The Celtic druids probably taught the same doctrine as the Greek philosophers. The metempsychosis, a prominent article of this creed, may have been derived from the Pythagoreans, but more probably it was one of the many relics of patriarchal belief which were engrafted on all pagan religions. They also taught that the universe would never be entirely destroyed, supposing that it would be purified by fire and water from time to time. This opinion may have been derived from the same source. The druids had a _pontifex maximus_, to whom they yielded entire obedience,--an obvious imitation of the Jewish custom. The nation was entirely governed by its priests, though after a time, when the kingly power developed itself, the priestly power gave place to the regal. Gaul was the head-quarters of druidism; and thither we find the Britons, and even the Romans, sending their children for instruction. Eventually, Mona became a chief centre for Britain. The Gaedhilic druids, though probably quite as learned as their continental brethren, were more isolated; and hence we cannot learn so much of their customs from external sources. There is no doubt that the druids of Gaul and Britain offered human sacrifices; it appears almost certain the Irish druids did not. Our principal and most reliable information about this religion, is derived from Cæsar. His account of the learning of its druids, of their knowledge of astronomy, physical science, mechanics, arithmetic, and medicine, however highly coloured, is amply corroborated by the casual statements of other authors.[144] He expressly states that they used the Greek character in their writings, and mentions tables found in the camp of the Helvetii written in these characters, containing an account of all the men capable of bearing arms.