Analysis of information sources in references of the Wikipedia article "Bibracte" in English language version.
they have their own eloquence and wise men called Druids. They claim to know the size of the earth and cosmos, the movements of the heavens and stars, and the will of the gods. They teach, in caves or hidden groves, many things to the nobles in a course of instruction lasting up to twenty years. One of their doctrines has become commonly known to the populace so that warriors might fight more bravely, that the spirit is eternal and another life awaits the spirits of the dead.
There are some who say that the study of philosophy had its beginning among the barbarians. They urge that the Persians have had their Magi, the Babylonians or Assyrians their Chaldaeans, and the Indians their Gymnosophists; and among the Celts and Gauls there are the people called Druids or Holy Ones, for which they cite as authorities the Magicus of Aristotle and Sotion in the twenty-third book of his Succession of Philosophers. [...] As to the Gymnosophists and Druids we are told that they uttered their philosophy in riddles, bidding men to reverence the gods, to abstain from wrongdoing, and to practise courage.
Philosophers, as we may call them, and men learned in religious affairs are unusually honoured among them and are called by them Druids. [...] the belief of Pythagoras prevails among them, that the souls of men are immortal and that after a prescribed number of years they commence upon a new life, the soul entering into another body.
Among all the Gallic peoples, generally speaking, there are three sets of men who are held in exceptional honour; the Bards, the Vates and the Druids. The Bards are singers and poets; the Vates, diviners and natural philosophers; while the Druids, in addition to natural philosophy, study also moral philosophy. ... not only the Druids, but others as well, say that men's souls, and also the universe, are indestructible, although both fire and water will at some time or other prevail over them.
The Druids usually hold aloof from war, and do not pay war‑taxes with the rest; they are excused from military service and exempt from all liabilities. Tempted by these great rewards, many young men assemble of their own motion to receive their training; many are sent by parents and relatives. Report says that in the schools of the Druids they learn by heart a great number of verses, and therefore some persons remain twenty years under training. And they do not think it proper to commit these utterances to writing, although in almost all other matters, and in their public and private accounts, they make use of Greek letters. ... The cardinal doctrine which they seek to teach is that souls do not die, but after death pass from one to another; and this belief, as the fear of death is thereby cast aside, they hold to be the greatest incentive to valour. Besides this, they have many discussions as touching the stars and their movement, the size of the universe and of the earth, the order of nature, the strength and the powers of the immortal gods, and hand down their lore to the young men.
I think now a suitable time to describe the regions and situation of the Gauls [...] Throughout these regions men gradually grew civilised and the study of the liberal arts flourished, initiated by the Bards, the Euhages and the Druids. Now, the Bards sang to the sweet strains of the lyre the valorous deeds of famous men composed in heroic verse, but the Euhages, investigating the sublime, attempted to explain the secret laws of nature. The Druids, being loftier than the rest in intellect, and bound together in fraternal organisations, as the authority of Pythagoras determined, were elevated by their investigation of obscure and profound subjects, and scorning all things human, pronounced the soul immortal.
