Cleanthes regards this as ‘building entirely in the air’.) But if we are arguing a posteriori, then ‘these difficulties become all real’. Furthermore, even if nature were perfect, the analogy with human design could suggest that the product results from trial and error: ‘Many worlds might have been botched and bungled, throughout an eternity, ere this system was struck out.’ Again, arguing from analogy cannot establish the unity of God. It would be even more analogous to human creation to suppose that so great a creation as the universe required the cooperation of a number of deities.

While Philo and Demea seek to make evident the incompatibility of Cleanthes’ anthropomorphism with the religious idea of a single, transcendent, infinitely perfect God, Cleanthes attacks the idea that God’s nature is incomprehensible as ‘mysticism’. At the beginning of Part IV he asks how the thesis that the nature of God is incomprehensible differs from the view of ‘sceptics or atheists’ that the first cause is unknown. Similarly, in Part XI, he says that ‘if we abandon all human analogy… I am afraid we abandon all religion, and retain no conception of the great object of our adoration.’ Cleanthes is here as before depending on the empiricist account of the origin of ideas: that all our ideas must be derived from experience. Although Demea and Philo are initially made to appear in agreement, Demea is affronted by Philo’s radical scepticism. Philo is concerned above all to establish a negative conclusion, that argument from analogy cannot yield the traditional view. Their divergence, hinted early in the Dialogues, becomes most apparent in Parts X and XI, which deal with the problem of evil.

Demea and Philo combine to paint a powerful picture of moral and natural evil. But Demea and not Philo thinks that in some way the imperfections of nature support religious belief. His attempt at arguing a priori having proved a failure, he now suggests:

that each man feels, in a manner, the truth of religion within his own breast; and from a consciousness of his imbecility and misery, rather than from any reasoning, is led to seek protection from that being, on whom he and all nature is dependent.

Philo gives to this a characteristically Humean twist. Nature appears unconcerned about the happiness of individuals, and to seek only the preservation of species. And while man can through society guard against many natural enemies, ‘does he not immediately raise up to himself imaginary enemies, the demons of his fancy, who haunt him with superstitious terrors, and blast every enjoyment of life?’ Demea’s view, that men look to religion for consolation in the vale of tears, and believe that evil and suffering are, from the point of view of eternity, reconcilable with divine power and benevolence, is attacked by both Philo and Cleanthes. Philo remarks that the religious imagination is moved as much by fear as by hope. Cleanthes objects that Demea’s position is entirely speculative.

Philo’s principle target, however, is the possibility of deriving the moral attributes of God from the nature of the world. When Cleanthes presented the design argument, we were invited to ‘look round the world’, and to infer divine intelligence and design from its order and purpose. Here, where the attribute of divine benevolence is in question, Hume brilliantly balances Cleanthes’ speech with Philo’s:

Look round this universe. What an immense profusion of beings, animated and organized, sensible and active! You admire this prodigious variety and fecundity. But inspect a little more narrowly these living existences, the only beings worth regarding. How hostile and destructive to each other! How insufficient all of them for their own happiness! How contemptible or odious to the spectator! The whole presents nothing but the idea of a blind nature, impregnated by a great vivifying principle, and pouring forth from her lap, without discernment or parental care, her maimed and abortive children.

Philo claims that in arguing from nature to God, the mixture of good and evil to be found in nature prevents any inference to a wholly good, or wholly evil, cause of the universe. He allows that the idea of there being both a good and an evil principle has some probability, but rejects it on the ground of ‘the uniformity and steadiness of general laws’; and consequently, he says, we are left with the idea of ‘blind’ nature–the cause or causes of the universe have no moral attributes at all. Earlier, in Part II, Philo had accepted that the existence of God is ‘unquestionable and self-evident. Nothing exists without a cause; and the original cause of this universe (whatever it be) we call GOD; and piously ascribe to him every species of perfection.’ Now he makes clear that the ascription of moral perfection to God can only be a matter of faith. So far as reasoning from the nature of the world goes, the most probable view is that God (i.e. whatever is the cause of the universe) has no moral nature. This conclusion is as unacceptable to Demea as it is to Cleanthes, who points out in Part X that ‘there is an end at once of all religion. For to what purpose establish the natural attributes of the deity, while the moral are still doubtful and uncertain?’

These are some of the themes of the Dialogues. But there are others, also important to the question of the possibility of natural theology. I have not attempted a detailed analysis; for this there are a number of helpful studies, mentioned later. In any case, a systematic exposition of arguments, let alone a critical assessment, would not be in harmony with Hume’s intention in writing in dialogue form. His general position about natural theology is sceptical, and his style is carefully adopted in order to achieve his sceptical aims.