Cleanthes responds to Philo's objections by asserting that the analogy between the universe and works of human ingenuity is obvious. He sets out to show that all the lines of reasoning which Philo made use of in the last chapter lead him to absurd consequences when applied to other inferences.
First, he asks his friends to imagine that an awesome voice came out of the heavens and spoke to all nations simultaneously, giving them some godly instruction. Would Philo doubt for an instant that this voice belonged to an intelligent being with some design or purpose? However, this voice is not at all like a human voice, since it is louder, more beautiful, universally understood, and awe-inspiring. Cleanthes claims that based on the first line of reasoning Philo produced in the previous chapter—that the universe is not very similar to a machine and thus that the analogy between the universe and a machine fails—Philo could not conclude that the voice was caused by an intelligent purpose, because the voice would be too unlike a human voice. And failure to draw this conclusion, of course, would be absurd. Therefore, Cleanthes claims, Philo's similar line of reasoning in the case of a machine is absurd: just because the universe is much more awesome than any other machine, this does not mean that we cannot draw any conclusions from the obvious analogy.
Next, he asks his friends to imagine picking up some ancient book like the Iliad. We have no direct experience of this book's being written by a human being, nor is there any other event exactly like this particular book's being written. Nonetheless, we do not doubt, while reading the book, that the cause of the book is an intelligent author. Given Philo's line of reasoning in the last chapter, though, we should doubt this: Philo said that we must reject an inference if we have no direct experience of the cause conjoined to the effect, and if the cause and effect are unique. Since this line of reasoning leads to such absurd consequences in the case of the book, it must be equally silly in the case of the universe.
In conclusion, Cleanthes points out that skepticism, far from demolishing his argument, only strengthens it. This is because a true skeptic is only supposed to reject obscure, remote arguments, not plain common sense. In this case, common sense is on the side of intelligent design. Who could look at an eye, he asks, and not be immediately struck with the design of it? It is so perfectly, intricately adapted to the purpose of sight that to deny that it was created for this very purpose is utterly ridiculous.
Demea now breaks in again to complain about this persistent comparison between the mind of God mind and the mind of man. The analogy with the book, he suggests, is dangerous: when we read a book, we enter into the mind of the author and completely understand the author's purpose. It cannot be so with God—his book, the universe, contains inexplicable riddles.
Demea then attempts to demonstrate why it is impossible for us to be the model of God. The sentiments of the human mind (such as gratitude, love, hatred, and envy) only make sense within the context of our position in the world, so they cannot apply to God. And all of the ideas we obtain from sensation are illusory and so they cannot have any place in the divine mind (since God cannot harbor any illusions). Even our very mode of thought is essentially flawed: it is uncertain, fleeting, and frequently full of errors. These properties of our thought are so central to its nature, that if we tried to abstract them away, we would be left with nothing: in other words, we cannot even imagine what thought would be like without these imperfect features. Therefore, God's thought cannot be like ours.
Various versions of the argument from design have been around for a long time. St. Thomas Aquinas was the first to present this line of thought. His argument was very different from Cleanthes' version of the argument, in large part because Aquinas' version was based on a very different picture of the world. Aquinas believed in an Aristotelian teleological picture of the world, on which all phenomena are explained in terms of their ultimate purpose. (Every event and every object is assumed on this picture to have some larger purpose that it exists to fulfill.) Given this picture of the world, Aquinas reasoned that there had to be a divine creative mind, in roughly this way: (1) Natural bodies act for an end or purpose. (2) Natural bodies are not themselves aware of acting for this end or purpose. (3) Therefore, there must be some other cause that explains why they act for this end or purpose. (4) This cause is God.
By Hume's day the picture of the world had altered significantly. René Descartes and Sir Isaac Newton had changed the way people thought about the nature of the physical world, replacing the teleological picture with a mechanistic picture. In the mechanistic picture the world is seen as operating based on immutable laws of nature, and not for any ultimate goal. The new version of the argument from design developed in response to this change in worldview. Sir Isaac Newton was himself vociferous in defending this version of the argument from design (Descartes, on the other hand, tried to give a priori proofs for religious truths).
The new mechanistic picture of the world does not only play into Cleanthes' arguments, but into Demea's as well. When Demea claims that God cannot have ideas of sensation it is because he is heavily influenced by Cartesian and Newtonian science: according to these pictures (which anticipate the current prevailing views) the world around us is made up of colorless, odorless, tasteless, soundless particles of matter which join together in various ways. Our sensations, therefore, do not give us access to the world as it really exists; they are thus misleading. God, as he cannot be deceived, could not have any sensations. It is interesting to note that Cleanthes uses analogical arguments to defend his argument by analogy. He says that the case of the voice from the sky is analogous to the question of the creation of the universe, and then says that if the result of the application of Philo's rules is absurd in one case, it must be absurd in the other.