The next step in Cleanthes argument is to show that God's existence is not a demonstrable truth. Nothing that is distinctly conceivable, he tells us, involves a contradiction. This is reasonable, for it is impossible for us to imagine anything that involves a contradiction, such as a ball which is all one color and is blue and not-blue. Next, Cleanthes claims that whatever we conceive of as existing we could also conceive of as not existing. For instance, we can imagine that the sun does not exist, despite the fact that it does. Thus, any statement which denies the existence of anything will not involve a contradiction. Therefore, there is no being whose existence is demonstrable. Therefore, Cleanthes thinks that there can be no contradiction in the statement, "God does not exist."
However, St. Anselm argued that it is impossible to conceive of God as not existing, for existence is part of God's nature (whereas it is not part of the sun's nature to exist), because anything that exists is more perfect than anything which does not, and God is the most perfect thing that can be thought of, so God must exist. A denial of God's existence, according to St. Anselm, would go as follows, "God, who exists, does not exist," and this statement clearly does contain a contradiction. So for Cleanthes's first objection to hold, he must either deny that existence is a perfection or that God is not the most perfect being that can be thought of. Many philosophers, including Kant, believed that existence is not a perfection.
Cleanthes's second objection may be similarly vulnerable. He says that the material universe may be the necessarily existing being, but according to his own logic this seems impossible. For there is no contradiction (or at least no trivial contradiction) in the statement, "the universe does not exist," and so the existence of the universe does not seem to be necessary.
Cleanthes's third objection, that the chain of causes does not exist except as an abstraction of our mind, is the most convincing. However, it is possible that someone who wants to defend the ontological argument could say that even though the chain of causes does not exist at any one time, it certainly does exist.