Galen, the physician, asserts that experience is the only source of knowledge; lays great stress on the culture of mathematics and logic, observing that he himself should have been a Pyrrhonist had it not been for geometry. In the teleological doctrine of physiology he considers that the foundations of a true theology must be laid. The physicians of the times exerted no little influence on the promotion of such views; for the most part they embraced the Pantheistic doctrine. As one of them, Sextus Empiricus may be mentioned; his works, still remaining, indicate to us the tendency of this school to materialism. [Sidenote: Philosophical atheism among the educated.] Such was the tone of thought among the cultivated Romans; and to this philosophical atheism among them was added an atheism of indifference among the vulgar. But, since man is so constituted that he cannot live for any length of time without a form of worship, it is evident that there was great danger, whenever events should be ripe for the appearance of some monotheistic idea, that it might come in a base aspect. At a much later period than that we are here considering, one of the emperors expressed himself to the effect that it would be necessary to give liberty for the exercise of a sound philosophy among the higher classes, and provide a gorgeous ceremonial for the lower; he saw how difficult it is, by mere statesmanship to co-ordinate two such requirements, in their very nature contradictory. Though polytheism had lost all intellectual strength, the nations who had so recently parted with it could not be expected to have ceased from all disposition to an animalization of religion and corporealization of God. In a certain sense the emperor was only a more remote and more majestic form of the conquered and vanished kings, but, like them, he was a man. There was danger that the theological system, thus changing with the political, would yield only expanded anthropomorphic conceptions. [Sidenote: Principles, to be effective, must coincide with existing tendencies.]