Cleanthes tells Demea that his "mystical" view of God (i.e. that God is entirely beyond comprehension) is practically atheism: it forces you to say that you have no idea what is out there controlling the world. Demea responds by bringing up some more reasons why the attempt to model God on man is untenable. God is supposed to be perfectly immutable (unchanging and eternal) and simple (that is, he has no separate parts; one cannot imagine God's leg or something like that because God is not spatially extended). But Demea thinks that the human mind is a composition of various faculties and thoughts constantly giving way to one another, and is thus neither immutable nor simple.
Cleanthes is not impressed with this line of attack: he does not think that God is perfectly immutable and simple and so he does not care that his theory contradicts this account of God's nature. To claim that God is immutable and simple, he thinks, is to claim that God has no mind at all.
Philo now breaks in with a wholly new line of attack. He says that if the universe has an intelligent designer, the order of the universe is no more explicable than it would be if there were no God. That is, for God's thoughts to order the universe, there must be a high degree of order in God's thoughts. If order requires an explanation, then we have only replaced one question, "why is there order in the universe?" with another, "why is there order in God's thoughts?" Order does not belong more essentially to thought than to matter, for in both we have examples of order (sane thought in ideas and animal systems in matter) and disorder (in thought, madness, in matter, monstrous mutations) arising without immediately obvious intelligent cause. Thus, by saying that God created the order, we do not gain any knowledge, but only displace our ignorance into a realm about which we could never learn. Cleanthes replies that he is not interested in the cause of the order of God's thoughts: he is satisfied, he says, to know that there is an anthropomorphic God and does not need to go further.
Cleanthes thinks that Philo is objecting to the design argument on the basis of the claim that God's intelligent design cannot be the final cause and that, therefore, it cannot be the cause at all. He, therefore, responds that all philosophers must confess that ultimate causes are unknown. Cleanthes is right in this claim: even modern science's most sturdy theories do not assign final causes. For instance (to use an example that Cleanthes could not have possibly used), we consider the Big Bang theory a good explanation of how our universe came into being even though we don't know what caused the Big Bang. Our lack of a plausible cause for the event that caused everything subsequent does not keep us from putting our faith in this very satisfying and well-confirmed theory of the origins of the universe.
Philo does not claim that questions are left unanswered by the design hypothesis; rather, says that this hypothesis does not provide any explanation of the order of the universe. If we accept the design argument, the question of "what creates the order of the universe?" has simply become, "what creates the order of God's thoughts?" Philo thinks that because Cleanthes replaces a particular question with another particular question, he does not actually explain anything about the order of the universe, and the reason for that order is no more intelligible to us than it would be if we did not assume an intelligent designer.
Philo thinks that explaining our ordered universe by appealing to the order of God's ideas is much like explaining why you punched your friend by the fact that you wanted to punch your friend. It might well be true that you wanted to punch your friend, but this information does not really explain why you punched your friend. All it does it push the mystery from the realm of action to the realm of emotion or ideas. To really give an explanation of the action in question one would need to tell a story about why you wanted to punch your friend (or how the accident occurred, or why and how someone forced you to do it). Philo sees that the design argument has the same problem: all it does is push the mystery from the realm of matter to the realm of ideas. What Philo thinks we really need in order to explain the high level of order in our universe is some general principle or law that tells us how order arises, either in matter or in thought. It is only when we have a general law that we will be able to begin to understand why and how our universe became ordered the way it did. Philo is demanding the sort of principle that Charles Darwin finally provided over a century later when he struck on the theory of natural selection. Natural selection is a general law that explains and illuminates how order arises in the natural world. Instead of pushing the mystery onto some other realm, the theory sticks to the realm of matter and shows us that through the operations of certain universal laws involving survival advantage and reproductive success, order naturally arises from chance and disorder. Interestingly, in Part VIII of the Dialogues Philo himself will propose a theory of the origins of the cosmos that closely anticipates Darwin's theory of natural selection.