Fountain-head of the Son and the Holy Ghost. Dionysius says, that in
the Father there is an outflowing of the Godhead, which naturally
communicates itself to the Word or Son. He also freely and lovingly
pours Himself out into the Son; and the Son in turn pours Himself
out freely and lovingly into the Father; and this love of the Father
for the Son, and of the Son for the Father, is the Holy Ghost. This
is truly said, but it is made clearer by that glorious Doctor of the
Church, St Thomas, who says as follows: In the outpouring of the
Word from the Father's heart, God the Father must contemplate
Himself with His own mind, bending back, as it were, upon His Divine
essence; for if the reason of the Father had not the Divine essence
for its object, the Word so conceived would be a creature instead of
God; which is false. But in the way described He is "God of God."
Again, this looking back upon the Divine essence, which takes place
in the mind of God, must, in a manner, produce a natural likeness;
else the Word would not be the Son of God. So here we have the unity
of essence in the diversity of Persons; and a clear proof of this
distinction may be found in the word of that soaring eagle St John:
"The Word was in the beginning with God."

Thus the Father is the Fountain-head of the Son, and the Son is the
outflowing of the Father; and the Father and Son pour forth the
Spirit; and the Unity, which is the essence of the Fountain-head, is
also the substance of the three Persons. But as to how the Three are
One, this cannot be expressed in words, on account of the simplicity
of that Abyss. Into this intellectual Where, the spirits of men made
perfect soar and plunge themselves, now flying over infinite
heights, now swimming in unfathomed depths, marvelling at the high
and wonderful mysteries of the Godhead. Nevertheless, the spirit
remains a spirit, and retains its nature, while it enjoys the vision