"The three books on the Holy Spirit are, as St. Ambrose says himself, a sequel to those on the Faith, and the two treatises together have been sometimes quoted as if one, with the title, De Trinitate. But we see from Gratian's letter to St. Ambrose, and from the reply, that each treatise is separate, and the De Spiritu Sancto was written some years later, a.d. 381. ...]"The truth is that St. Ambrose being a good Greek scholar, and having undertaken to write on the Holy Spirit, studied what others had written before him, and made use of what had been urged by SS. Basil, Didymus, and others. The opinion of the great St. Augustine concerning this treatise may be set against that of St. Jerome. "St. Ambrose when treating of the deep subject of the Holy Spirit, and showing that He is equal with the Father and the Son, yet makes use of a simple style of discourse; inasmuch as his subject required no the embellishments of language, but proofs to move the minds of his readers."" - Introduction