The plant that springs from the root is something distinct from that whence it grows up; and yet it is of one nature with it. And the river which flows from the fountain is something distinct from the fountain. For we cannot call either the river a fountain, or the fountain a river. Nevertheless we allow that they are both one according to nature, and also one in substance; and we admit that the fountain may be conceived of as father, and that the river is what is begotten of the fountain.
Reproduced in full. Translation: Ante-Nicene Fathers, vol. 6 (Roberts and Donaldson, 1886), public domain (Public domain).