“For what man knoweth the things which be in God, but the Spirit which is in Him?” But the Word was formed by the Spirit, and (if I may so express myself) the Spirit is the body of the Word. The Word, therefore, is both always in the Father, as He says, “I am in the Father;” and is always with God, according to what is written, “And the Word was with God;” and never separate from the Father, or other than the Father, since “I and the Father are one.”
…Following, therefore, the form of these analogies, I confess that I call God and His Word—the Father and His Son—two. For the root and the tree are distinctly two things, but correlatively joined; the fountain and the river are also two forms, but indivisible; so likewise the sun and the ray are two forms, but coherent ones. Everything which proceeds from something else must needs be second to that from which it proceeds, without being on that account separated. Where, however, there is a second, there must be two; and where there is a third, there must be three. Now the Spirit indeed is third from God and the Son; just as the fruit of the tree is third from the root, or as the stream out of the river is third from the fountain, or as the apex of the ray is third from the sun. Nothing, however, is alien from that original source whence it derives its own properties. In like manner the Trinity, flowing down from the Father through intertwined and connected steps, does not at all disturb the Monarchy, whilst it at the same time guards the state of the Economy.
Tertullian
Against Praxeas
Chapter VIII