In his "Meditations on the First Philosophy" philosopher René Descartes offers three proofs of the existence of God. These arguments are highly important for Descartes' process of meditation, since after proving the I can believe in my own existence (meditation 2) Descartes now needs to prove how he can believe in anything else, and this is where God come in.
Descartes' proofs of God are called the anthropological argument, the cosmological argument and the ontological argument.
Descartes' anthropological argument of God
Descartes' anthropological proof of god is offered in meditation 3. Being a doubting entity as Descartes is, he must be aware of not being aware to everything. We know that we are imperfect beings. But knowing imperfection must be the result of having a concept of perfection in our mind. Since I am imperfect I cannot be the source of this knowledge about perfection. This must mean that there is a greater, unbounded, power which precedes me.
Descartes' cosmological argument of God
Descartes' cosmological proof is also offered in meditation 3 and goes as follows: If I exist, and I exist since I think, then there must be a reason for my existence. I cannot be the reason for my existence since that would make me perfect and in want of nothing. If it try to explain my limited existence as the result of other limited beings, such as my parents, I will be forced to account for their origins and so forth. In the end of the chain, say Descartes, there must reside something which is perfect and is the source of all imperfect things such as myself.
Descartes' ontological argument of God
Descartes' ontological proof of God is found in meditation 5. This argument holds that existence is a necessary trait of something which is perfect. Since God is by definition perfect it follows that he must exist.