19 Thus we do not deny a unanimity between the Father and the Son—for heretics are accustomed to utter this falsehood, that since we do not accept concord by itself as the bond of unity we declare Them to be at variance. But let them listen how it is that we do not deny such a unanimity. The Father and the Son are one in nature, honour, power, and the same nature cannot will things that are contrary. Moreover, let them listen to the testimony of the Son as touching the unity of nature between Himself and the Father, for He says, When that advocate has come, Whom I shall send to you from the Father, the Spirit of truth Who proceeds from the Father, He shall testify of Me. The Advocate shall come and the Son shall send Him from the Father, and He is the Spirit of truth Who proceeds from the Father.
Let the whole following of heretics arouse the keenest powers of their wit; let them now seek for what lies they can tell to the unlearned, and declare what that is which the Son sends from the Father. He Who sends manifests His power in that which He sends. But as to that which He sends from the Father, how shall we regard it, as received or sent forth or begotten? For His words that He will send from the Father must imply one or other of these modes of sending. And He will send from the Father that Spirit of truth which proceeds from the Father; He therefore cannot be the Recipient, since He is revealed as the Sender. It only remains to make sure of our conviction on the point, whether we are to believe an egress of a co-existent Being, or a procession of a Being begotten.
Source: On the Trinity (New Advent)