18. You see, then, that He Who went with him, Himself departed from him. The Same is, then, the Lord, Who is the Spirit of the Lord, that is, he called the Spirit of God, Lord, as also the Apostle says: “The Lord is the Spirit, now where the Spirit of the Lord is, there is liberty.” You find, then, the Holy Spirit called the Lord; for the Holy Spirit and the Son are not one Person [unus] but one Substance [unum].