Objection 1. It would seem that the notional acts are voluntary. For Hilary says (De Synod.): "Not by natural necessity was the Father led to beget the Son."
Objection 2. Further, the Apostle says, "He transferred us to the kingdom of the Son of His love" (Colossians 1:13). But love belongs to the will. Therefore the Son was begotten of the Father by will.
Objection 3. Further, nothing is more voluntary than love. But the Holy Ghost proceeds as Love from the Father and the Son. Therefore He proceeds voluntarily.
Objection 4. Further, the Son proceeds by mode of the intellect, as the Word. But every word proceeds by the will from a speaker. Therefore the Son proceeds from the Father by will, and not by nature.
Objection 5. Further, what is not voluntary is necessary. Therefore if the Father begot the Son, not by the will, it seems to follow that He begot Him by necessity; and this is against what Augustine says (Ad Orosium qu. vii).
On the contrary, Augustine says, in the same book, that, "the Father begot the Son neither by will, nor by necessity."
I answer that, When anything is said to be, or to be made by the will, this can be understood in two senses. In one sense, the ablative designates only concomitance, as I can say that I am a man by my will--that is, I will to be a man; and in this way it can be said that the Father begot the Son by will; as also He is God by will, because He wills to be God, and wills to beget the Son. In the other sense, the ablative imports the habitude of a principle as it is said that the workman works by his will, as the will is the principle of his work; and thus in that sense it must be said the God the Father begot the Son, not by His will; but that He produced the creature by His will. Whence in the book De Synod, it is said: "If anyone say that the Son was made by the Will of God, as a creature is said to be made, let him be anathema." The reason of this is that will and nature differ in their manner of causation, in such a way that nature is determined to one, while the will is not determined to one; and this because the effect is assimilated to the form of the agent, whereby the latter acts. Now it is manifest that of one thing there is only one natural form whereby it exists; and hence such as it is itself, such also is its work. But the form whereby the will acts is not only one, but many, according to the number of ideas understood. Hence the quality of the will's action does not depend on the quality of the agent, but on the agent's will and understanding. So the will is the principle of those things which may be this way or that way; whereas of those things which can be only in one way, the principle is nature. What, however, can exist in different ways is far from the divine nature, whereas it belongs to the nature of a created being; because God is of Himself necessary being, whereas a creature is made from nothing. Thus, the Arians, wishing to prove the Son to be a creature, said that the Father begot the Son by will, taking will in the sense of principle. But we, on the contrary, must assert that the Father begot the Son, not by will, but by nature. Wherefore Hilary says (De Synod.): "The will of God gave to all creatures their substance: but perfect birth gave the Son a nature derived from a substance impassible and unborn. All things created are such as God willed them to be; but the Son, born of God, subsists in the perfect likeness of God."
Reply to Objection 1. This saying is directed against those who did not admit even the concomitance of the Father's will in the generation of the Son, for they said that the Father begot the Son in such a manner by nature that the will to beget was wanting; just as we ourselves suffer many things against our will from natural necessity--as, for instance, death, old age, and like ills. This appears from what precedes and from what follows as regards the words quoted, for thus we read: "Not against His will, nor as it were, forced, nor as if He were led by natural necessity did the Father beget the Son."
Reply to Objection 2. The Apostle calls Christ the Son of the love of God, inasmuch as He is superabundantly loved by God; not, however, as if love were the principle of the Son's generation.
Reply to Objection 3. The will, as a natural faculty, wills something naturally, as man's will naturally tends to happiness; and likewise God naturally wills and loves Himself; whereas in regard to things other than Himself, the will of God is in a way, undetermined in itself, as above explained (19, 3). Now, the Holy Ghost proceeds as Love, inasmuch as God loves Himself, and hence He proceeds naturally, although He proceeds by mode of will.
Reply to Objection 4. Even as regards the intellectual conceptions of the mind, a return is made to those first principles which are naturally understood. But God naturally understands Himself, and thus the conception of the divine Word is natural.
Reply to Objection 5. A thing is said to be necessary "of itself," and "by reason of another." Taken in the latter sense, it has a twofold meaning: firstly, as an efficient and compelling cause, and thus necessary means what is violent; secondly, it means a final cause, when a thing is said to be necessary as the means to an end, so far as without it the end could not be attained, or, at least, so well attained. In neither of these ways is the divine generation necessary; because God is not the means to an end, nor is He subject to compulsion. But a thing is said to be necessary "of itself" which cannot but be: in this sense it is necessary for God to be; and in the same sense it is necessary that the Father beget the Son.
Source: Summa Theologica (New Advent)