De Synodis 98

VIII. "And if any one understanding that the Son is like in essence to Him whose Son He is admitted to be, says that the Son is the same as the Father, or part of the Father, or that it is through an emanation or any such passion as is necessary for the procreation of corporeal children that the incorporeal Son draws His life from the incorporeal Father: let him be anathema." 21. We have always to beware of the vices of particular perversions, and countenance no opportunity for delusion. For many heretics say that the Son is like the Father in divinity in order to support the theory that in virtue of this similarity the Son is the same Person as the Father: for this undivided similarity appears to countenance a belief in a single monad. For what does not differ in kind seems to retain identity of nature. 22. But birth does not countenance this vain imagination; for such identity without differentiation excludes birth. For what is born has a father who caused its birth. Nor because the divinity of Him who is being born is inseparable from that of Him who begets, are the Begetter and the Begotten the same Person; while on the other hand He who is born and He who begets cannot be unlike. He is therefore anathema who shall proclaim a similarity of nature in the Father and the Son in order to abolish the personal meaning of the word Son: for while through mutual likeness one differs in no respect from the other, yet this very likeness, which does not admit of bare union, confesses both the Father and the Son because the Son is the changeless likeness of the Father. For the Son is not part of the Father so that He who is born and He who begets can be called one Person. Nor is He an emanation so that by a continual flow of a corporeal uninterrupted stream the flow is itself kept in its source, the source being identical with the flow in virtue of the successive and unbroken continuity. But the birth is perfect, and remains alike in nature; not taking its beginning materially from a corporeal conception and bearing, but as an incorporeal Son drawing His existence from an incorporeal Father according to the likeness which belongs to an identical nature.