Chapters two and three of the Proslogium contain Anselm’s famous proof. In chapter two Anselm gives three premises and a conclusion. They are numbered in this paper as follows: 1) God “art a being than which nothing greater can be conceived,” 2) “it is one thing for an object to be in the understanding, and another to understand that the object exists,” 3) “that, than which nothing greater can be conceived, cannot exist in the understanding alone,” and therefore 4) “there exists a being, than which nothing greater can be conceived, and it exists both in the understanding and in reality” (Anselm 39).
God by definition is a being in which nothing greater can be conceived; thus premise 1) is true by definition. Premise 2) is more controversial, depending upon how it is interpreted. Existence can either be a property of an object or it can be defined as existing in W possible worlds; in either case it is making the claim that existence is different from and better than nonexistence. Premise 3) is making an inference from premise 1) and 2); if God is the greatest conceivable being and existence is better than nonexistence, then the greatest conceivable being has the property of existence or exists in all possible worlds W. Conclusion 4) follows directly from premise 3), viz. God exists.
In chapter three Anselm gives a slightly different formulation of the argument. In chapter three Anselm has two premises and one conclusion; they are numbered in this paper as follows: 5) God “cannot be conceived not to exist,” 6) “there is […] a being than which nothing greater can be conceived to exist, that it cannot even be conceived not to exist,” and therefore 7) God necessarily exists (Anselm 39). The justification for premise 5) is, “it is possible to conceive of a being which cannot be conceive not to exist; and this is greater than one which can be conceived not to exist” (Anselm 39). This is saying that a necessary being is greater than a contingent being. Since, God cannot be conceived not to exist, and there is not a being conceivable that is greater than God, God exists necessarily.
Both of these arguments work together to form Anselm’s proof. The argument in chapter two shows that the greatest possible being that can be conceived actually exists. The argument in chapter three shows that this being is a necessary being. The argument also claims that God is the only necessary being.
An objection to this argument that Gaunilon raises is, it is possible to think of an island that is the greatest possible island, but that does not mean the island actually exists. Anselm replies that Gaunilon is correct in thinking about the island because it is a contingent object: it is possible to conceive of the island not existing. God, however, is not a contingent being since it is not possible to conceive of God’s non-existence - this is demonstrated in chapter two.
Even if it is objected that existence is not a property that can be had by an object, Anselm could respond that the being than which nothing greater can be conceived that exists in all possible worlds is God (Look). This formulation is also helpful in understanding how this being is also necessary, viz. in every possible world it is not possible to conceive of the non-existence of God.
Anselm’s argument is valid and sound in proving that God exists. It should be noted that this proof does not say anything about the nature of God; if one wanted to know about God, they would have to look for empirical evidence or provide different proofs for God’s attributes. This proof, however, clearly demonstrates that God exists and could not be otherwise.
Quotations taken from: St. Anselm. Proslogium. Readings in Medieval Philosophy. Ed. and introd. Andrew B. Schoedinger. New York: Oxford Press, 1996. 36-40.
Look, Brandon C. “Anselm, Aquinas, and Pascal.” U of Kentucky Philosophy Department. 5 March, 2009. http://www.uky.edu/~look/AnselmAquinasPascal.pdf