the concept of something. He proposes that existence is not a predicate, or quality. This is because existence does not add to the essence of a being, but merely indicates its occurrence in reality. He states that by taking the subject of God with all its predicates and then asserting that God exists, "I add no new predicate to the conception of God". He argues that the ontological argument works only if existence is a predicate; if this is not so, he claims the ontological argument is invalidated, as it is then conceivable a completely perfect being doesn't exist.
In addition, Kant claims that the concept of God is not of one a particular sense; rather, it is an "object of pure thought". He asserts that God exists outside the realm of experience and nature. Because we cannot experience God through experience, Kant argues that it is impossible to know how we would verify God's existence. This is in contrast to material concepts, which can be verified by means of the senses.
However, although Kant was sceptical of the power of Descartes' and Anselm's formulations of the argument, he nonetheless stated that he believed his formulation to be potent in The Only Possible Argument in Support of a Demonstration of the Existence of God.
Australian philosopher Douglas Gasking (1911–1994) developed a version of the ontological argument meant to prove God's non-existence. It was not intended to be serious; rather, its purpose was to illustrate the problems Gasking saw in the ontological argument.
Gasking asserted that the creation of the world is the most marvellous achievement imaginable. The merit of such an achievement is the product of its quality and the creator's disability: the greater the disability of the creator, the more impressive the achievement. Non-existence, Gasking asserts, would be the greatest handicap. Therefore, if the universe is the product of an existent creator, we could conceive of a greater being—one which does not exist. A non-existent creator is greater than one which exists, so God does not exist. Gasking's proposition that the greatest disability would be non-existence is a response to Anselm's assumption that existence is a predicate and perfection. Gasking uses this logic to assume that non-existence must be a disability.
Oppy criticized the argument, viewing it as a weak parody of the ontological argument. He stated that, although it may be accepted that it would be a greater achievement for a non-existent creator to create something than a creator who exists, there is no reason to assume that a non-existent creator would be a greater being. He continued by arguing that there is no reason to view the creation of the world as "the most marvellous achievement imaginable". Finally, he stated that it may be inconceivable for a non-existent being to create anything at all.
Coherence of a maximally great being
In his development of the ontological argument, Leibniz attempted to demonstrate the coherence of a supremely perfect being. C. D. Broad countered that if two characteristics necessary for God's perfection are incompatible with a third, the notion of a supremely perfect being becomes incoherent. The ontological argument assumes the definition of God purported by classical theism: that God is omnipotent, omniscient, and morally perfect. Kenneth Einar Himma claimed that omniscience and omnipotence may be incompatible: if God is omnipotent, then he should be able to create a being with free will; if he is omniscient, then he should know exactly what such a being will do (which may technically render them without free will). This analysis would render the ontological argument incoherent, as the characteristics required of a maximally great being cannot coexist in one being, thus such a being could not exist.
Existence vs. essence
Bertrand Russell, during his early Hegelian phase, accepted the argument; he once exclaimed: "Great God in Boots!—the ontological argument is sound!" However, he later criticized the argument, asserting that "the argument does not, to a modern mind, seem very convincing, but it is easier to feel convinced that it must be fallacious than it is to find out precisely where the fallacy lies." He drew a distinction between existence and essence, arguing that the essence of a person can be described and their existence still remain in question.
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