While Anaxagoras, his trial notwithstanding, is not generally designated an atheist, probably because there was nothing in his writings to which he might be pinned down, that fate befell two of his contemporaries, Hippo of Rhegium and Diogenes of Apollonia. Very little, however, is known of them. Hippo, who is said to have been a Pythagorean, taught that water and fire were the origin of everything; as to the reason why he earned the nickname _atheos_, it is said that he taught that Water was the primal cause of all, as well as that he maintained that nothing existed but what could be perceived by the senses. There is also quoted a (fictitious) inscription, which he is said to have caused to be put on his tomb, to the effect that Death has made him the equal of the immortal gods (in that he now exists no more than they). Otherwise we know nothing special of Hippo; Aristotle refers to him as shallow. As to Diogenes, we learn that he was influenced by Anaximenes and Anaxagoras; in agreement with the former he regarded Air as the primary substance, and like Anaxagoras he attributed reason to his primary substance. Of his doctrine we have extensive accounts, and also some not inconsiderable fragments of his treatise _On Nature_; but they are almost all of them of purely scientific, mostly of an anatomical and physiological character. In especial, as to his relation to popular belief, it is recorded that he identified Zeus with the air. Indirectly, however, we are able to demonstrate, by the aid of an almost contemporary witness, that there must have been some foundation for the accusation of “atheism.” For in _The Clouds_, where Aristophanes wants to represent Socrates as an atheist, he puts in his mouth scraps of the naturalism of Diogenes; that he would hardly have done, if Diogenes had not already been decried as an atheist.
It is of course impossible to base any statement of the relation of the two philosophers to popular belief on such a foundation. But it is, nevertheless, worth noticing that while not a single one of the earlier naturalists acquired the designation atheist, it was applied to two of the latest and otherwise little-known representatives of the school. Take this in combination with what has been said above of Anaxagoras, and we get at any rate a suspicion that Greek naturalism gradually led its adherents beyond the naïve stage where many individual phenomena were indeed ascribed to natural causes, even if they had formerly been regarded as caused by divine intervention, but where the foundations of the popular belief were left untouched. Once this path has been entered on, a point will be arrived at where the final conclusion is drawn and the existence of the supernatural completely denied. It is probable that this happened towards the close of the naturalistic period. If so early a philosopher as Anaxagoras took this point of view, his personal contribution as a member of the Periclean circle may have been more significant in the religious field than one would conjecture from the character of his work.