It must be known that different things have different modes of generation. The generation of God is different from that of other things. Hence, we cannot arrive at a notion of divine generation except through the generation of that created thing which more closely approaches to a likeness to God. We have seen that nothing approaches in likeness to God more than the human soul. The manner of generation in the soul is effected in the thinking process in the soul of man, which is called a conceiving of the intellect. This conception takes its rise in the soul as from a father, and its effect is called the word of the intellect or of man. In brief, the soul by its act of thinking begets the word. So also the Son of God is the Word of God, not like a word that is uttered exteriorly (for this is transitory), but as a word is interiorly conceived; and this Word of God is of the one nature as God and equal to God.
--St. Thomas Aquinas,
The Catechetical Instruction of St. Thomas Aquinas
