Objection 1. It would seem that the Son and the Father are not in each other. For the Philosopher (Phys. iv, text. 23) gives eight modes of one thing existing in another, according to none of which is the Son in the Father, or conversely; as is patent to anyone who examines each mode. Therefore the Son and the Father are not in each other.
Objection 2. Further, nothing that has come out from another is within it. But the Son from eternity came out from the Father, according to Micheas v. 2: "His going forth is from the beginning, from the days of eternity." Therefore the Son is not in the Father.
Objection 3. Further, one of two opposites cannot be in the other. But the Son and the Father are relatively opposed. Therefore one cannot be in the other.
On the contrary, It is said (John 14:10): "I am in the Father, and the Father is in Me."
I answer that, There are three points of consideration as regards the Father and the Son; the essence, the relation, and the origin; and according to each the Son and the Father are in each other. The Father is in the Son by His essence, forasmuch as the Father is His own essence, and communicates His essence to the Son not by any change on His part. Hence it follows that as the Father's essence is in the Son, the Father Himself is in the Son; likewise, since the Son is His own essence, it follows that He Himself is in the Father in Whom is His essence. This is expressed by Hilary (De Trin. v), "The unchangeable God, so to speak, follows His own nature in begetting an unchangeable subsisting God. So we understand the nature of God to subsist in Him, for He is God in God." It is also manifest that as regards the relations, each of two relative opposites is in the concept of the other. Regarding origin also, it is clear that the procession of the intelligible word is not outside the intellect, inasmuch as it remains in the utterer of the word. What also is uttered by the word is therein contained. And the same applies to the Holy Ghost.
Reply to Objection 1. What is contained in creatures does not sufficiently represent what exists in God; so according to none of the modes enumerated by the Philosopher, are the Son and the Father in each other. The mode the most nearly approaching to the reality is to be found in that whereby something exists in its originating principle, except that the unity of essence between the principle and that which proceeds therefrom is wanting in things created.
Reply to Objection 2. The Son's going forth from the Father is by mode of the intereior procession whereby the word emerges from the heart and remains therein. Hence this going forth in God is only by the distinction of the relations, not by any kind of essential separation.
Reply to Objection 3. The Father and the Son are relatively opposed, but not essentially; while, as above explained, one relative opposite is in the other.
Source: Summa Theologica (New Advent)