See Athan., Contra Arian., Orat. 3; Greg. Naz., Orat. 35. So Augustine (Contr. Max. iii. 14, De Trin. xv.). Epiphanius (Anchor.), and Gregory of Nyssa (Epist. ad Ablab.) teach that the Spirit proceeds, and is not begotten, because He is both of the Father and the Son, while the Son is only of the Father.