What the theory of human nature should do is enable us to recognize that the basis of a proper methodology, in any science, should be the natural workings of the human mind. That follows from the discovery that all reasoning is, in essence, a kind of natural instinct. In realizing this, we see what we can hope to achieve in any area. For example, we realize that we simply cannot discover natural laws in science by a priori reasoning. We understand that ‘philosophical decisions are nothing but the reflections of common life, methodized and corrected.’17

As any natural theologian must, each character in the Dialogues assumes an epistemology. An epistemology not grounded in the theory of human nature is, for Hume, mistaken in itself, and therefore incapable of supporting conclusions in natural theology. Although both Demea and Cleanthes are made to employ principles drawn from Hume’s own theory in criticism of the other speakers, Philo’s methods of reasoning are the closest of the three to Hume’s. By skilfully putting themes from his own philosophy into the mouths of his characters, and equally by having them also represent other types of philosophy opposed to his own, Hume aims to achieve the ‘changes and improvements’ which he considers so desirable in natural religion. He considers it especially desirable that his theory of human nature is applied to natural religion because the subject is not purely theoretical, but has a practical relation to how we should live.

In Part II, Cleanthes proposes a version of the Design Argument. He says that the argument is a posteriori, that is, from matters of fact established by experience, and that it is the only argument needed to prove both ‘the existence of a deity and his similarity to human mind and intelligence’. An analogy can be seen between the structure of the world, and machines. The world resembles ‘one great machine, subdivided into an infinite number of lesser machines’. As in a machine, the parts are ‘adjusted to each other’ with astonishing accuracy. In the case of machines, we know that the adjustment of parts to each other, and the ‘adapting of means to ends’, are caused by ‘human design, thought, wisdom, and intelligence’. Since the ordered structure of a machine resembles the ordered structure of the world, and that of a machine is caused by intelligent design, we may infer ‘by all the rules of analogy’ that the cause of the ordered structure of the world is also intelligent design. As the structure of the world so much surpasses that of a machine, so, by analogy, the mind of the author of nature surpasses that of a man. He is proved to be ‘somewhat similar to the mind of man, though possessed of much larger faculties, proportioned to the grandeur of the work which he has executed’.

In giving this argument, Cleanthes has claimed to be inferring a cause from an effect, and to be arguing analogically. But he has not, of course, given any detailed account of these principles of reasoning. By the device of having Philo suggest that Demea’s vehement objection to the argument arises in part from his failing to see its logical structure, Hume introduces some of his own theory of such reasoning through Philo’s ‘restatement’ of it. Philo paraphrases Hume to establish that ‘experience alone can point out to [anyone] the true cause of any phenomenon’. It follows that whether or not such phenomena as ‘order, arrangement, or the adjustment of final causes’ are caused by design is something which can be established only by experience. If we consider only what is a priori conceivable, then it could be that the material world has a cause of its order inherent within it. Subtly, Philo brings out that Cleanthes must be assuming that the ideas in God’s mind cause order in the world by themselves being ordered and structured, and similarly the ideas in the mind of a human being who makes a watch or a house ‘arrange themselves so as to form the plan’. So Cleanthes is committed to the possibility of some things being inherently ordered. A priori, it could be matter as much as mind which has a cause of order inherent in it. So Cleanthes is claiming that it is from experience that we know that matter is not, but mind is, inherently ordered. Philo implies that all of this is already tacitly contained in Cleanthes’ initial argument. By thus ‘restating’ it in terms of Hume’s philosophy, Philo in fact identifies some of the weak points in it. But Cleanthes is invited to confirm that Philo has ‘made a fair representation of it’, and does so.

Throughout Part II Philo raises a series of difficulties with Cleanthes’ method of argument, some of which are explored further in other parts. He objects, for example, that the analogy between the world and a machine is not close enough to permit the inference to design in the case of the world to be anything more than a conjecture.