A less well known priestly class among the Celtic people are the Vates (literally, “seer”). Vates are rare on the continent and are most often found in Ireland and sometimes in Albion. They interpret sacrifices and natural phenomena to determine the will of the gods. Vates keep the great calendars, and maintain shrines in sacred groves. They have a great practical knowledge of mathematics, but refuse to apply this to worldly pursuits. Instead, they carefully study flights of birds, clouds, dreams, storms, and sticks thrown on the ground to sense divine will. To be touched by the gods is to court madness, and many Vates are not quite right as a result.
Unlike Druids, Vates (called Ovates in some Celtic dialects) do not truly fit in with Celtic society. Warriors fear what a drunken Vates might spill and are uncomfortable with their attendance at feasts. Tradition ties a Vates to his or her sacred grove, and they rarely wander from this site. Everyone likes it better this way.
The earliest Latin writers used vates to denote "prophets" and soothsayers in general; the word fell into disuse in Latin until it was revived by Virgil. Thus Ovid could describe himself as the vates of Eros (Amores 3.9).
According to the Ancient Greek writers Strabo, Diodorus Siculus, and Poseidonius, the vates (ουατεις) were one of three classes of Celtic priesthood, the other two being the druids and the bards. The Vates had the role of seers and performed sacrifices (in particular administering human sacrifice), under the presidence of a druid. Their role therefore corresponded to that of an Adhvaryu in Vedic religion. Celtic vates is continued by Irish fáith "prophet, seer," and ofydd in Welsh.
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 Lyres, sing 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 they 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.