Julius Cesar, The Gallic Wars
6.13 Throughout all Gaul there are only two orders of men who are of any rank and dignity: for the common people are held almost in the condition of slaves, who dare to undertake nothing, and are not admitted to any councils. The greater part, when they are pressed either by debt, or the large amount of their tributes, or the oppression of the more powerful, give themselves up in vassalage to the nobles, who possess over them the same rights without exception as masters over their slaves. But of these two orders, one is that of the Druids, the other that of the knights.
The Druids are engaged in things sacred, conduct the public and the private sacrifices, and interpret all matters of religion. To these a large number of the young men come for instruction, for Druids are held in great honor among them. They make the decisions on almost all controversies, public and private; and if any crime has been perpetrated, or if a murder has been committed, or if there is any dispute about an inheritance or any property boundary, these same persons decide it. They decree rewards and punishments. If anyone, either in a private or public capacity, does not submit to their decision, they exclude him from their sacrifices. This among them is the most heavy punishment. Those who have been thus excluded are counted as impious and criminal: all shun them, and avoid their society and conversation, to avoid receiving some evil from their contact. No justice is administered to these outcasts, when they seek it. No dignities are bestowed on them.
Over all these Druids one presides, who possesses supreme authority among them. Upon his death, if any individual among the rest is pre-eminent in dignity, he succeeds; but, if there are many equal, an election is made, or sometimes they contend for the presidency with arms. They assemble at a fixed period of the year in a consecrated place in the territories of the Carnutes [between the Seine and the Loire rivers, in the region of modern Chartres, Orleans and Blois], which is reckoned the central region of the whole of Gaul. All who have disputes assemble in that place from every part, and submit to their decrees and judgments. This institution is supposed to have been invented first in Britain, and then brought over from there into Gaul, and even now any who want to gain a more accurate knowledge of that system generally go to Britain to study it.
6.14 The Druids do not go to war, nor do they pay taxes with the rest. They have an exemption from military service and other duties. Induced by such great advantages, many choose to embrace this profession, and many others are sent to it by their parents and relations. They are required to memorize a great number of verses; accordingly some remain in the course of training for twenty years. Nor do they regard it lawful to commit these verses to writing, though in almost all other matters, in their public and private transactions, they use Greek characters. That practice of not writing their religious teachings they seem to me to have adopted for two reasons. First, they do not want their doctrines to be known among the mass of the people. Second, those who learn would devote themselves less to the efforts of memory if they could rely on writing. This objection to writing generally occurs to most men, that dependence on writing relaxes diligence in learning and employment of the memory. They try to promote, as one of their leading beliefs, that souls do not perish, but pass after death from one body into another, and they think that men who believe this are inspired with courage, the fear of death being disregarded. They likewise discuss and teach the youth many things about the stars and their movements, about the size of the universe and of our earth, about the nature of things, and about the power and majesty of the immortal gods.
Roman Descriptions of Ancient Britain