5 But how did it come to pass that the Æon was both dissolved [into her component parts], and became subject to passion? She was undoubtedly of the same substance as the Pleroma; but the entire Pleroma was of the Father. Now, any substance, when brought in contact with what is of a similar nature, will not be dissolved into nothing, nor will be in danger of perishing, but will rather continue and increase, such as fire in fire, spirit in spirit, and water in water; but those which are of a contrary nature to each other do, [when they meet,] suffer and are changed and destroyed.
And, in like manner, if there had been a production of light, it would not suffer passion, or recur any danger in light like itself, but would rather glow with the greater brightness, and increase, as the day does from [the increasing brilliance of] the sun; for they maintain that Bythus [himself] was the image of their father (Sophia). Whatever animals are alien [in habits] and strange to each other, or are mutually opposed in nature, fall into danger [on meeting together], and are destroyed; whereas, on the other hand, those who are accustomed to each other, and of a harmonious disposition, suffer no peril from being together in the same place, but rather secure both safety and life by such a fact.
If, therefore, this Æon was produced by the Pleroma of the same substance as the whole of it, she could never have undergone change, since she was consorting with beings similar to and familiar with herself, a spiritual essence among those that were spiritual. For fear, terror, passion, dissolution, and such like, may perhaps occur through the struggle of contraries among such beings as we are, who are possessed of bodies; but among spiritual beings, and those that have the light diffused among them, no such calamities can possibly happen.
But these men appear to me to have endowed their Æon with the [same sort of] passion as belongs to that character in the comic poet Menander, who was himself deeply in love, but an object of hatred [to his beloved]. For those who have invented such opinions have rather had an idea and mental conception of some unhappy lover among men, than of a spiritual and divine substance.
Source: Adversus haereses (New Advent)