St. Thomas on the Trinity:
"God understands and loves himself; likewise, understanding and willing in him are not something distinct from his essence. Since God understands himself, and since all that is understood is in the person who understands, God must be in himself as the object understood is in the person understanding.
"But the object understood, so far as it is in the one who understands, is a certain word of the intellect. We signify by an exterior word what we c omprehend interiorly in our intellect. For words, according to Aristotle, are signs of intellectual concepts. Hence we must acknowledge in God the existence of his Word." (Compendium of Theology, ch. 37).
Whew! Dense stuff! Thomas is saying that the Word or Logos, the second Person of the Trinity, proceeds from the Father by way of an intellectual generation. The Father knows himself and as God, knows himself perfectly. His idea of himself cannot lack anything; so it perfectly expresses his Being to the point that the Word is another Person of the Trinity. The idea of generation leads to the concept of the Son, which Thomas will take up later.