de bello gallico, 6.14

The Druids are accustomed to be apart from war, and they do not pay tribute together with the rest; they have an exemption from military service and a immunity from all matters. Induced by such rewards and by their own free will many come into the discipline and are sent to it by their parents and relations. There they are said to learn by heart a great number of verses. And so not none remain in the instruction twenty years. Nor do they think it right to commit these to writing, although generally in remaining matters, in their public and private matters, they use Greek characters. They seem to me to have adopted it for two reasons, because they neither want their discipline to be born among the common people, nor those who learn, having relied on writing, to cultivate their memory less; because it generally happens to most that with the help of writing they relax their diligence in learning thoroughly and their memory. They wish to inculcate this among the first, that souls do not die, but pass after death from some to others, and they think that by this (men) are most excited to virtue, the fear of death having been neglected. Many things besides about the stars and their motion, about the size of the world and lands, about the nature of things, about the power and the majesty of the immortal gods they debate and hand down to the youth.