Alcuin of York — as excerpted in the Catena Aurea on John 5:31-40

“Or thus; Christ, being both God and man, He shews the proper existence of both, by sometimes speaking according to the nature he took from man, sometimes according to the majesty of the Godhead. If I bear witness of Myself, My witness is not true: this is to be understood of His humanity; the sense being, If I, a man, bear witness of Myself, i. e. without God, My witness is not true: and then follows, There is another that beareth witness of Me. The Father bore witness of Christ, by the voice which was heard at the baptism, and at the transfiguration on the mount. And I know that His witness is true; because He is the God of truth. How then can His witness be otherwise than true?”