1 The words of the Lord, I in the Father, and the Father in Me, confuse many minds, and not unnaturally, for the powers of human reason cannot provide them with any intelligible meaning. It seems impossible that one object should be both within and without another, or that (since it is laid down that the Beings of whom we are treating, though They do not dwell apart, retain their separate existence and condition) these Beings can reciprocally contain One Another, so that One should permanently envelope, and also be permanently enveloped by, the Other, whom yet He envelopes.
This is a problem which the wit of man will never solve, nor will human research ever find an analogy for this condition of Divine existence. But what man cannot understand, God can be. I do not mean to say that the fact that this is an assertion made by God renders it at once intelligible to us. We must think for ourselves, and come to know the meaning of the words, I in the Father, and the Father in Me: but this will depend upon our success in grasping the truth that reasoning based upon Divine verities can establish its conclusions, even though they seem to contradict the laws of the universe.
Source: On the Trinity (New Advent)