  Descartes often compares the ontological argument to a geometric demonstration, arguing that necessary existence cannot be excluded from idea of God anymore than the fact that its angles equal two right angles, for example, can be excluded from the idea of a triangle. The analogy underscores once again the argument’s supreme simplicity. God’s existence is purported to be as obvious and self-evident as the most basic mathematical truth. It also attempts to show how the “logic” of the demonstration is rooted in our ordinary reasoning practices.