This treatise, On the Ogdoad, is no longer extant, though it is probable that we have a few fragments of it (see Harvey, I. clxvi.). The importance which Irenæus attached to this work is seen from the solemn adjuration with which he closed it. It must have been largely identical in substance with the portions of his Adv. Hær. which deal with the æons of the Valentinians. It may have been little more than an enlargement of those portions of the earlier work. The Ogdoad (Greek, ὀγδόας, a word signifying primarily a thing in eight parts) occupied a prominent place in the speculations of the Gnostics. Valentinus taught eight primary æons, in four pairs, as the root and origin of the other æons and of all beings. These eight he called the first or primary Ogdoad; and hence a work upon the Ogdoad, written against a Valentinian, must, of course, be a general discussion of the Valentinian doctrine of the æons. The word Ogdoad was not used by all the Gnostics in the same sense. It was quite commonly employed to denote the supercelestial region which lay above the seven planetary spheres (or Hebdomad), and hence above the control of the seven angels who severally presided over these spheres. In the Valentinian system a higher sphere, the Pleroma, the abode of the æons, was added, and the supercelestial sphere, the Ogdoad of the other systems, was commonly called the Mesotes. or middle region. For further particulars in regard to the Ogdoad, see Salmon’s articles Hebdomad and Ogdoad in the Dict. of Christ. Biog.