​TREATISE, a written composition, dealing fully and systematically with the principles of some subject of serious importance. The M. Eng. tretis, O. Fr. tretis, or treitis, is a doublet of "treaty," which also meant a discourse or account. Both words are to be referred to Lat. tractare, to treat, handle, frequentative of trahere, tractus, to draw. " Treatise " thus would mean, by etymology, something well handled, nicely made.