A. That Polytheism was the Primary Religion of Man. The prior work that perhaps most closely approaches the sort of thing Hume is attempting here was the De Religione Gentilium (1663) of Edward Herbert of Cherbury. Herbert argued that all actual religions in existence were corruptions of an original rational worship consisting of five notions:
(1) That there is a supreme God;
(2) That this God ought to be worshipped;
(3) That the primary form this worship takes is piety combined with virtue;
(4) That men should repent of their sins;
(5) That vice and virtue are punished and rewarded (respectively).
Herbert's book was an attempt to argue that religions exhibited these principles in more or less faithful forms. Hume takes an entirely contrary approach, arguing that human beings were originally polytheistic. The farther back we go in history, Hume argues, the more polytheistic societies become. The "savage tribes" of America, Asia, and Africa are all polytheists. Hume allows that it is possible to develop an argument for monotheism from an accurate investigation into the frame of nature, but suggests that common people of any time don't really pay much attention to the data that would be required to make and appreciate this argument.
B. Religion originally arises out of hope and fear in matters of everyday life. If we aren't going to locate the origin of religion in speculative reason, then, Hume argues, we must locate it in the passions; in particular, in "the anxious concern for happiness, the dread of future misery, the terror of death, the thirst of revenge, the appetite for food and other necessaries" (p. 28).
The way in which these passions lead to polytheism is this. Actuated by hope and fear about the future, human beings begin to worry about the unknown causes that govern things like sickness, famine, and the like; "and while the passions are kept in perpetual alarm by an anxious expectation of the events, the imagination is equally employed in forming ideas of those powers, on which we have so entire a dependance" (p. 29). In the absence of a developed theory of the relevant causes, the human mind anthropomorphizes. Not only does it happen in poetry, but common people often go farther than the poet and begin believing in the hamadryad in the wood or the spirit of the waters. Even philosophers, Hume thinks, are not immune, giving the standard list of supposed scholastic anthropomorphisms (the horror of the vacuum, sympathies, antipathies, natural appetite).
No wonder, then, that mankind, being placed in such an absolute ignorance of causes, and being at the same time so anxious concerning their future fortune, should immediately acknowledge a dependence on invisible powers, possessed of sentiment and intelligence. The unknown causes which continually employ their thought, apeparing always in the same aspect, are all apprehended to be of the same kind or species. Nor is it long before we ascribe to them thought and reason and apassion, and sometimes even the limbs and figures of men, in order to bring them nearer to a resemblance with ourselves. (p. 30)
Any human passion, Hume thinks, may lead to the belief in "invisible intelligent power"; but Hume thinks that the less agreeable passions -- fear, grief, and the like -- are more conducive to the development of religion, since they lead to attempts to appease the invisible powers.
Interestingly, the bulk of this explanation is not found exclusively in Hume. One also finds it in Malebranche. Malebranche's The Search after Truth discusses a very similar type of mechanism in explaining how original sin creates a pagan mindset in people; this mechanism plays an important role in Malebranche's account of how we fall into the error of believing that things have their own causal powers. Hume would certainly have known of the passages in which Malebranche discusses this; he quotes elsewhere from one of the most important. Whether Hume's eighteenth-century naturalistic account is adapted from Malebranche's seventeenth-century Catholic account is unclear, however; very little work has been done on the historical influences for The Natural History of Religion.
C. Monotheism arises through a propensity to flattery. After discussing various other issues (e.g., the limited non-creative function of polytheistic gods, allegory, hero-worship, and the like), Hume goes on to discuss the origin of theism from polytheism. His view is that it does not arise by any process of reasoning but by a sycophantic tendency in human nature. Polytheists tend to have special devotions to a particular god; as time goes on they tend to flatter and eulogize that deity, whose characteristics become greater and greater until all the other gods are edged out. Gods become supreme through bootlicking.
D. Polytheism and monotheism are engaged in a perpetual cycle of flux and reflux. The propensity to anthropomorphize forces the mind in a polytheistic direction; the propensity to adulation forces the mind in a monotheistic direction.
From this Hume passes on to discussion of how the two, polytheism and monotheism compare. Polytheism makes men tolerant, monotheism makes them intolerant; polytheism makes men heroic, monotheism makes them servile; monotheism, while initially more rational, is more likely to corrupt philosophy for precisely that reason; both are a bad influence on morals. Hume finds this all quite puzzling, and thus ends the work with a famous passage:
The whole is a riddle, an aenigma, an inexplicable mystery. Doubt, uncertainty, suspence of judgment appear the only result of our most accurate scrutiny, concerning this subject. But such is the frailty of human reason, and such the irresistible contagion of opinion, that even this deliberate doubt could scarcely be upheld; did we not enlarge our view, and opposing one species of superstition to another, set them a quarrelling; while we ourselves, during their fury and contention, happily make our escape into the calm, though obscure, regions of philosophy. (p. 76)
One of the interesting limitations of Hume's approach in The Natural History of Religion is that his knowledge of the "savage" societies is very indirect. Virtually all of his knowledge of pagan life is drawn from classical writings, particularly from Greek and Roman historians. This means that his knowledge of other religious cultures is a curious mix of idealization and traveler's tales. In this sense, Hume's attempt to a psychological history of religion runs into a problem that is common in this period: newly interested in other cultures, Europeans of the time are nonetheless almost wholly dependent on ancient sources, traveler's tales, and missionary reports.
[Page numbers are from David Hume, The Natural History of Religion. H. E. Root, editor. Stanford University Press (Stanford: 1956).]
[For a more modern approach that Hume would have loved, and which occasionally shows some surprising agreement with his speculations, see Chris's January posts at "Mixing Memory" here, here, and here.]