The time in which the Druid priesthood was instituted is unknown. It probably rose, like other institutions of that kind, from low and obscure beginnings, and acquired from time, and the labors of able men, a form by which it extended itself so far, and attained at length so mighty an influence over the minds of a fierce and otherwise ungovernable people. Of the place where it arose there is somewhat less doubt: Cæsar mentions it as the common opinion that this institution began in Britain, that there it always remained in the highest perfection, and that from thence it diffused itself into Gaul. I own I find it not easy to assign any tolerable cause why an order of so much authority and a discipline so exact should have passed from the more barbarous people to the more civilized, from the younger to the older, from the colony to the mother country: but it is not wonderful that the early extinction of this order, and that general contempt in which the Romans held all the barbarous nations, should have left these matters obscure and full of difficulty.
The Druids were kept entirely distinct from the body of the people; and they were exempted from all the inferior and burdensome offices of society, that they might be at leisure to attend the important duties of their own charge. They were chosen out of the best families, and from the young men of the most promising talents: a regulation which placed and preserved them in a respectable light with the world. None were admitted into this order but after a long and laborious novitiate, which made the character venerable in their own eyes by the time and difficulty of attaining it. They were much devoted to solitude, and thereby acquired that abstracted and thoughtful air which is so imposing upon the vulgar; and when they appeared in public, it was seldom, and only on some great occasion,--in the sacrifices of the gods, or on the seat of judgment. They prescribed medicine; they formed the youth; they paid the last honors to the dead; they foretold events; they exercised themselves in magic. They were at once the priests, lawgivers, and physicians of their nation, and consequently concentred in themselves all that respect that men have diffusively for those who heal their diseases, protect their property, or reconcile them to the Divinity. What contributed not a little to the stability and power of this order was the extent of its foundation, and the regularity and proportion of its structure. It took in both sexes; and the female Druids were in no less esteem for their knowledge and sanctity than the males. It was divided into several subordinate ranks and classes; and they all depended upon a chief or Arch-Druid, who was elected to his place with great authority and preeminence for life. They were further armed with a power of interdicting from their sacrifices, or excommunicating, any obnoxious persons. This interdiction, so similar to that used by the ancient Athenians, and to that since practised among Christians, was followed by an exclusion from all the benefits of civil community; and it was accordingly the most dreaded of all punishments. This ample authority was in general usefully exerted; by the interposition of the Druids differences were composed, and wars ended; and the minds of the fierce Northern people, being reconciled to each other under the influence of religion, united with signal effect against their common enemies.