Ancient Greek naturalism is essentially calculated to collide with the popular belief. It seeks a natural explanation of the world, first and foremost of its origin, but in the next place of individual natural phenomena. As to the genesis of the world, speculations of a mythical kind had already developed on the basis of the popular belief. They were not, however, binding on anybody, and, above all, the idea of the gods having created the world was altogether alien to Greek religion. Thus, without offence to them it might be maintained that everything originated from a primary substance or from a mixture of several primary substances, as was generally maintained by the ancient naturalists. On the other hand, a conflict arose as soon as the heavenly phenomena, such as lightning and thunder, were ascribed to natural causes, or when the heavenly bodies were made out to be natural objects; for to the Greeks it was an established fact that Zeus sent lightning and thunder, and that the sun and the moon were gods. A refusal to believe in the latter was especially dangerous because they were _visible_ gods, and as to the person who did not believe in their divinity the obvious conclusion would be that he believed still less in the invisible gods. That this inference was drawn will appear before long. But the epithet “atheist” was very rarely attached to the ancient naturalists; only a few of the later (and those the least important) were given the nickname _atheos_. Altogether we hear very little of the relation of these philosophers to the popular belief, and this very silence is surely significant. No doubt, most of them bestowed but a scant attention on this aspect of the matter; they were engrossed in speculations which did not bring them into conflict with the popular belief, and even their scientific treatment of the “divine” natural phenomena did not make them doubt the _existence_ of the gods. This is connected with a peculiarity in their conception of existence. Tradition tells us of several of them, and it applies presumably also to those of whom it is not recorded, that they designated their primary substance or substances as gods; sometimes they also applied this designation to the world or worlds originating in the primary substance. This view is deeply rooted in the Greek popular belief and harmonises with its fundamental view of existence. To these ancient thinkers the primary substance is at once a living and a superhuman power; and any living power which transcended that of man was divine to the Greeks. Hylozoism (the theory that matter is alive) consequently, when it allies itself with popular belief, leads straight to pantheism, whereas it excludes monotheism, which presupposes a distinction between god and matter. Now it is a matter of experience that, while monotheism is the hereditary foe of polytheism, polytheism and pantheism go very well together. The universe being divine, there is no reason to doubt that