The argument continues in Proslogion 3, and it is that it’s simply impossible for God not to exist. I can acknowledge that the existence of someone while agreeing that the person does not have to exist. In the same way, Anselm assumes that even if we know that God exists both in the intellect and outside it, it does not follow that there is no possibility of God not existing. And so he concludes that God has to exist and cannot fail to exist. This is known as necessary existence, anything which has to exist and cannot fail to exist is said to exist by ‘necessity’. Most things that exist depend on something else for their existence. For example, Costa in Aylesbury only exists in reality because someone built it. We cannot say that Costa had to exist, because it was up to the owners of Costa to decide whether or not to build it. This type of existence is called contingent existence. Anselm argues that God must necessarily exist because if God existed only contingently, God would depend on something else for existence, and therefore would not be as great as a being that had to exist and could not fail to exist.
The word ‘predicate’ is used to indicate an intrinsic property or quality of something. For example, a predicate of a particular species of goat might be its form or its horns. In other words, predicates tell us something about the nature of a thing. Anselm’s claim is that existence is a predicate of God (a property or quality of God’s nature). Therefore, God, being the greatest possible being, has to exist, since an idea in the mind is not as great as an idea that exists in reality. The idea that exists in reality has the extra property of ‘existence’. To be the greatest possible being, God must, necessarily, have his property of existence. The conclusion that Anselm reaches is that because God is the greatest being that can be thought of, part of a ‘being’ or ‘thing’ of any sort is that you exist. So God must exist. For Anselm, God’s existence is thus analytic.
Descartes formulation of the Ontological Argument is based on the idea that God exists to guarantee what can be known. There are important differences between Descartes’ argument and that of Anselm. Anselm argues from a definition of God as “the greatest thought that can be thought”. He argues that the existence of God is “self-evident”, based on a sense of intuitive certainty. From his experience as a mathematician, Descartes set out to demonstrate the existence of God in the manner of a geometrical exercise. God’s existence is as necessary as a triangle has to have three sides. Descartes thought that this was as self-evident as 2+2=4. Descartes is pointing out that we do not need to establish complex logical proofs that a triangle had three sides – it is self-evident. The same can be said about the existence of God.
In the same way as a triangle needs three sides, Descartes argues that the “essence” of God requires that He exists. Necessary existence cannot be divorced from the concept of a supremely perfect being. We intuit this in exactly the same way that we might intuit the concept of the number 2, or the fact that a triangle has 3 sides.
In the previous chapter of Meditations, Descartes had argued that if a person can clearly see that something is intrinsic to the concept of a thing, then that “something” must be true. There are some qualities that a thing must have for it to be that thing.
A triangle must have three sides.
A hill must have slopes.
A Bachelor must be unmarried.
These “facts” are self evident – they do not require empirical proofs in order for a person to accept them as truths. In Other Words: ß Whatever I perceive to be contained in the concept of a thing is true of that thing. ß I clearly perceive that necessary existence is a part of the concept of God. ß God therefore exists.