Gödel's ontological proof

Gödel's ontological proof is a formalization of Saint Anselm's ontological argument for God's existence by the mathematician Kurt Gödel. St. Anselm's ontological argument, in its most succinct form, is as follows: "God, by definition, is that than which none is greater. 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; this is the version that Gödel studied and attempted to clarify with his ontological argument. While Gödel was religious, he never published his\nproof because he feared that it would be mistaken as establishing\nGod's existence beyond doubt. Instead, he only saw it as a logical\ninvestigation and a clean formulation of Leibniz' argument with all\nassumptions spelled out. He repeatedly showed the argument to friends around 1970 and it was published after his death in 1987.\nAn outline of the proof follows.

Table of contents

1 Modal logic 2 Axioms 3 Derivation 4 Critique of definitions and axioms 5 Related articles 6 Related articles (objections) 7 External links 8 References

Modal logic

The proof uses modal logic, which distinguishes between necessary truths and contingent truths. A truth is necessary if it cannot be avoided, such as 2 + 2 = 4;\nby contrast, a contingent truth just happens to be the case,\nfor instance "more than half of the earth is covered by water".\nIn the most common interpretation of modal logic, one considers "all possible worlds". If a statement is true in all possible worlds, then it is a necessary truth. If a statement happens to be true in our world, but is not true in all other worlds, then it is a contingent truth. A statement that is true in some world (not necessarily our own) is called a possible truth. A property assigns to each object in every possible world a truth value (either true or false). Note that not all worlds have the same objects: some objects exist in some worlds and not in others. A property only has to assign truth values to those objects that exist in a particular world. As an example, consider the property\n:P(x) = x is grey\nand consider the object \n:s = my shirt\nIn our world, P(s) is true because my shirt happens to be grey; in some other world, P(s) is false, while in still some other world, P(s) wouldn't make sense because my shirt doesn't exist there. We say that the property P entails the property Q, if any object in any world that has the property P in that world, also has the property Q in that same world. For example, the property \n:P(x) = x is taller than 2 meters \nentails the property\n:Q(x) = x is taller than 1 meter.

Axioms

We first assume the following axiom:

Axiom 1: It is possible to single out positive properties from among all properties. Gödel defines a positive property rather vaguely: "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)

We then assume that the following three conditions hold for all positive properties (which can be summarized by saying "the positive properties form an ultrafilter"):

Axiom 2: If P is positive and P entails Q, then Q is positive.\n:Axiom 3: If P₁, P₂, P₃, ..., P_n are positive properties, then the property (P₁ AND P₂ AND P₃ ... AND P_n) is positive as well.\n:Axiom 4: If P is a property, then either P or its negation is positive, but not both.

Finally, we assume:

Axiom 5: Necessary existence is a positive property (Pos(NE)). This mirrors the key assumption in Anselm's argument.

Now we define a new property G: if x is an object in some possible world, then G(x) is true if and only if P(x) is true in that same world for all positive properties P. G is called the "God-like" property. An object x that has the God-like property is called God.

Derivation

Subject to the assumptions, it is asserted that one can now already show that in some possible world there exists God. We want to show that necessarily, in every world there exists a unique God. In order to do this, Gödel first defines essences: if x is an object in some world, then the property P is said to be an essence of x if P(x) is true in that world and if P entails all other properties that x has in that world. We also say that x strongly exists if for every essence P of x the following is true: in every possible world, there is an element y with P(y). From these hypotheses, it is now possible to prove that there is one and only one God in each world. Gödel did not attempt to do so however, as he purposely limited his proof to the issue of existence, rather than uniqueness. This was more to preserve the logical precision of the argument than due to a penchant for polytheism. [Some explanation of how, rather than "it is now possible to do so" would make this article more useful.]

Critique of definitions and axioms

There are several reasons Gödel's axioms may not be realistic, including the following:\n* The selection of positive properties may be arbitrary except for the requirement that necessary existence must be positive (and G(x) must be positive in some versions). Although a given individual may not define certain properties as positive, another might do so.\n* It may be impossible to satisfy the second axiom. Since it could not be proven consistent, this axiom was replaced in some later versions of the proof by the assumption that G(x) is positive (Pos(G(x)). \n* Gödel does not clearly state whether he intends positiveness to be an attribute of properties independent from objects or of properties when combined with objects. Thus we may interpret him as saying that a property that is positive for one object must be positive for all, or that a property that is positive for one object may or may not be positive for all.\n* Unless precisely the same set of properties is defined in every possible world, the God of some worlds will be distinct from the God in others. This is not consistent with monotheism.\n*Gödel's definition of God is not shown to correspond to any specific religious concept of God.\n*It was pointed out by Jordan Sobel that Gödel's axioms are too strong: they imply that all possible worlds are identical. C. Anthony Anderson gave a slightly different axiomatic system which attempts to avoid this problem.

Related articles

\nAbsolute Infinite, Arguments for the existence of God, Modality, Philosophy of religion, Mathematics and God, Synthetic proposition, Teleological argument, Theism

Related articles (objections)

\nArguments against the existence of God, Logical and evidential arguments from evil, The problem of evil

External links

\n*Kurt Gödel's Ontological Argument\n*Stanford Encyclopedia of Philosophy: Gödel's Ontological Argument

References

\n* C. Anthony Anderson, "Some Emendations of Gödel's Ontological Proof", Faith and Philosophy, Vol. 7, No 3, pp. 291-303, July 1990\n* Kurt Gödel (1995). "Ontological Proof". Collected Works: Unpublished Essays & Lectures, Volume III. pp. 403-404. Oxford University Press. ISBN 0195147227 \n* A. P. Hazen, "On Gödel's Ontological Proof", Australasian Journal of Philosophy, Vol. 76, No 3, pp. 361-377, September 1998\n* Jordan Howard Sobel, "Gödel's Ontological Proof" in On Being and Saying. Essays for Richard Cartwright, ed. Judith Jarvis Thomson (MIT press, 1987)