Demea now breaks into the conversation, and asks whether it is only God's nature and not God's existence that is being subjected to skeptical doubt? His friends assure him that this is the case. Well, he continues, as far as the latter is concerned, it is almost as impious to claim that we can actually understand God's nature as it is to claim that there is no God at all. God, he declares, is inherently beyond human comprehension, and must necessarily remain mysterious to us.
Philo agrees that God's existence is beyond doubt and he also agrees that God's nature cannot be known. He provides arguments for both of these claims. First, God must exist because every effect has some cause and so there must be some ultimate cause of the universe. We call this ultimate cause God and we "piously ascribe to him" every possible type of perfection. But there is no reason to think his perfection bears any resemblance to anything we know, so there is no reason to think that we have any idea what God is like. We say that he is wise, knowledgeable etc., but we only use these words for lack of any others. We have no idea what any of these attributes could mean when put in the divine context. Philo points out that this conclusion is based on the most common-sense reasoning: our ideas are produced by our experience, and we have no experience of divine attributes and operations. Therefore, we have no idea what God could be like.
Cleanthes disagrees with Demea and Philo. He thinks he can prove that, even though we do not have any direct experience of God's attributes and operations, there is enough evidence in nature to allow us to draw justified conclusions about what God is like. By looking at the natural world we see that it resembles nothing so much as a work of human artifice (which, for the sake of ease, we can just call a "machine"). Though we have never experienced God, we have experienced machines and we know a thing or two about them. Most relevantly, we know that wherever there is a machine, there is some intelligent designer behind it. Machines do not just come together by chance; they are created by skilled human beings. Given that the universe is obviously just an elaborate machine, with each part from smallest to largest perfectly adapted to the harmony of the whole, we can reasonably infer that, just like any other machine, the universe was created by an intelligent designer. That intelligent designer, i.e. God, must be similar to a human designer, only much more perfect, in proportion with the greater perfection of his art.
Demea is the first to react to Cleanthes' argument from design. He does not approve of the claim that God and man are at all similar. Second, he is unhappy that Cleanthes is trying to use an a posteriori proof rather than a priori for God's existence (since the argument by design proves both God's nature and his being). A posteriori proofs are only probable proofs, not definite proofs. That is, when we give an argument from experience we can only prove that our conclusion is more likely than not; we can never prove that it is definitely true. It is only when we give an a priori demonstration that we can prove something with certainty.
Philo does not mind that the argument is a posteriori; his only complaint is that it is a bad argument. He will spend the rest of the book showing just how bad the argument is. In this chapter all of Philo's objections aim to show either that the argument from design does not really have the proper form necessary for an inductive inference or that it is an improper use of an argument by analogy.
Philo brings up several ways in which the argument from design fails as an inductive inference. First, the analogy between the universe and a machine is weak, for he claims that the world does not really resemble a machine all that well. Second, the analogy between the universe and a machine does not necessarily work because it is not an analogy between two separately existing entities, but between the universe as a whole and certain parts of the universe (i.e. man and the objects he manipulates). Thus, drawing an analogy between a machine and the universe might be like trying to figure out how an entire man develops by looking at how a single hair in his head grows. Third, it seems to be false to claim that all order in the world is the result of intelligence. The process responsible for the intricate order and adaptability in the case of organic bodies seems to be animal and vegetable reproduction, not design. So why suppose that the order of the universe is like the order of man-made machines, and not like the order of organic bodies? Finally what makes a causal inference work is that we repeatedly experience examples of A-like events followed by B- like events. But here A, God, is a unique cause and B, the universe, is a unique effect. Thus Philo concludes that the argument from design is not so much an inductive inference, as a whimsical conjecture.