Gödel's ontological proof
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Gödel's ontological proof is a formal argument by the mathematician Kurt Gödel (1906–1978) for God's existence. More precisely, it presupposes the notion of positive and negative properties, and proves the necessary existence of an object which each positive property, but no negative property, applies to.
The argument is in a line of development that goes back to Anselm of Canterbury (1033–1109). St. Anselm's ontological argument, in its most succinct form, is as follows: "God, by definition, is that for which no greater can be conceived. God exists in the understanding. If God exists in the understanding, we could imagine Him to be greater by existing in reality. Therefore, God must exist." A more elaborate version was given by Gottfried Leibniz (1646–1716); this is the version that Gödel studied and attempted to clarify with his ontological argument.
Gödel left a fourteen-point outline of his philosophical beliefs in his papers. Points relevant to the ontological proof include
- 4. There are other worlds and rational beings of a different and higher kind.
- 5. The world in which we live is not the only one in which we shall live or have lived.
- 13. There is a scientific (exact) philosophy and theology, which deals with concepts of the highest abstractness; and this is also most highly fruitful for science.
- 14. Religions are, for the most part, bad—but religion is not.
The first version of the ontological proof in Gödel's papers is dated "around 1941". Gödel is not known to have told anyone about his work on the proof until 1970, when he thought he was dying. In February, he allowed Dana Scott to copy out a version of the proof, which circulated privately. In August 1970, Gödel told Oskar Morgenstern that he was "satisfied" with the proof, but Morgenstern recorded in his diary entry for 29 August 1970, that Gödel would not publish because he was afraid that others might think "that he actually believes in God, whereas he is only engaged in a logical investigation (that is, in showing that such a proof with classical assumptions (completeness, etc.) correspondingly axiomatized, is possible)." Gödel died January 14, 1978. Another version, slightly different from Scott's, was found in his papers. It was finally published, together with Scott's version, in 1987.
Morgenstern's diary is an important and usually reliable source for Gödel's later years, but the implication of the August 1970 diary entry—that Gödel did not believe in God—is not consistent with the other evidence. In letters to his mother, who was not a churchgoer and had raised Kurt and his brother as freethinkers, Gödel argued at length for a belief in an afterlife. He did the same in an interview with a skeptical Hao Wang, who said: "I expressed my doubts as G spoke [...] Gödel smiled as he replied to my questions, obviously aware that his answers were not convincing me." Wang reports that Gödel's wife, Adele, two days after Gödel's death, told Wang that "Gödel, although he did not go to church, was religious and read the Bible in bed every Sunday morning." In an unmailed answer to a questionnaire, Gödel described his religion as "baptized Lutheran (but not member of any religious congregation). My belief is theistic, not pantheistic, following Leibniz rather than Spinoza."[note 1]
The proof uses modal logic, which distinguishes between necessary truths and contingent truths. In the most common semantics for modal logic, many "possible worlds" are considered. A truth is necessary if it is true in all possible worlds. By contrast, a truth is contingent if it just happens to be the case. For instance, "more than half of this planet is covered by water" is a contingent truth, that relies upon which planet "this planet" is. If a statement happens to be true in our world, but is false in another world, then it is a contingent truth. A statement that is true in some world (not necessarily our own) is called a possible truth.
First, Gödel axiomatizes the notion of a "positive property":[note 2] for each property φ, either φ or its negation ¬φ must be positive, but not both (axiom 2). If a positive property φ implies a property ψ in each possible world, then ψ is positive, too (axiom 1). Gödel then argues that each positive property is "possibly exemplified", i.e. applies at least to some object in some world (theorem 1). Defining an object to be Godlike if it has all positive properties (definition 1), and requiring that property to be positive itself (axiom 3),[note 3] Gödel shows that in some possible world a Godlike object exists (theorem 2), called "God" in the following.[note 4] Gödel proceeds to prove that a Godlike object exists in every possible world.
To this end, he defines essences: if x is an object in some world, then a property φ is said to be an essence of x if φ(x) is true in that world and if φ necessarily entails all other properties that x has in that world (definition 2). Requiring positive properties being positive in every possible world (axiom 4), Gödel can show that Godlikeness is an essence of every Godlike object (theorem 3). Now, x is said to exist necessarily if for every essence φ of x the following is true: in every possible world, there is an element y with property φ (definition 3). Axiom 5 requires necessary existence to be a positive property.
Hence, it must follow from Godlikeness. Moreover, Godlikeness is an essence of God, since it entails all positive properties, and any nonpositive property is the negation of some positive property, so God cannot have any nonpositive properties. Since any Godlike object is necessarily existent, it follows that any Godlike object in one world is a Godlike object in all worlds, by the definition of necessary existence. Given the existence of a Godlike object in one world, proven above, we may conclude that there is a Godlike object in every possible world, as required (theorem 4). Besides axiom 1-5 and definition 1-3, a few other axioms from modal logic[clarification needed] were tacitly used in the proof.
From these hypotheses, it is also possible to prove that there is only one God in each world by Leibniz's law, the identity of indiscernibles: two or more objects are identical (the same) if they have all their properties in common, and so, there would only be one object in each world that possesses property G. Gödel did not attempt to do so however, as he purposely limited his proof to the issue of existence, rather than uniqueness.
There is an ongoing open-source effort to formalize Gödel's proof to a level that is suitable for automated theorem proving or at least computer verification via proof assistants. The effort made headlines in German newspapers. According to the authors of this effort, they were inspired by Melvin Fitting's book.
Most criticism of Gödel's proof is aimed at its axioms: As with any proof in any logical system, if the axioms the proof depends on are doubted, then the conclusions can be doubted. This is particularly applicable to Gödel's proof, because it rests on five axioms that are all questionable. The proof does not say that the conclusion has to be correct, but rather that if you accept the axioms, then the conclusion is correct.
Many philosophers have questioned the axioms. The first layer of attack is simply that there are no arguments presented that give reasons why the axioms are true. A second layer is that these particular axioms lead to unwelcome conclusions. This line of thought was argued by Jordan Howard Sobel, showing that if the axioms are accepted, they lead to a "modal collapse" where every statement that is true is necessarily true, i.e. the sets of necessary, of contingent, and of possible truths all coincide (provided there are accessible worlds at all).[note 5] According to Koons,:9 Sobel suggested that Gödel might have welcomed modal collapse.
There are suggested amendments to the proof, presented by C. Anthony Anderson, but argued to be refutable by Anderson and Michael Gettings. Sobel's proof of modal collapse has been questioned by Robert Koons,[note 6] but a counter-defence by Sobel has been given. In 2014, Christoph Benzmüller and Bruno Woltzenlogel-Paleo gave a computer-checked proof of modal collapse.:97 lf In the same paper, they suspected Gödel's original version of the axioms[note 7] to be inconsistent; in 2016, they gave a computer proof that this version implies , i.e. is inconsistent in every modal logic with a reflexive or symmetric accessibility relation.:940 lf
Gödel's proof has also been questioned by Graham Oppy, asking whether lots of other almost-gods would also be "proven" by Gödel's axioms. This counter-argument has been questioned by Gettings, who agrees that the axioms might be questioned, but disagrees that Oppy's particular counter-example can be shown from Gödel's axioms.
There are many more criticisms, most focusing on the philosophically interesting question of whether these axioms must be rejected to avoid odd conclusions. The broader criticism is that even if the axioms cannot be shown to be false, that does not mean that they are true. Hilbert's famous remark about interchangeability of the primitives' names applies to those in Gödel's ontological axioms ("positive", "god-like", "essence") as well as to those in Hilbert's geometry axioms ("point", "line", "plane"). According to André Fuhrmann (2005) it remains to show that the dazzling notion prescribed by traditions and often believed to be essentially mysterious satisfies Gödel's axioms. This is not a mathematical, but merely a theological task.:364–366 It is this task which decides which religion's god has been proven to exist.
A humorous variant of Gödel's ontological proof is mentioned in Quentin Canterel's novel The Jolly Coroner.[vague]
- Gödel's answer to a special questionnaire sent him by the sociologist Burke Grandjean. This answer is quoted directly in Wang 1987, p. 18, and indirectly in Wang 1996, p. 112. It's also quoted directly in Dawson 1997, p. 6, who cites Wang 1987. The Grandjean questionnaire is perhaps the most extended autobiographical item in Gödel's papers. Gödel filled it out in pencil and wrote a cover letter, but he never returned it. "Theistic" is italicized in both Wang 1987 and Wang 1996. It is possible that this italicization is Wang's and not Gödel's. The quote follows Wang 1987, with two corrections taken from Wang 1996. Wang 1987 reads "Baptist Lutheran" where Wang 1996 has "baptized Lutheran". "Baptist Lutheran" makes no sense, especially in context, and was presumably a typo or mistranscription. Wang 1987 has "rel. cong.", which in Wang 1996 is expanded to "religious congregation".
- This assumes that it is possible to single out positive properties from among all properties. Gödel comments that "Positive means positive in the moral aesthetic sense (independently of the accidental structure of the world)... It may also mean pure attribution as opposed to privation (or containing privation)." (Gödel 1995), see also manuscript in (Gawlick 2012).
- Axioms 1, 2 and 3 can be summarized by saying that positive properties form a principal ultrafilter.
- By removing all modal operators from axioms, definitions, proofs, and theorems, a modified version of theorem 2 is obtained saying "∃x G(x)", i.e. "There exists an object which has all positive, but no negative properties". Nothing more than axioms 1-3, definition 1, and theorems 1-2 needs to be considered for this result.
- Formally, for all p implies for all p by indirect proof, and holds for all p whenever there are accessible worlds.
- Since Sobel's proof of modal collapse uses lambda abstraction, but Gödel's proof does not, Koons suggests to forbid this property-construction operation as the "most conservative" measure, before "rejecting or emending ... axioms (as Anderson does)".
- differing from those shown here by omitting the first conjunct, , in Df.2
- Quoted in Gödel 1995, p. 388. The German original is quoted in Dawson 1997, p. 307. The nested parentheses are in Morgenstern's original diary entry, as quoted by Dawson.
- The publication history of the proof in this paragraph is from Gödel 1995, p. 388
- Dawson 1997, pp. 6.
- Dawson 1997, pp. 210–212.
- Wang 1996, p. 317. The ellipsis is Wikipedia's.
- Wang 1996, p. 51.
- Fitting, 2002, p. 139
- Knight, David (23 October 2013). "Scientists Use Computer to Mathematically Prove Gödel's God Theorem". Der Spiegel. Retrieved 28 October 2013.
- Jordan Howard Sobel (Nov 1987). "Gödel's ontological proof". In Judith Jarvis Thomson. On Being and Saying: Essays for Richard Cartwright. Cambridge/MA & London, England: MIT Press. pp. 241–261. ISBN 978-0262200639.
- Robert C. Koons (Jul 2005). Sobel on Gödel’s Ontological Proof (PDF) (Unpublished Paper). University of Texas at Austin.
- Kurt Gödel (Mar 1995). "Texts Relating to the Ontological Proof". In Solomon Feferman; John W. Dawson jr.; Warren Goldfarb; Charles parsons; Robert M. Solovay. Unpublished Essays and Lectures (PDF). Collected Works. III (1st ed.). Oxford: Oxford University Press. pp. 429–437. ISBN 0-19-507255-3. Here: p.435; probably, Sobel referred to Gödel's note: "If is assumed [as following from the essence of ], ... but that is the inferior way. Rather, should follow first from the existence of God." The note might indicate that Gödel was aware of his axioms implying modal collapse.
- Curtis Anthony Anderson (Jul 1990). "Some Emendations of Gödel's Ontological Proof" (PDF). Faith and Philosophy. 7 (3): 291—303.
- Curtis Anthony Anderson and Michael Gettings (Aug 1996). "Gödel's ontological proof revisited". In Petr Hájek. Proc. Gödel '96: Logical Foundations of Mathematics, Computer Science and Physics — Kurt Gödel's Legacy. Lecture Notes in Logic. 6. Springer. pp. 167—172.
- Christoph Benzmüller and Bruno Woltzenlogel-Paleo (2014). "Automating Gödel's Ontological Proof of God's Existence with Higher-Order Automated Theorem Provers" (PDF). Proc. European Conference on Artificial Intelligence. Frontiers in Artificial Intelligence and Applications. 263. IOS Press. pp. 93—98.
- Christoph Benzmüller and Bruno Woltzenlogel-Paleo (Jul 2016). "The Inconsistency in Gödel's Ontological Argument: — A Success Story for AI in Metaphysics" (PDF). In Subbarao Kambhampati. Proc. 25th International Joint Conference on Artificial Intelligence. AAAI Press. pp. 936—942.
- Graham Oppy (Oct 1996). "Godelian ontological arguments". Analysis. 54 (4): 226—230. — Longer version (2005)
- Gettings, Michael. "Gödel's ontological argument: a reply to Oppy." Analysis 59.264 (1999): 309–313.
- André Fuhrmann (2005). "Existenz und Notwendigkeit — Kurt Gödels axiomatische Theologie" [Existence and Necessity — Kurt Gödel's Axiomatic Theology] (PDF). In W. Spohn. Logik in der Philosophie [Logic in Philosophy] (in German). Heidelberg: Synchron. pp. 349—374.
- Bjørdal, Frode. "Understanding Gödel's Ontological Argument", in T. Childers (ed.), The Logica Yearbook 1998, Prague 1999, 214-217.
- Bromand, Joachim. "Gödels ontologischer Beweis und andere modallogische Gottesbeweise", in J. Bromand und G. Kreis (Hg.), Gottesbeweise von Anselm bis Gödel, Berlin 2011, 381-491.
- John W. Dawson, Jr (1997). Logical Dilemmas: The Life and Work of Kurt Godel. Wellesley, Mass: AK Peters, Ltd. ISBN 1-56881-025-3.
- Melvin Fitting, "Types, Tableaus, and Godel's God" Publisher: Dordrecht Kluwer Academic, 2002, ISBN 1-4020-0604-7, ISBN 978-1-4020-0604-3
- Kurt Gödel (Mar 1995). Solomon Feferman; John W. Dawson jr.; Warren Goldfarb; Charles parsons; Robert M. Solovay, eds. Unpublished Essays and Lectures (PDF). Collected Works. III (1st ed.). Oxford: Oxford University Press. ISBN 0-19-507255-3. — See Chapter "Ontological Proof", pp. 403–404, and Appendix B "Texts Relating to the Ontological Proof", pp. 429–437.
- Goldman, Randolph R. "Gödel's Ontological Argument", PhD Diss., University of California, Berkeley 2000.
- Hazen, A. P. "On Gödel's Ontological Proof", Australasian Journal of Philosophy, Vol. 76, No 3, pp. 361–377, September 1998
- Small, Christopher. "Reflections on Gödel's Ontological Argument" (PDF). University of Waterloo.
- Wang, Hao (1987). Reflections on Kurt Gödel. Cambridge, Mass: MIT Press. ISBN 0-262-23127-1.
- Wang, Hao (1996). A Logical Journey: from Gödel to Philosophy. Cambridge, Mass: MIT Press. ISBN 0-262-23189-1.
- Oppy, Graham. "Ontological arguments". Stanford Encyclopedia of Philosophy.
- Annotated bibliography of studies on Gödel's Ontological Argument
- Thomas Gawlick, Was sind und was sollen mathematische Gottesbeweise?, Jan. 2012 — shows Gödel's original proof manuscript on p. 2-3