No name in connection with Celtic religion is more familiar to the average reader than that of the Druids, yet there is no section of the history of Celtic religion that has given rise to greater discussion than that relating to this order.

Even the association of the name with the Indo-European root _dru_-, which we find in the Greek word _drus_, an oak, has been questioned by such a competent Celtic scholar as M.
d'Arbois de Jubainville, but on this point it cannot be said that his criticism is conclusive.

The writers of the ancient world who refer to the Druids, do not always make it sufficiently clear in what districts the rites, ceremonies, and functions which they were describing prevailed.

Nor was it so much the priestly character of the Druids that produced the deepest impression on the ancients.

To some philosophical and theological writers of antiquity their doctrines and their apparent affinities with Pythagoreanism were of much greater interest than their ceremonial or other functions.