Saint Thomas Aquinas rejected this proof. He argued that the human mind cannot conceive of a being such as God. He held to a negative theology, in which we can only understand God by deducing what he is not. For instance, God cannot be composed of matter, for that would mean that God could be moved, but God as an all powerful being cannot be moved. We have no means of comprehending God, and so the argument cannot be used to prove his existence. Better to just stick with the Five Ways, or something similar.