Vates were a class of seers and sacrificers among the ancient Gauls. Greek and Roman writers describe them as one of three specially honoured groups in Gaulish society, ranked beside the bards and the druids. They conducted sacrifices, foretold the future, and concerned themselves with the natural world. The account goes back to the lost Gaulish ethnography of the Greek philosopher Posidonius. It survives in three later writers, Diodorus Siculus, Strabo and Ammianus Marcellinus, who name and describe the class in terms that do not fully agree. The native word behind the Greek and Latin forms is reconstructed as Proto-Celtic *wātis, also continued by Old Irish fáith.