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.