37. But, as we have often said, the inadequacy of human ideas has no corresponding inadequacy in the unity of God the Father and God the Son: as though there were extension, or series, or flux, like a spring pouring forth its stream from the source, or a tree supporting its branch on the stem, or fire giving out its heat into space. In these cases we have expansion without any separation: the parts are bound together and do not exist of themselves, but the heat is in the fire, the branch in the tree, the stream in the spring. So the thing itself alone has an independent existence; the one does not pass into the other, for the tree and the branch are one and the same, as also the fire and the heat, the spring and the stream. But the Only-begotten God is God, subsisting by virtue of a perfect and ineffable birth, true Scion of the Unbegotten God, incorporeal offspring of an incorporeal nature, living and true God of living and true God, God of a nature inseparable from God. The fact of birth does not make Him God with a different nature, nor did the generation, which produced His substance, change its nature in kind.

37. Unitas Patris et Filii non humano more cogitanda. 0308C Filii nativitas.---Non est autem, ut saepe jam commemoravimus, in unitate Dei patris et Dei filii humanarum vitium opinionum: ut sit vel extensio, vel series, vel fluxus; ut aut rivum fons effundat ab origine, aut ramum arbor teneat in caudice, aut calorem ignis emittat in spatium. Haec enim ab se inseparabili protensione manent potius detenta, quam sibi sunt: dum et calor in igne est, et 0309A in arbore ramus est, et rivus in fonte est. Et haec ipsa res sola sibi est potius, quam res ex re substituta est: quia non aliud arbor quam ramus, neque ignis quam calor, neque fons possit esse quam rivus. At vero unigenitus Deus ex perfecta atque inenarrabili nativitate subsistens Deus est, et vera progenies innascibilis Dei est, et incorporalis naturae generatio incorporalis, et Deus vivus et verus a vivente Deo vero, et inseparabilis a Deo naturae Deus: dum 286 subsistens nativitas non alterius naturae Deum perfecit, neque generatio, quae substantiam provehebat, substantiae naturam demutavit in genere.