A (Simplified) Supreme Being Necessarily Exists — Says the Computer!
A simplified variant of Kurt Gödel’s modal ontological argument is presented. Some of Gödel’s, resp. Dana Scott’s, premises are modified, others are dropped, and modal collapse is avoided. The emended argument is shown valid already in quantified modal logic K.
The presented simplifications have been computationally explored utilising latest knowledge representation and reasoning technology based on higher-order logic. The paper thus illustrates how modern symbolic AI technology can contribute new knowledge to formal philosophy and theology.
Variants of Kurt Gödel’s \shortciteGoedelNotes, resp. Dana Scott’s
\shortciteScottNotes, modal ontological argument have previously
been analysed and verified on the computer by Benzmüller and
Woltzenlogel \shortciteC40,C55 and Benzmüller and Fuenmayor
\shortciteJ52, and even some unknown flaws were
revealed in these works.
In this paper a simplified and improved variant of Gödel’s modal ontological argument is presented. This simplification has been explored in collaboration with the proof assistant Isabelle/HOL [\citeauthoryearNipkow et al.2002], while employing Benzmüller’s \shortciteJ41,J23 shallow semantical embedding (SSE) approach as enabling technology. This technology supports the reuse of automated theorem proving (ATP) tools for classical higher-order logic (HOL) to represent and reason with a wide range of non-classical logics and theories, including higher-order modal logic (HOML) and Gödel’s modal ontological argument, which are in the focus of this paper.
The new, simplified modal argument is as follows. Gödel’s definition of a Godlike entity remains unchanged ( is an uninterpreted constant denoting positive properties):
A Godlike entity thus possesses all positive properties.
The three single axioms of the new theory are
where the following defined terms are used ( is a possibilist quantifier and is an actualist quantifier for individuals):
Informally we have:
Self-identity is a positive property, self-difference is not.
A property entailed or necessarily entailed by a positive property is positive.
The conjunction of any collection of positive properties is positive. (Technical reading: if is any set of positive properties, then the property obtained by taking the conjunction of the properties in is positive.)
From these premises it follows, in a few argumentations steps in base modal logic K, that a Godlike entity possibly and necessarily exists. Modal collapse, which expresses that there are no contingent truths and which thus eliminates the possibility of alternative possible worlds, does not follow from these axioms. These observations should render the new theory interesting to formal philosophy and theology.
Compare the above with Gödel’s premises of the modal ontological argument (we give the consistent variant of Scott):
Necessary existence () and essence () are defined as (the other definitions are as above):
Informally: property is the essence of an entity if, and only if, (i) holds for and (ii) necessarily entails every property of . Moreover, an entity has the property of necessary existence if, and only if, the essence of is necessarily instantiated.
We also give informal readings of Gödel’s axioms:
A1 says that one of a property or its complement is
positive. A2 states that a property necessarily
entailed by a positive property is positive, and A3 is as before.
A4 expresses that any positive property is necessarily
A5 postulates that necessary existence is a
positive property. Axiom B (symmetry of the accessibility
relation associated with modal -operator) is added to ensure that we are in modal logic
KB instead of just
Using Gödel’s premises as stated it can be proved that a Godlike entity possibly and necessarily exists [\citeauthoryearBenzmüller and Woltzenlogel Paleo2014]. However, as is well known [\citeauthoryearSobel1987], modal collapse is implied; see also Fitting \shortcitefitting02:_types_tableaus_god and Sobel \shortcitesobel2004logic for further details on this.
Benzmüller and Fuenmayor \shortciteJ52 showed that different modal ultrafilter properties can be deduced from Gödel’s premises. These insights are key to the new argument presented in this paper: If Gödel’s premises entail that positive properties form a modal ultrafilter, then why not turning things around, and start with an axiom U1 postulating ultrafilter properties for ? Then use U1 instead of other axioms for proving that a Godlike entity necessarily exists, and on the fly explore what further simplifications of the argument are triggered. This research plan worked out and it ultimately led to the new modal ontological argument presented above.
The proof assistant Isabelle/HOL and its integrated ATP systems have supported our exploration work surprisingly well, despite the undecidability and high complexity of the underlying logic setting. As usual, we here only present the main steps of the exploration process, and various interesting eureka and frustration steps in between are dropped.
Paper structure: An SSE of HOML in HOL is introduced in Sect. 2. The foundations outlined there ensure that the paper is sufficiently self-contained; readers familiar with the SSE approach may simply skip it. Modal ultrafilter are defined in Sect. 3. Section 4 recaps the Gödel/Scott variant, and then an ultrafilter-based modal ontological argument is presented in Sect. 5. This new argument is further simplified in Sect. 6, leading to our new proposal based on axioms A1’, A2’ and A3 as presented above. Related work is mentioned in Sect. 7. Further variants are presented in the Appendix.
Since we develop, explain and discuss our formal encodings directly in Isabelle/HOL, some familiarity with this proof assistant and its underlying logic HOL [\citeauthoryearAndrews2002, \citeauthoryearBenzmüller and Andrews2019] is assumed. The entire sources of our formalization are presented and explained.
2 Modeling HOML in HOL
Related work focused on the development of various SSEs, cf. [\citeauthoryearBenzmüller2019, \citeauthoryearKirchner et al.2019] and the references therein. These contributions, among others, show that the standard translation from propositional modal logic to first-order (FO) logic can be concisely modeled (i.e., embedded) within HOL theorem provers, so that the modal operator , for example, can be explicitly defined by the -term , where denotes the accessibility relation associated with . Then one can construct FO formulas involving and use them to represent and proof theorems. Thus, in an SSE, the target logic is internally represented using higher-order (HO) constructs in a theorem proving system such as Isabelle/HOL. Benzmüller and Paulson \shortciteJ23 developed an SSE that captures quantified extensions of modal logic. For example, if is shorthand in HOL for , then would be represented as , where stands for the -term , and the gets resolved as above.
To see how these expressions can be resolved to produce the right representation, consider the following series of reductions:
Thus, we end up with a representation of in HOL. Of course, types are assigned to each (sub-)term of the HOL language. We assign individual terms (such as variable above) the type e, and terms denoting worlds (such as variable above) the type i. From such base choices, all other types in the above presentation can actually be inferred.
An explicit encoding of HOML in Isabelle/HOL, following the above
ideas, is presented in Fig. 1.
The modal logic connectives are defined in lines 11–24. In line 16, for example, the definition of the world-lifted -connective of type is given; explicit type information is presented after the ::-token for ’c5’, which is the ASCII-denominator for the (right-associative) infix-operator as introduced in parenthesis shortly after. is then defined as abbreviation for the truth-set , respectively. In the remainder we generally use bold-face symbols for world-lifted connectives (such as ) in order to rigorously distinguish them from their ordinary counterparts (such as ) in meta-logic HOL.
Further modal logic connectives, such as , , , , and are introduced analogously. The operator , introduced in lines 22, is inverting properties of types ; this operation occurs in some Gödel’s axiom A1. and are defined in lines 23–24 as world-independent, syntactical equality.
The already discussed, world-lifted modal -operator is introduced in lines 19–20; accessibility relation is now named r. The definition of in line 21 is analogous.
The world-lifted (polymorphic) possibilist quantifier as discussed before is introduced in line 27. In line 28, user-friendly binder-notation for is additionally defined. Instead of distinguishing between and as in our illustrating example, -symbols are overloaded here. The introduction of the possibilist -quantifier in lines 29–30 is analogous.
Further actualist quantifiers, and , are introduced in lines 33–37; their definition is guarded by an explicit, possibly empty, existsAt (@) predicate, which encodes whether an individual object actually “exists” at a particular possible world, or not. These additional actualist quantifiers are declared non-polymorphic, and they support quantification over individuals only. In the subsequent study of the ontological argument we will indeed apply and for different types in the type hierarchy of HOL, while we use and only for quantification over individuals.
Global validity of a world-lifted formula , denoted as , is introduced in line 40 as an abbreviation for .
Consistency of the introduced concepts is confirmed by the model finder nitpick [\citeauthoryearBlanchette and Nipkow2010] in line 43. Since only abbreviations and no axioms have been introduced so far, the consistency of the Isabelle/HOL theory HOML as displayed in Fig. 1 is actually evident.
In line 44–47 it is studied whether instances of the Barcan and the converse Barcan formulas are implied. As expected, both principles are valid only for possibilist quantification, while they have countermodels for actualist quantification.
The SSE of HOML in HOL is faithful for base modal logic K.
Follows [\citeauthoryearBenzmüller and Paulson2013]. ∎
Theory HOML thus successfully models base modal logic K in HOL. To arrive at logic KB the symmetry axiom B as shown earlier can be postulated.
3 Modal Ultrafilter
Theory ModalUltrafilter, see Fig. 2, imports theory HOML and adapts the topological notions of filter and ultrafilter to our modal logic setting. For an introduction to filter and ultrafilter see the literature, e.g., [\citeauthoryearBurris and Sankappanavar1981].
Modal ultrafilter are introduced in lines 18–19 as world-lifted characteristic functions of type . A modal ultrafilter is thus a world-dependent set of intensions of -type properties; in other words, a -subset of the -powerset of -type property extensions. An ultrafilter is defined as a filter satisfying an additional maximality condition: , where is elementhood of -type objects in -sets of -type objects (see line 4), and where is the relative set complement operation on sets of entities (line 9).
A filter , see lines 12–15, is required to
be large: , where U denotes the full set of -type objects we start with,
exclude the empty set: , where is the world-lifted empty set of -type objects,
be closed under supersets: (world-lifted -relation is defined in line 7), and
be closed under intersections: (where is defined in line 8).
Benzmüller and Fuenmayor \shortciteJ52 have studied two different notions of modal ultrafilter (called - and -ultrafilter) which are defined on intensions and extensions of properties, respectively. This distinction is not needed in this paper; what we call modal ultrafilter here corresponds to their -ultrafilter.
4 Gödel’s Modal Ontological Argument
The full formalization of Scott’s variant of Gödel’s argument, which relies on theories ModalUltrafilter and HOML, is presented in Fig. 3. Line 3 starts out with the declaration of the uninterpreted constant symbol , for positive properties, which is of type . is thus an intensional, world-depended concept.
The premises of Gödel’s argument, as already discussed earlier,
are stated in lines 5–24.
An abstract level “proof net” for theorem T6, the necessary existence of a Godlike entity, is presented in lines 26–34. Following the literature the proof goes as follows: From A1 and A2 infer T1: positive properties are possibly exemplified. From A3 and the defn. of obtain T2: being Godlike is a positive property (Scott actually directly postulated T2). Using T1 and T2 show T3: possibly a Godlike entity exists. Next, use A1, A4, the defns. of and to infer T4: being Godlike is an essential property of any Godlike entity. From this, A5, B and the defns. of and have T5: the possible existence of a Godlike entity implies its necessary existence. T5 and T3 then imply T6.
The six subproofs and their dependencies have been automatically
explored using state-of-the-art ATP system integrated with
Isabelle/HOL via its sledgehammer tool; sledgehammer then identified and returned the abstract
level proof justifications as displayed here, e.g. “using T1 T2 by
simp”. The mentioned proof engines/tactics blast, metis, simp and
smt are trustworthy components of Isabelle/HOL’s, since they internally
reconstruct and check each (sub-)proof in the proof assistants small and trusted
proof kernel. Using the definitions from Sect. 2, one can
also reconstruct and formally verify all
proofs with pen and paper directly in meta-logic HOL.
The presented theory is consistent, which is confirmed in line 37 by model finder nitpick; nitpick reports a model (not shown here) consisting of one world and one Godlike entity.
Validity of modal collapse (MC) is confirmed in lines 40–47; a proof net displaying the proofs main idea is shown.
Most relevant for this paper is that the ATP systems were able to quickly prove that Gödel’s notion of positive properties constitutes a modal ultrafilter, cf. lines 50–57. This was key to the idea of taking the modal ultrafilter property of as an axiom U1 of the theory; see the next section.
5 Ultrafilter Modal Ontological Argument
Taking U1 as an axiom for Gödel’s theory in fact leads to a significant simplification of the modal ontological argument; this is shown in lines 16–28 in Fig. 4: not only Gödel’s axiom A1 can be dropped, but also axioms A4 and A5, together with defns. and . Even logic KB can be given up, since K is now sufficient for verifying the proof argument.
The proof is similar to before: Use U1 and A2 to infer T1 (positive properties are possibly exemplified). From A3 and defn. of have T2 (being Godlike is a positive property). T1 and T2 imply T3 (a Godlike entity possibly exists). From U1, A2, T2 and the defn. of have T5 (possible existence of a Godlike entity implies its necessary existence). Use T5 and T3 to conclude T6 (necessary existence of a Godlike entity).
Consistency of the theory is confirmed in line 31; again a model with one world and one Godlike entity is reported.
Most interestingly, modal collapse MC now has a simple countermodel as nitpick informs us. This countermodel consists of a single entity and two worlds and with . Trivially, formula is such that holds in but not in , which invalidates MC at world . is the Godlike entity in both worlds, i.e., is the property that holds for in and , which we may denote as . Using tuple notation we may write .
Remember that , which is of type
an intensional, world-depended concept. In our countermodel for MC the
extension of for world has the above and
as its elements, while
in world we have and
tuple notation we note
In order to verify that is a modal ultrafilter we have to check whether the respective modal ultrafilter conditions are satisfied in both worlds. in and also in , since both and are in ; in and also in , since both and are not in . It is also easy to verify that is closed under supersets and intersection in both worlds.
Note that in our countermodel for MC, also Gödel’s axiom A4 is invalidated. Consider , i.e., is true for in , but false for in . We have in , but we do not have in , since does not hold in , which is reachable from .
nitpick is capable of computing all partial
modal ultrafilters as part of its countermodel exploration: out of 512
candidates, nitpick identifies 32 structures of form
, for , in which satisfies the ultrafilter
conditions in the specified world . An example for such an is
is not a proper modal ultrafilter, since fails to be an ultrafilter in world .
6 Simplified Modal Ontological Argument
What modal ultrafilters properties of are actually needed to support T6? Which ones can be dropped? Experiments with the computer confirm that, in modal logic K, the filter conditions from Sect. 3 must be upheld for , while maximality can be dropped. It is possible to merge condition 3 (closed under supersets) for with Gödel’s A2 into A2’ as shown in line 18 of Fig. 5. Moreover, instead of requiring the universal set/property to be a positive property, we postulate that self-identity , which is extensionally equal to U, is in . Analogously, we replace in ultrafilter condition 2 for by self-difference . Self-identity and self-difference have been used frequently in the history of the ontological argument, which is part of the motivation for this switch. Filter condition 4 is now implied by the theory.
In summary, it is sufficient to ensure that is a filter, and this is what our simplified axioms do.
Now, from the definition of (line 13) and the axioms A1’, A2’ and A3 (lines 17–19) theorem T6 immediately follows: in line 22 several theorem provers integrated with sledgehammer report a proof in about one second when running the experiments on a standard notebook. Moreover, a more detailed “proof net” is presented in lines 23–28; the proof argument is analogous to what has been discussed before.
Consistency is confirmed by nitpick in line 31, and a counter model (similar to the one discussed in the previous section) is reported to MC in line 34.
In lines 37–45 further questions are answered experimentally: neither A1, nor A4 or A5, of the premises we dropped from Gödel’s theory are implied anymore. Also Monotheism is not implied (line 42); by postulating A1 it can be enforced. In lines 44-45 we see that is a filter, but no ultrafilter. Since some of these axioms, e.g. the strong A1, have been discussed controversially in the history of Gödel’s argument and since also MC is independent, it is justified to claim that we have arrived at a philosophically and theologically potentially relevant simplification of the modal ontological argument.
7 Related Work
Fitting \shortcitefitting02:_types_tableaus_god has suggested to carefully distinguish between intensions and extensions of positive properties in the context of Gödel’s modal ontological argument, and, in order to do so within a single framework, he introduces a sufficiently expressive HOML enhanced with means for the explicit representation of intensional terms and their extensions; see also the intensional operations used by Fuenmayor and Benzmüller \shortciteC65,J52 in the course of their verification of the work of Fitting and Anderson.
The application of computational methods to philosophical problems was initially limited to first-order theorem provers. Fitelson and Zalta \shortciteFitelsonZalta used Prover9 to find a proof of the theorems about situation and world theory in [\citeauthoryearZalta1993] and they found an error in a theorem about Plato’s Forms that was left as an exercise in [\citeauthoryearPelletier and Zalta2000]. Oppenheimer and Zalta \shortciteOppenheimerZalta2011 discovered, using Prover9, that one of the three premises used in their reconstruction of Anselm’s ontological argument (in [\citeauthoryearOppenheimer and Zalta1991]) was sufficient to derive the conclusion. The first-order conversion techniques that were developed and applied in these works are outlined in some detail in [\citeauthoryearAlama et al.2015].
More recent related work makes use of higher-order proof assistants. Besides the already mentioned work of Benzmüller and colleagues, this includes Rushby’s \shortciteRushby study on Anselm’s ontological argument in the PVS proof assistant and Blumson’s \shortciteBlumson related study in Isabelle/HOL.
Gödel’s modal ontological argument stands in prominent tradition of western philosophy. The ontological argument has its roots in the Proslogion (1078) of Anselm of Canterbury and it has been picked up in the Fifth Meditation (1637) of Descartes and in the works of Leibniz, which in turn inspired and informed the work of Gödel.
In this paper we have linked Gödel’s theory to a suitably adapted mathematical theory (modal filter and ultrafilter), and subsequently we have developed a significantly simplified modal ontological argument that avoids some axioms and consequences in the new theory, including modal collapse, that have led to criticism in the past.
While data scientists apply subsymbolic AI techniques to obtain approximating and rather opaque models in their application domains of interest, we have in this paper applied modern symbolic AI techniques to arrive at sharp, explainable and verifiable models of the metaphysical concepts we are interested in. In particular, we have illustrated how state of the art theorem proving systems, in combination with latest knowledge representation and reasoning technology, can be fruitfully utilized to explore and contribute new knowledge to theoretical philosophy and theology.
I am grateful to all friends, colleagues and students who have supported this line of research in the past, including (in alphabetical order) C. Brown, D. Fuenmayor, T. Gleißner, D. Kirchner, X. Parent, R. Rojas, D. Scott, A. Steen, L. van der Torre, E. Weydert, D. Streit, B. Woltzenlogel-Paleo, E. Zalta.
Appendix: Further Variants
Further variants of the simplified ontological argument are presented in this appendix. They show that axiom A3 can be replaced by T2 (cf. footnote 2) and that A2’ can be further simplified into
A property entailed by a positive property is positive.
The latter has the consequence that T3, the possible existence of a Godlike entity, is no longer implied due to an undesired countermodel, which can again be fixed by postulating modal axiom T.
Obviously, also axiom A1’ could be replaced by
The universal property is a positive property, the empty property is not.
We leave it to the reader to choose his personally preferred technical variant of the simplified modal ontological argument as introduced in this paper.
Appendix A Postulating T2 instead of A3
Instead of working with third-order axiom A3 to infer T2, we directly postulate T2 as an axiom of the theory. This variant of the simplified modal ontological argument is displayed in Fig. 6.
Theorem T6 can be proved as before: from A1’ and A2’ we get T1 (positive properties are possibly exemplified); from T1 and T2 we infer T3 (possible existence of a Godlike entity); from A1’, A2’ and T2 obtain T5 (the possible existence of a Godlike entity implies its necessary existence); combining T3 and T5 have T6 (the necessary existence of a Godlike entity).
Appendix B Simple Entailment in Axiom A2’
Instead of using a disjunction of simple entailment and necessary entailment in axiom A2’ we may in fact only require simple entailment in A2’; see axiom A2” in line 15 of Fig. 7. A proof for T6, the necessary existence of a Godlike entity, and also T7, the existence of a Godlike entity, can still be found (lines 19–20). However, after replacing A2’ by A2”, T3 (the possible existence of a Godlike entity) is no longer implied; see line 23. T3 now has an undesired countermodel (not depicted here) which consists of one single world that is not connected to itself.
However, when assuming modal axiom T (what is necessary true is true in the given world), this countermodel is eliminated, see lines 25–26.
Appendix C Simple Entailment in Modal Logic T
The above discussion motivates the following, final alternative of the simplified modal ontological argument; see also Fig. 8. This argument, which is formulated in modal logic T (which comes with axiom T as discussed above), has the following axioms; cf. lines 17–19 in Fig. 8:
Self-identity is a positive property, self-difference is not.
A property entailed by a positive property is positive.
Being Godlike is a positive property.
The proof argument as used before is applicable: from A1’, A2” and modal axiom T deduce T1 (positive properties are possibly exemplified, line 24); from T1 and T2 infer T3 (possible existence of a Godlike entity, line 25); from A1’, A2” and T2 obtain T5 (the possible existence of a Godlike entity implies its necessary existence, line 26); combining T3 and T5 have T6 (the necessary existence of a Godlike entity, line 27).
- E.g., the theorem prover Leo-II detected that Gödel’s \shortciteGoedelNotes variant of the argument is inconsistent; this inconsistency had, unknowingly, been fixed in the variant of Scott \shortciteScottNotes; cf. [\citeauthoryearBenzmüller and Woltzenlogel Paleo2016] for more details on this.
- An alternative to A1’ would be: The universal property () is a positive property, and the empty property () is not. The third-order formalization of A3 given here has been proposed by Anderson and Gettings \shortciteAndersonGettings, see also Fitting \shortcitefitting02:_types_tableaus_god. Axiom A3, together with the definition of , implies that being Godlike is a positive property. Since supporting this inference is the only role this axiom plays in the argument, could be (and has been; cf. Scott \shortciteScottNotes) taken as an alternative to our A3; cf. the Appendices A–C.
- B’s counterpart, , is implied and could be used instead.
- In Isabelle/HOL explicit type information can often be omitted due the system’s internal type inference mechanism. This feature is exploited in our formalization to improve readability. However, for all new abbreviations and definitions, we usually explicitly declare the types of the freshly introduced symbols. This supports a better intuitive understanding, and it also reduces the number of polymorphic terms in the formalization (heavy use of polymorphism may generally lead to decreased proof automation performance).
- Remark: whether we use actualist or possibilist quantifiers for individuals, e.g., in the defns. of or T4 turned out irrelevant in this paper, and we consistently use actualist quantifiers in the remainder.
- Reconstruction of proofs from such proof nets within direct proof calculi for quantified modal logics, cf. Kanckos and Woltzenlogel-Paleo \shortciteKanckos2017VariantsOG or Fitting \shortcitefitting02:_types_tableaus_god, is ongoing work.
- Jesse Alama, Paul E. Oppenheimer, and Edward N. Zalta. Automating leibniz’s theory of concepts. In Automated Deduction – CADE-25, volume 9195 of LNCS, pages 73–97. Springer, 2015.
- C. Anthony Anderson and M. Gettings. Gödel’s ontological proof revisited. In Gödel’96: Logical Foundations of Mathematics, Computer Science, and Physics: Lecture Notes in Logic 6, pages 167–172. Springer, 1996.
- Peter B Andrews. An Introduction to Mathematical Logic and Type Theory: To Truth Through Proof, volume 27 of Applied Logic Series. Springer, 2002.
- Christoph Benzmüller and Peter Andrews. Church’s type theory. In Edward N. Zalta, editor, The Stanford Encyclopedia of Philosophy, pages pp. 1–62 (in pdf version). Metaphysics Research Lab, Stanford University, summer 2019 edition, 2019.
- Christoph Benzmüller and David Fuenmayor. Computer-supported analysis of positive properties, ultrafilters and modal collapse in variants of Gödel’s ontological argument. Bulletin of the Section of Logic, 2020. To appear, preprint: https://arxiv.org/abs/1910.08955.
- Christoph Benzmüller and Lawrence Paulson. Quantified multimodal logics in simple type theory. Logica Universalis, 7(1):7–20, 2013.
- Christoph Benzmüller and Bruno Woltzenlogel Paleo. Automating Gödel’s ontological proof of God’s existence with higher-order automated theorem provers. In ECAI 2014, volume 263 of Frontiers in Artificial Intelligence and Applications, pages 93–98. IOS Press, 2014.
- Christoph Benzmüller and Bruno Woltzenlogel Paleo. The inconsistency in Gödel’s ontological argument: A success story for AI in metaphysics. In S. Kambhampati, editor, IJCAI 2016, volume 1-3, pages 936–942. AAAI Press, 2016.
- Christoph Benzmüller. Universal (meta-)logical reasoning: Recent successes. Science of Computer Programming, 172:48–62, March 2019.
- Jasmin C. Blanchette and Tobias Nipkow. Nitpick: A counterexample generator for higher-order logic based on a relational model finder. In Interactive Theorem Proving — ITP 2010, volume 6172 of LNCS, pages 131–146. Springer, 2010.
- Ben Blumson. Anselm’s God in Isabelle/HOL. Archive of Formal Proofs, 2017.
- S. Burris and H.P. Sankappanavar. A Course in Universal Algebra. Springer, 1981.
- Branden Fitelson and Edward N. Zalta. Steps toward a computational metaphysics. Journal Philosophical Logic, 36(2):227–247, 2007.
- Melvin Fitting. Types, Tableaus, and Gödel’s God. Kluwer, 2002.
- David Fuenmayor and Christoph Benzmüller. Automating emendations of the ontological argument in intensional higher-order modal logic. In KI 2017: Advances in Artificial Intelligence, volume 10505 of LNAI, pages 114–127. Springer, 2017.
- James Garson. Modal logic. In Edward N. Zalta, editor, The Stanford Encyclopedia of Philosophy. Metaphysics Research Lab, Stanford University, fall 2018 edition, 2018.
- Kurt Gödel. Appendix A. Notes in Kurt Gödel’s Hand. In Sobel \shortcitesobel2004logic, pages 144–145.
- Annika Kanckos and Bruno Woltzenlogel Paleo. Variants of gödel’s ontological proof in a natural deduction calculus. Studia Logica, 105:553–586, 2017.
- Daniel Kirchner, Christoph Benzmüller, and Edward N. Zalta. Computer science and metaphysics: A cross-fertilization. Open Philosophy, 2:230–251, 2019.
- Tobias Nipkow, Lawrence C. Paulson, and Markus Wenzel. Isabelle/HOL — A Proof Assistant for Higher-Order Logic, volume 2283 of LNCS. Springer, 2002.
- Paul E. Oppenheimer and Edward N. Zalta. On the logic of the ontological argument. Philosophical Perspectives, 5:509–529, 1991.
- P. E. Oppenheimer and Edward N. Zalta. A computationally-discovered simplification of the ontological argument. Australasian Journal of Philosophy, 89(2):333–349, 2011.
- Francis J. Pelletier and Edward N. Zalta. How to Say Goodbye to the Third Man. Noûs, 34(2):165–202, 2000.
- John Rushby. A mechanically assisted examination of begging the question in Anselm’s ontological argument. J. Applied Logics, 5(7):1473–1496, 2018.
- Dana S. Scott. Appendix B: Notes in Dana Scott’s Hand. In Sobel \shortcitesobel2004logic, pages 145–146.
- Jordan H. Sobel. Gödel’s ontological proof. In Judith Jarvis Tomson, editor, On Being and Saying. Essays for Richard Cartwright, pages 241–261. MIT Press, 1987.
- Jordan H. Sobel. Logic and Theism: Arguments for and Against Beliefs in God. Cambridge University Press, 2004.
- Edward N. Zalta. Twenty-Five Basic Theorems in Situation and World Theory. Journal of Philosophical Logic, 22(4):385–428, 1993.