In Dialogues Concerning Natural Religion Hume explores whether religious belief can be rational. Because Hume is an empiricist (i.e. someone who thinks that all knowledge comes through experience), he thinks that a belief is rational only if it is sufficiently supported by experiential evidence. So the question is really, is there enough evidence in the world to allow us to infer an infinitely good, wise, powerful, perfect God? Hume does not ask whether we can rationally prove that God exists, but rather whether we can rationally come to any conclusions about God's nature. He asserts that the first question is beyond doubt; the latter is initially undecided.
Hume presents three characters, each of whom represent a different position on this issue, engaged in a dialogue together. Demea argues for the position of religious Orthodoxy, and insists that we cannot possibly come to know the nature of God through reason. He believes, in fact, that we cannot ever know the nature of God at all because God's nature is inherently beyond the capacity of human comprehension. Philo, the philosophical skeptic, agrees with Demea that God is incomprehensible and provides the most convincing arguments for this position. Cleanthes argues the position of empirical theism—the position that we can come to know about God by reasoning from the evidence afforded us by nature—against these two opponents.
Cleanthes bases his belief in empirical theism on the argument from design. According to this argument, the complex order and beauty of our universe can only be explained by positing the existence of an intelligent designer, that is, God. The argument is supposed to work by way of analogy (an argument of this form is called an argument by analogy): (1) The world resembles a finely tuned machine. (2) All machines we know of are created by intelligence (human intelligence). (3) Therefore, the world must also be caused by intelligence (divine intelligence). By looking at nature, in other words, we obtain overwhelming evidence that God's intelligence resembles human intelligence (though of course, in much more perfect form). The argument from design is supposed to be the best case that can be made for the claim that religious belief can be rational. By showing that the argument from design fails, Hume hopes to prove that religious belief cannot possibly be based on reason.
Philo the skeptic delivers Hume's objections to the argument from design. In part II he attempts to demonstrate that the argument from design is not even an actual instance of the sort of argument it purports to be, and as such is faulty. The argument from design seems to be an argument by analogy, but it does not work even under this rubric. First, the analogy between machines and the universe is weak at best, and as such any reasoning based on this analogy must also be weak. Second, the universe and a machine are not strictly analogous phenomena because they are not independently existing entities, rather the universe is a whole and a machine is a part of it.
Philo also argues that it is not true that all order we experience is caused by intelligence that we can sense. Some order, such as that found in organic bodies, is caused by generation and vegetation. There is no reason, then, to think that just because the world is ordered, it is necessarily a result of intelligent design. Finally, an inductive argument (that is, an argument that argues for a conclusion based on past evidence), which the argument from design certainly is, requires repeated experience of the phenomena in question (i.e. repeated experience of the cause followed by the effect). However here the relevant cause (God) and effect (the universe) are both wholly unique, so there is no way that we could have such repeated experience of their existences or anything that resembles them.
In section IV, Philo takes up another line of attack. He argues that the claim that God is an intelligent designer does not even succeed in explaining why the world is ordered. It is no easier to understand how God's thoughts might set the world in order than it is to understand how the material world might be its own source of order. In either case we have to ask how and why this happens. Nothing is gained, therefore, by positing God as an intelligent designer.
In part V, Philo argues that even if we can infer anything from the argument from design, it is not what we want to be able to infer. Given the evidence we have from nature we have no grounds on which to conclude that God is infinite, that God is perfect, that there is only one God, or even that God lacks a physical body. Thus even if the argument from design were valid, the evidence we get from the nature of the universe provides us with no knowledge about God's nature.
In parts VI through VIII, Philo attempts to show that there are many other possible analogies, other than the analogy to machines, that are equally well supported by the evidence we find in nature. For instance, the universe can be analogized to an animal body and God to its soul. It is therefore almost random to choose the analogy between the universe and a machine.
In parts X and XI, Philo gives his most famous and most decisive arguments against empirical theism. Until this point, the discussion has centered around God's natural attributes—his infiniteness, his eternality, and his perfection. Now Philo examines the idea of God's moral attributes (for instance, his goodness) and asks whether these can be inferred through an investigation of nature. Together, Demea and Philo paint a bleak picture of our universe. In stark contrast to the perfectly harmonious machine that Cleanthes considers the universe to be, they tell us that our world is actually a miserable place, filled with evil. As Philo puts it, if the universe is a machine, its only goal is the bare survival of each species, not that any species be happy. Given how much evil there is in the world, we cannot possibly look at the world and infer that God is infinitely good, infinitely wise, and infinitely powerful. In fact, we cannot even look at the world and infer from the evidence that he is at all good, wise, and powerful. If we were to try to infer God's moral attributes from the evidence in nature (which, of course, Philo does not think that we should do), the only reasonable conclusion to draw would be that God is morally neutral.
At this point, it seems that Philo has shown that the argument from design is manifestly invalid. However, in the last chapter Philo does an about face and provisionally accepts the argument from design. It is wholly obvious, he declares, that the ordered world has some intelligence behind it and that this intelligence bears some resemblance to the human mind. The only real point of disagreement, he continues, is how strong this resemblance really is; what separates the atheist from the theist is only a question over the degree of analogy between man and God. Philo then goes on to attack organized religion as morally and psychologically harmful, and to urge that only true religion (that is, a philosophical belief in some higher power) should be accepted. Finally, he ends by espousing a fideist position that would have made Demea proud if he had not already exited in a huff at the end of the previous chapter: philosophical skepticism, Philo tells Cleanthes, is the only proper route to true Christianity, it forces us to turn toward revelation by undermining our faith in reason. It is only through revelation that we come to worship God in the right way. However, it is questionable whether this last surprising assertion is an expression of Hume's own opinion, as he was a notorious skeptic and critic of organized Christianity.