The dialogue begins with a letter from Pamphilus to Hermippus. The letter discusses the advantages and disadvantages of dialogue as a form. Pamphilus argues that questions in philosophy that are obscure and uncertain, such as natural religion, lead naturally into the style of dialogue. There seem to be two reasons for this. First "reasonable men may be allowed to differ, where no one can reasonably be positive." So, presumably, the dialogue style displays these disagreements. Interestingly he also believes that this sort of debate will "afford an agreeable amusement" and that such a book will provide these two purest pleasures of life, "study and society." Now he makes a big point that we are not here to be questioning the existence of God, since after all, the existence of God provides the "surest foundation of morality" and "the firmest support of society." The importance of this truth is such that it should always be present in our thoughts. I suspect, as others have, that Hume is being extremely cautious here. He does not want to be accused of being an atheist even though his dialogue might encourage atheist conclusions. Even Philo the skeptic will assume that the existence of God is unassailable. The paragraph ends with the admission that "nothing but doubt, uncertainty, and contradiction, have as yet been the result of our most accurate researches." His last paragraph refers to Cleanthes' "accurate philosophical turn" to Philo's "careless skepticism and to Demea's "rigid inflexible orthodoxy." As a student of Cleanthes it is natural that he would be more favorable to him. Philo's skepticism seems hardly careless.
Part 1 is mainly a debate between Cleanthes and Philo about skepticism generally. As with Aristotle's Nicomachean Ethics, the dialogue begins with discouraging younger students from entering into these debates. But mainly here we are talking about children. Demea insists that students only study the nature of the gods after logic, ethics and physics: "this science of natural theology...being the most profound and abstruse of any, required the maturest judgment in its students..." and this requires that their minds should be "enriched with all the other sciences."
Philo worries that this means that the children have not been taught "the principles of religion." But Demea is only holding them back from religion as a science, i.e. as something that can be debated. He will teach them piety through precept and example so that they will have "habitual reverence." Also, as they are studying the other sciences he will point out "the uncertainty of each part" the debates surrounding them and "the obscurity of all philosophy." He will also observe the "strange, ridiculous conclusions" derived from mere reason. It is only then that they were learn the "greatest mysteries of religion." He will now feel that they are safe from the "assuming arrogance of philosophy" that might lead them to reject "the most established doctrines," for example the existence of God. So, interestingly, Demea is committed to promoting a kind of skepticism, at least about the sciences.
Philo is excited (perhaps ironically) about this early teaching of piety, which is needed in our "irreligious age." But mainly he admires Demea's drawing from principles of philosophy in this, principles which people have always seen as being destructive of religion. Vulgar people who have not studied science and observing the disputes of learned people will have contempt for philosophy and will settle on religious belief too quickly. People who have studied only a little philosophy think that very little is "too difficult for human reason." They then presumptuously question religious doctrine. So the way to avoid this is to follow and even improve on Demea's principles, i.e. becoming "thoroughly sensible of the weakness, blindness, and narrow limits of human reason." Philo even goes so far, in this skeptical vein, to note the uncertainty of reason "even in subjects of common life and practice." He then brings up the common skeptical theme that the senses deceive us. (It looks here that he is no longer talking about advice in the upbringing of children.) "Let the errors and deceits of our very senses be set before us" and in addition, all the difficulties entailed in the first principles of philosophical systems. These include contradictions involved in natural science, i.e. those which "adhere to the very ideas of matter, cause and effect, extension, space, time, motion" i.e. all areas that are quantifiable. This skepticism is extreme since this area, physics, is the only one "that can fairly pretend to any certainty or evidence." At the end of this, no one can retain confidence in "this frail faculty of reason" especially on such sublime and abstruse points that are far from everyday life. If we cannot explain how a stone hold together then how can we decide about the origin of the universe.
Pamphilus notes that Demea and Cleanthes were both smiling during this speech. But whereas Demea was happy, Cleanthes seemed to think that Philo was joking or just being artificially mean. Cleanthes says to Philo that he is proposing to "erect religious faith on philosophical scepticism." And he believes Cleanthes thinks that if we reject certainty in every other discipline it will still exist in theology, even acquiring "superior force and authority." But he questions whether Philo is as absolute and sincere in his scepticism as he pretends. If he were then he might well go out of the room through a window and might doubt that he would fall or be harmed by such a fall, since he questions the senses and experience. But if skeptics were earnest then we would not have to worry about them long. And if they are jokers they are not dangerous. Sure, a man might "after intense reflection on the many contradictions and imperfections of human reason, may entirely renounce all belief and opinion" he cannot continue in total scepticism or act by it for more than a few hours. External object, internal feelings, lightening of spirit will intervene: he just can't impose such violence on himself. The followers of Pyrrho should have confined their scepticism to their schools. Actually they are like the Stoics in having their system "founded on this erroneous maxim" viz. "that what a man can perform sometimes, and in some dispositions, he can perform always, and in every disposition." The Stoic thinks that the mind can withstand utmost bodily pain, but he cannot support such enthusiasm.
Philo replies even the Stoic when his mind cannot "support the highest flights of philosophy" still retains its disposition, and this will appear in how he lives his life. Similarly a sceptic will continue as a sceptic. Sure he must "live, and converse like other men." Also he notes that everyone has this philosophy to some extent in order to make advances, and that philosophy is the "methodical operation" of this kind. But when we look to "the properties of surrounding [nonhuman] bodies" and speculate about the "creation and formation of the universe" as well as the "existence and properties of spirits" and "the powers and operations of one universal spirit, existing without beginning and without end" [God] "omnipotent, omniscient, immutable, infinite, and incomprehensible" we will, like the skeptics, think we might have gotten "quite beyond the reach of our faculties." With more practical matters of business, morals, politics, criticism, we appeal to "common sense or experience" and these "strengthen our philosophical conclusions" and this partly removes sceptical worries. But in theology we cannot do this.