Adv. Hær. iii. 26. The allusion is to the gnostics and mainly to Valentinus and his school who imagined seven heavens, and a supercelestial space termed “Ogdoad.” “The doctrine of an Ogdoad of the commencement of finite existence having been established by Valentinus, those of his followers who had been imbued with the Pythagorean philosophy introduced a modification. In that philosophy the tetrad was regarded with peculiar veneration, and held to be the foundation of the sensible world.” Cf. Hippolytus Ref. vi. 23, p. 179 “We read there (Iren. i. xi.) of Secundus as a Valentinian who divided the Ogdoad into a right hand and a left hand tetrad, and in the case of Marcus who largely uses Pythagorean speculations about numbers, the tetrad holds the highest place in the system.” Dr. Salmon, Dict. Christ. Biog. iv. 72. Irenæus wrote a work, no longer extant, “on the Ogdoad.” Euseb. H.E. v. 20.