In his meditations, Descartes points out that there are three types of ideas; they can be innate, adventitious, caused by things outside of one's self, and others that can be invented by us, such as ideas of mermaids or unicorns. After he concludes that God must necessarily exist, which will be discussed in more detail later on in this paper, he closes in to the fact that his idea of the Perfect Being cannot be adventitious, coming from without, nor can it be invented by him. Thus, this idea must be innate since he has a clear and distinct perception of God's existence, and that Descartes must have been created by God with such an idea already in him. The idea of a Perfect Being in Descartes' mind consists of an infinite, eternal, immutable, independent, supremely intelligent as well as a powerful substance, which created him and everything else. Thus he realizes that the idea of God must have far more objective reality than he has formal reality because God is an infinite substance where as he himself is only a finite substance. Having explained the idea of Descartes' perfect being lets go into more detail on the process he uses to prove that such being actually exists.
Descartes' proof of the existence of God occurs in the Third Meditation. He builds his entire argument upon his proof in the previous meditation that in order for him to think, he must exist. From this single observation, Descartes notices that the idea of his existence is very clear and distinct in his mind; based upon this clarity and the fact that he has just determined his own existence, he deduces a rule such that the things that he sees as very clear and very distinct are all true. .
Descartes starts his proof by dividing "thought- into four categories; ideas (concepts), volitions (choices), emotions (desires), and judgments (beliefs). He then breaks down these categories to discover which types of thoughts can yield error.