Objection 1. It would seem that no procession in God can be called generation. For generation is change from non-existence to existence, and is opposed to corruption; while matter is the subject of both. Nothing of all this belongs to God. Therefore generation cannot exist in God.
Objection 2. Further, procession exists in God, according to an intelligible mode, as above explained (1). But such a process is not called generation in us; therefore neither is it to be so called in God.
Objection 3. Further, anything that is generated derives existence from its generator. Therefore such existence is a derived existence. But no derived existence can be a self-subsistence. Therefore, since the divine existence is self-subsisting (3, 4), it follows that no generated existence can be the divine existence. Therefore there is no generation in God.
On the contrary, It is said (Psalm 2:7): "This day have I begotten Thee."
I answer that, The procession of the Word in God is called generation. In proof whereof we must observe that generation has a twofold meaning: one common to everything subject to generation and corruption; in which sense generation is nothing but change from non-existence to existence. In another sense it is proper and belongs to living things; in which sense it signifies the origin of a living being from a conjoined living principle; and this is properly called birth. Not everything of that kind, however, is called begotten; but, strictly speaking, only what proceeds by way of similitude. Hence a hair has not the aspect of generation and sonship, but only that has which proceeds by way of a similitude. Nor will any likeness suffice; for a worm which is generated from animals has not the aspect of generation and sonship, although it has a generic similitude; for this kind of generation requires that there should be a procession by way of similitude in the same specific nature; as a man proceeds from a man, and a horse from a horse. So in living things, which proceed from potential to actual life, such as men and animals, generation includes both these kinds of generation. But if there is a being whose life does not proceed from potentiality to act, procession (if found in such a being) excludes entirely the first kind of generation; whereas it may have that kind of generation which belongs to living things. So in this manner the procession of the Word in God is generation; for He proceeds by way of intelligible action, which is a vital operation:--from a conjoined principle (as above described):--by way of similitude, inasmuch as the concept of the intellect is a likeness of the object conceived:--and exists in the same nature, because in God the act of understanding and His existence are the same, as shown above (Question 14, Article 4). Hence the procession of the Word in God is called generation; and the Word Himself proceeding is called the Son.
Reply to Objection 1. This objection is based on the idea of generation in the first sense, importing the issuing forth from potentiality to act; in which sense it is not found in God.
Reply to Objection 2. The act of human understanding in ourselves is not the substance itself of the intellect; hence the word which proceeds within us by intelligible operation is not of the same nature as the source whence it proceeds; so the idea of generation cannot be properly and fully applied to it. But the divine act of intelligence is the very substance itself of the one who understands (14, 4). The Word proceeding therefore proceeds as subsisting in the same nature; and so is properly called begotten, and Son. Hence Scripture employs terms which denote generation of living things in order to signify the procession of the divine Wisdom, namely, conception and birth; as is declared in the person of the divine Wisdom, "The depths were not as yet, and I was already conceived; before the hills, I was brought forth." (Proverbs 8:24). In our way of understanding we use the word "conception" in order to signify that in the word of our intellect is found the likeness of the thing understood, although there be no identity of nature.
Reply to Objection 3. Not everything derived from another has existence in another subject; otherwise we could not say that the whole substance of created being comes from God, since there is no subject that could receive the whole substance. So, then, what is generated in God receives its existence from the generator, not as though that existence were received into matter or into a subject (which would conflict with the divine self-subsistence); but when we speak of His existence as received, we mean that He Who proceeds receives divine existence from another; not, however, as if He were other from the divine nature. For in the perfection itself of the divine existence are contained both the Word intelligibly proceeding and the principle of the Word, with whatever belongs to His perfection (4, 2).
Source: Summa Theologica (New Advent)