Extant Fragments: Epistle to Dionysius Bishop of Rome. 3From the Same First Book.

7. It was said above that God is the spring of all good things, but the Son was called the river flowing from Him; because the word is an emanation of the mind, and--to speak after human fashion--is emitted from the heart by the mouth. But the mind which springs forth by the tongue is different from the word which exists in the heart. For this latter, after it has emitted the former, remains and is what it was before; but the mind sent forth flies away, and is carried everywhere around, and thus each is in each although one is from the other, and they are one although they are two. And it is thus that the Father and the Son are said to be one, and to be in one another.