Start with: Descartes’ method of doubt by which he tries to establish foundations beyond doubt upon which to build knowledge.
In his book Meditations, arrives at one certainty – that he is a thinking being (as he could be deceived about what he is thinking, but not about that he is thinking). Asks if there is anything else he can be certain of.
Decides that he can form clear and distinct impressions of mathematical objects and numbers, but these things exist in some sense independently of his mind. He can “draw the idea of something from my thought”.
Explain the method he uses to do this, which is to meditate upon the essence of something, ie. what makes it what it is. With a triangle this is that its internal angles add up to 180 degrees.
This truth about triangles exists in his mind clearly and distinctly, and he argues that he can be certain of it.
How does he do this? From the preceding thought experiment he found out that the one thing he could be certain of was that he was a thinking thing, but that he might be deceived about having a body etc.
Therefore, he argues, things of the mind are more certain than physical things.
They are much more clearly and distinctly known than that of the body.
One such idea is the idea of God that Descartes finds in his mind. He says:
1. I have an idea of God, a perfect being.
2. There must be as much reality or perfection in the cause of any thing as in the effect.
a. This applies not only to the existence of ideas, but also to the reality of what they represent. Not only must the existence of the idea be explained, but also what it represents.
3. The idea of God represents something so perfect that I could not have been the cause of this idea.
Therefore, God must exist as the only possible cause of the perfection found in my idea of Him.
So far, this is all background to get you as far as Descartes’ idea of God in the mind. If you understand how he arrives at that, then you will be better equipped to answer a part b question.
Descartes then uses his version of the ontological argument – the idea of God can be said to exist in reality, because existence is a perfection God cannot lack.
Use the example of rivers not being able to exist without banks – they logically have to go together. Triangles have to have three internal angles. God necessarily exists because he has all perfections (that’s what we mean by God) therefore God must have existence, as that is a perfection.
To explain this further talk about God’s essence including existence. Descartes tries to show that it is impossible for God not to exist.