Diodorus Siculus, ¶31
The Gauls are terrifying in appearance and speak with deep, harsh voices. They speak together in few words, using riddles which leave much of the true meaning to be understood by the listener. They frequently exaggerate their claims to raise their own status and diminish another's. They are boastful, violent, and melodramatic, but very intelligent and learn quickly. They have Iyric poets called Bards, who, accompanied by instruments resembling Iyres, slng both praise and satire. They have highly-honoured philosophers and theologians [those who speak about the gods] called Druids. They also make use of seers, who are greatly respected. These seers, having great authority, use auguries and sacrifices to foresee the future. When seeking knowledlre of great importance, they use a strange and unbelievable method: they choose a person for death and stab him or her in the chest above the diaphragm. By the convulsion of the victim's limbs and spurtinp of blood, they foretell the future, trusting in this ancient method. They do not sacrifice or ask favours from the Gods without a Druid present, as thev believe sacrifice should be made only by those supposedly skilled in divine communication. Not only during peacetime but also in war, the Gauls obey with great care these Druids and singing poets, both friend and enemy alike. Often when two armies have come together with swords drawn these men have stepped between the battle-lines and stopped the conflict, as if they held wild animals spell-bound. Thus even among most brutal barbarians angry passion yields to wisdom and Ares stands in awe of the Muses.
Strabo, Geography, 4.4.4
As a rule, among all the Gallic peoples three sets of men are honoured above all others: the Bards, the vates, and the Druids. The bards are singers and poets, the vates overseers of sacred rites and philosophers of nature, and the Druids, besides being natural philosophers, practice moral philosophy as well. They are considered to be the most just and therefore are entrusted with settling both private and public disputes, so that in earlier times they even arbitrated wars and could keep those intending to draw themselves up for battle from so doing and it was to these men most of all that cases involving murder had been entrusted for adjudication. And whenever there is a big yield from these cases, they believe that there will come a yield from the land too. Both these men and others aver that and the universe are imperishable, although both fire and water will at some times prevail over them.