Hume relates a conversation he had with a friend concerning the opposition between speculative philosophy and religion. He suggests to his friend that philosophy in ancient Greece and Rome was far better able to flourish since it was less countered by religious superstition. His friend replies that an Epicurus, known for disavowing traditional religious beliefs, would still be able to defend himself reasonably against religious zealots. In order to prove his point Hume's friend presents an imaginary speech on Epicurus' behalf made to the people of Athens.
Hume's friend, as Epicurus, suggests that religious philosophers have strayed from their traditions, trying to prove the existence of God by means of reason. Such a question, Epicurus insists, is beyond the capacity of reason. His non- religious principles do not contradict religious tradition, but merely suggest that religious traditions are not founded in reason.
Religious philosophy argues for God's existence from design. It perceives a certain order in the universe that could not have arisen by accident, and so infers the existence of God as an ordering principle. This kind of reasoning-- inferring by experience from effect to cause--requires that we infer no qualities in the cause beyond what are requisite for producing the effect, and that we infer no further effects from this cause beyond what we have observed already. Religious philosophers often make the mistake of inferring the existence of God from what they observe, and then inferring other, unobserved, effects from this inference of God's existence. We can perhaps infer God's existence from what we observe in the world, but we cannot then infer some greater design or perfection in the world from our inference of God's existence. Philosophers who claim to do so proceed not from reason but from imagination.
Hume objects that sometimes it is perfectly acceptable to infer further effects from a cause that one has inferred from observed effects. For instance, if we see a footprint in the sand, we can infer that it was caused by a person walking on the beach. Furthermore, we can infer that this person must have left other footprints as well, which have since been erased by the waves. It seems perfectly reasonable to infer that other footprints must have existed, and yet this inference comes from a cause that itself has been inferred from other effects.
Hume's friend replies that, in the case of the footprints, we can infer that further footprints existed based on our wider knowledge of human behavior. We know a great deal more about people than what we learn from the one footprint-- for instance, that people have two feet and usually leave consecutive footprints. Once we infer that a person was walking across the sand, we can bring to bear our wider knowledge of people and infer further effects than the one we observe. Unlike people, however, we know nothing of God beyond what we observe. We may infer his existence from certain observed effects, but this inference will not help us infer any further effects than what we have already observed. Hume's friend asserts that God's divinity is too far beyond our comprehension for us to reasonably ascribe any human motives to him.
Hume also points out that reasoned inferences are drawn from observing the constant conjunction between species of effects and species of causes. The inference of the existence of God is singular in nature, and thus the inference might be regarded with some suspicion.