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 proof because he feared that it would be mistaken as establishing God's existence beyond doubt. Instead, he only saw it as a logical investigation and a clean formulation of Leibniz' argument with all assumptions spelled out. He repeatedly showed the argument to friends around 1970 and it was published after his death in 1987. An outline of the proof follows.

Table of contents |

2 Axioms 3 Critique of Definitions and Axioms 4 Derivation 5 Related Articles 6 Related Articles (Objections) 7 External Links 8 References |

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;
by contrast, a contingent truth just happens to be the case,
for instance "more than half of the earth is covered by water".
In 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

*P*(*x*) =*x*is grey

*s*= my shirt

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

*P*(*x*) =*x*is taller than 2 meters

*Q*(*x*) =*x*is taller than 1 meter.

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)

**Axiom 2**: If*P*is positive and*P*entails*Q*, then*Q*is positive.**Axiom 3**: If*P*_{1},*P*_{2},*P*_{3}, ... are positive properties, then the property (*P*_{1}AND*P*_{2}AND*P*_{3}...) is positive as well.**Axiom 4**: If*P*is a property, then either*P*or its negation is positive, but not both.

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

There are several reasons these assumptions are not realistic, including the following:

- 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. - 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)*). - 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 a property that is positive for one object may be positive for all, and conversely.
- Unless precisely the same set of properties is defined in every possible world, each assignment of the "positive" attribute to properties results in different definitions of God in different worlds. This is not consistent with monotheism.
- Gödel's definition of God is not shown to correspond to any specific religious concept of God.

From these hypotheses, it is now possible to prove that there is one and only one God in each world.

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*Collected Works: Unpublished Essays & Lectures, Volume III*.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