1 The Bythus, therefore, whom they conceive of with his Pleroma, and the God of Marcion, are inconsistent. If indeed, as they affirm, he has something subjacent and beyond himself, which they style vacuity and shadow, this vacuum is then proved to be greater than their Pleroma. But it is inconsistent even to make this statement, that while he contains all things within himself, the creation was formed by some other. For it is absolutely necessary that they acknowledge a certain void and chaotic kind of existence (below the spiritual Pleroma) in which this universe was formed, and that the Propator purposely left this chaos as it was, either knowing beforehand what things were to happen in it, or being ignorant of them. If he was really ignorant, then God will not be prescient of all things. But they will not even [in that case] be able to assign a reason on what account He thus left this place void during so long a period of time. If, again, He is prescient, and contemplated mentally that creation which was about to have a being in that place, then He Himself created it who also formed it beforehand [ideally] in Himself.
2. Let them cease, therefore, to affirm that the world was made by any other; for as soon as God formed a conception in His mind, that was also done which He had thus mentally conceived. For it was not possible that one Being should mentally form the conception, and another actually produce the things which had been conceived by Him in His mind. But God, according to these heretics, mentally conceived either an eternal world or a temporal one, both of which suppositions cannot be true. Yet if He had mentally conceived of it as eternal, spiritual, and visible, it would also have been formed such. But if it was formed such as it really is, then He made it such who had mentally conceived of it as such; or He willed it to exist in the ideality of the Father, according to the conception of His mind, such as it now is, compound, mutable, and transient. Since, then, it is just such as the Father had [ideally] formed in counsel with Himself, it must be worthy of the Father. But to affirm that what was mentally conceived and pre-created by the Father of all, just as it has been actually formed, is the fruit of defect, and the production of ignorance, is to be guilty of great blasphemy. For, according to them, the Father of all will thus be [regarded as] generating in His breast, according to His own mental conception, the emanations of defect and the fruits of ignorance, since the things which He had conceived in His mind have actually been produced.
Source: Adversus haereses (New Advent)