Descartes argues that objects and the idea of objects have differing objective reality that result from the extent of perfection of the objects in question. Objects that rely on other objects have less objective reality; the same applies to ideas, e.g. the idea of bread has the same formal reality (they are all ideas) but less objective reality (bread does not exist without a baker) as the idea of a baker. Since God created everything, he must have the most (infinite) objective reality.
By his causal axiom, Descartes states that an idea’s objective reality must originate from something with as much formal reality. The objective reality of the idea of a tree must come from an actual tree.
From here, Descartes proves the existence of God:
- By the causal axiom, objective reality of one’s idea of God originates from something with as much formal reality.
- God has infinite formal reality, and the idea of God has infinite objective reality.
- The idea of God could not have originated from humans, since infinite objective reality (idea of God) could not be produced by finite formal reality (humans).
- The idea of God must have originated from an actual God.
- God exists
This argument is called the cosmetological proof of God’s existence, because the proof starts with something that exists in the world (in this case, the existence of the idea of God)