69 Should any one presume, not merely to speak thus of the Only-begotten God in the rashness of his tongue, but even to think so in the wickedness of his heart, let him hear what the Apostle thought of the Holy Ghost, from the words he wrote to the Corinthians, But unto us God revealed them through the Spirit: for the Spirit searches all things, yea the deep things of God. For who among men knows the things of a man, which are in him, save the spirit of the man which is in him?
Even so the things which are in God, none knows, save the Spirit of God. But let us cast aside these empty illustrations of material things, and measure God born of God, Spirit of Spirit, by His own powers and not by earthly conditions. Let us measure Him not by our own senses, but by His divine claims. Let us believe Him Who said, He that has seen Me has seen the Father also. Let us not forget that He said, Believe, if only by My works, that the Father is in Me, and I in the Father, and again, I and the Father are one. If the names which correspond to realities, when intelligibly used, impart to us any true information, then He Who is seen in Another by the eye of understanding is not different in nature from that Other; not different in kind, since He abides in the Father, and the Father in Him; not separate, since Both are One.
Perceive their unity in the indivisibility of their nature, and apprehend the mystery of that indivisible nature by regarding the One as the mirror of the Other. But remember that He is the mirror, not as the image reflected by the splendour of a nature outside Himself, but as being a living nature, indistinguishable from the Father's living nature, derived wholly from the whole of His Father's, having the Father's in Him because He is the Only begotten, and abiding in the Father, because He is God.
Source: On the Trinity (New Advent)