Whether or not something exists is a matter of whether there is in reality anything corresponding to our idea; it is not a matter of what the content of our idea is.

The design argument given by Cleanthes is supposed to establish the existence of God, and something of his nature – that he possesses great wisdom, for example. The a priori argument given by Demea is also intended to prove the existence of a deity. But at the beginning of the Dialogues it is said that the existence of God is not in question; what will be discussed is the divine attributes, the nature of God. However, that there is no dispute about the existence of God is asserted initially by Pamphilus. Although all three characters are prepared to say that there is a God, they are not agreed about either what this means or whether it can be established by reason. Given Hume’s view about existence, one could not believe that God exists without having an idea of God. So the distinction between questions about the existence and questions about the nature of God is shaky. Cleanthes holds that the design argument establishes similarity between God and human minds. This is attacked by both Philo and Demea. Demea’s objection is that God is transcendent; he is beyond our comprehension. God’s nature is a religious mystery. But, he says, the method of reasoning employed by Cleanthes suggests that we can understand the nature of God as analogous to our own. From his first statement of this position, in Part II, Demea expresses his position in religious rather than philosophical language:

Finite, weak, and blind creatures, we ought to humble ourselves in his august presence, and, conscious of our frailties, adore in silence his infinite perfections which eye hath not seen, ear hath not heard, neither hath it entered into the heart of man to conceive.

He goes on to quote from Malebranche’s De la recherche de la vérité (The Search after Truth, 1674–5), in order to suggest that there is here harmony between reason and faith. This allows Hume to introduce into the Dialogues the notion of anthropomorphism, that is, the attribution to God of human characteristics. Philo and Demea then can use this term to summarize their criticism of Cleanthes. Demea regards the anthropomorphic notion of God generated by the design argument as not a proper object of religious devotion. In Part III, Cleanthes tries to support his argument with thought experiments – the examples of ‘an articulate voice… heard in the clouds’, and of a library of books which are ‘natural volumes’, not produced by mankind, but reproducing themselves ‘in the same manner with animals and vegetables, by descent and propagation’. If there were such phenomena, he says, it would be absurd not to infer that they were the product of intelligence and design, for they would contain intelligible messages. He then draws an analogy with the actual structure of nature. (The idea of nature as a book, in which we can read the message of divine purpose, is an ancient one; it is found, for example, in the Natural Theology of Raymond Sebond, written around 1430.19) Demea objects that in reading a book we enter into the mind of the author, but we cannot enter into the mind of God. ‘His ways are not our ways.’ Besides, he says, the human mind, both in its sentiments and its ideas is wholly unlike the divine mind. Human sentiments, such as gratitude, love, pity, etc., ‘have a plain reference to the state and situation of man’. And human thought is ‘fluctuating, uncertain, fleeting, successive, and compounded’. But the God of faith cannot be thought to have sentiments or thoughts as we do.

In the main, however, it is left to Philo to develop the criticism of anthropomorphism. In Part V, he emphasizes that, insofar as Cleanthes is arguing from analogy, his conclusion will be better supported the more similar the inferred cause (of the order in nature) is to the known cause (of the order in machines). Thus the logic of Cleanthes’ position pushes him into a more and more anthropomorphic conception of the mind of God. Cleanthes agrees with Philo that, for his argument, comparing divine and human intelligence, ‘the liker the better’.

Philo exploits this in a series of arguments. He shows that the argument from analogy cannot establish any of God’s attributes to be infinite, for ‘the cause ought only to be proportioned to the effect’. Again, if we knew a priori that God is a perfect being, the imperfection in nature could be said to appear an objection only because of our limited understanding. (In Part X Demea expresses his faith that the wickedness and misery of mankind will at last be seen ‘in some future period of existence’ not to be inconsistent with divine power and benevolence.