The Greek original of the work of Irenæus is from time to time recovered through the numerous quotations made from it by subsequent writers, especially by the author’s pupil Hippolytus, and by Epiphanius. The latter preserves (Hær. xxxi. secs. 9–32) the preface of Irenæus, and most of the first book. An important difference of reading occurs between the Latin and Greek in the very first word. The translator manifestly read ἐπεί, quatenus, while in Epiphanius we find ἐπί, against. The former is probably correct, and has been followed in our version. We have also supplied a clause, in order to avoid the extreme length of the sentence in the original, which runs on without any apodosis to the words ἀναγκαῖον ἡγησάμην, “I have judged it necessary.”