In an entirely gratuitous act of love, God has given Himself to mankind through the Church. Part of this gift is his supernatural revelation through the Church, both in Scripture and Tradition, where He reveals both what is necessary for salvation, but also His own unimaginably sublime inner life. Psalm 33 says “By your word you made them,” John 1 gives evidence of the eternity of the Word, and Hebrews 4:12 says, “The Word of God is living and active … discerning the thoughts and intentions of the heart.”
Because it is an essentially supernatural mystery, the Trinity cannot be comprehended. It is a preamble of the faith that God is Truth: therefore the Trinity is not incomprehensible because it is a contradiction in some way, or somehow too vague. Rather, it overflows with Truth. Just as some good things are too good to be believed, some true things are, so to speak, too true to be known. We experience this in people we love, whose personhoods are, in a mysterious way, too true to be comprehended.
Nevertheless, people have fruitfully meditated on the Trinity in great depth. One such meditation is St. Thomas’ famous Psychological Analogy of the Trinity in his Summa Contra Gentiles. His object in the analogy is to clarify certain conceptions of Trinitarian doctrine, not so that the Trinity can be imagined but so that it can be understood with more lucidity and ultimately more wonder. More precisely speaking, Aquinas seeks to explain through a proper analogy (in which the terms do fully apply) how the Son proceeds from the Father, and how the Holy Spirit proceeds from the Father and the Son. For the sake of simplicity, I will only outline the first part concerning the procession of the Son.
As an analogy, Aquinas works from natural things upward. Every creature has an act of production, but the nature of the thing produced is more or less intimate with the one producing, depending on how perfect the creature is. As the creature is more perfect, its purest act of production is increasingly an intimate emanation of its nature. Thus merely physical parts of creation, such as rocks and dirt, only generate other non-living things through violent external action. Merely living creatures like plants, on the other hand, have an internal principle of motion whereby they reproduce. Life, however, is still external from the source because plants only produce other plants. Inasmuch as the produced plants have a separate act of existence from their parent, the two are not intimately connected. Animals have a higher mode of being than this, since they sense as well as live: there is thus an interior production of an image of what is sensed (otherwise what the animal perceives could not be processed or judged in any way). There is still a lack of intimacy, however, since the internal image only resembles what the animal is sensing, not the sense power itself. Thus the nature of what produces is not communicated to what is produced.
Human persons, who alone among creatures are called the image of God, are higher than all these in the order of perfection because of their intellects. Like the sense power, which produces an imaginative representation of what it perceives, the intellect also has within it an image of what it considers. For one thing, how else could it interact with the world, or abstract definitions and principles from concrete circumstances, such as mathematical expressions? Unlike the sense power, the intellect can consider itself. Therefore the emanation from the intellect is a word in the intellect which is in the image of the intellect. We cannot stop here, however, since there are different kinds of intellect, according to Aquinas. For human intellects cannot produce ideas without sense experience first, and are thus full of less penetrating words. Angelic intellects can contemplate truth directly—that is, without the mediation of experience. But even these lack the fulness of perfection, since the thought of the angel does not share the nature of the angel, being merely an act of the angel. Aquinas’ proper analogy, then, supposing that God has a Word, as Scripture says, asks what that Word may be like.
The analogy runs thus. The object of an act of understanding exists in the intellect as an image of the thing understood, as said above. Moreover, such an image, or “internal word,” has the mode of being of the intellect. Now God understands Himself perfectly, as it would be repugnant to His nature not to understand anything perfectly. Therefore in God there is an image of Himself vis-à-vis Word. However, because of God’s essentially perfect Unity, whatever is in Him is Him (for more on this, see Aquinas’ Summa Theologica, Part I, question 11). Therefore this internal Word as image has the same being and essence as God.
Everything said properly of God is thus also properly said of His Word except whatever regards their relation. For there is only One God: this relational distinction must be understood therefore as a relation of opposites. The Word as God is only not the Father as God insofar as He proceeds from the Father, and similarly the Father is not His Word only insofar as He does not proceed. If we admitted any other difference between them, they would not be One God but two.
Finally, since the Word is an emanation from God which shares His essence, He is properly called begotten, just as men beget children by giving them human nature. But insofar as the Word is eternally within the Father as image, He is also properly called conceived. This relation is therefore justly called the procession of “Son” from “Father,” which is the same language used by Scripture regarding this mystery.
With great precision and beauty, Aquinas thus attempts to lift up his students’ minds toward what is ultimately too sublime to be understood without God’s help. I will only add a few clarifying remarks. For one, we must remember that as helpful as analogical theological discourse is, it has its weaknesses. We may not infer from this attempt to represent the relation of the Father and the Son that the Son is God’s “intellect,” for example. We may also not suppose that Aquinas has here given a proof of the Son: what belongs to God ad intra cannot be discovered by reason. He is totally Other, beyond creation in every way. Having received the gift of Revelation, our minds ought to wonder without end at His beauty, and contemplate with a simple love the ever-welcoming depths of His mystery.