Objection 1. It would seem that the notional acts do not proceed from anything. For if the Father begets the Son from something, this will be either from Himself or from something else. If from something else, since that whence a thing is generated exists in what is generated, it follows that something different from the Father exists in the Son, and this contradicts what is laid down by Hilary (De Trin. vii) that, "In them nothing diverse or different exists." If the Father begets the Son from Himself, since again that whence a thing is generated, if it be something permanent, receives as predicate the thing generated therefrom just as we say, "The man is white," since the man remains, when not from white he is made white--it follows that either the Father does not remain after the Son is begotten, or that the Father is the Son, which is false. Therefore the Father does not beget the Son from something, but from nothing.
Objection 2. Further, that whence anything is generated is the principle regarding what is generated. So if the Father generate the Son from His own essence or nature, it follows that the essence or nature of the Father is the principle of the Son. But it is not a material principle, because in God nothing material exists; and therefore it is, as it were, an active principle, as the begetter is the principle of the one begotten. Thus it follows that the essence generates, which was disproved above (Question 39, Article 5).
Objection 3. Further, Augustine says (De Trin. vii, 6) that the three persons are not from the same essence; because the essence is not another thing from person. But the person of the Son is not another thing from the Father's essence. Therefore the Son is not from the Father's essence.
Objection 4. Further, every creature is from nothing. But in Scripture the Son is called a creature; for it is said (Sirach 24:5), in the person of the Wisdom begotten,"I came out of the mouth of the Most High, the first-born before all creatures": and further on (Sirach 24:14) it is said as uttered by the same Wisdom, "From the beginning, and before the world was I created." Therefore the Son was not begotten from something, but from nothing. Likewise we can object concerning the Holy Ghost, by reason of what is said (Zechariah 12:1): "Thus saith the Lord Who stretcheth forth the heavens, and layeth the foundations of the earth, and formeth the spirit of man within him"; and (Amos 4:13) according to another version [the Septuagint]: "I Who form the earth, and create the spirit."
On the contrary, Augustine (Fulgentius, De Fide ad Petrum i, 1) says: "God the Father, of His nature, without beginning, begot the Son equal to Himself."
I answer that, The Son was not begotten from nothing, but from the Father's substance. For it was explained above (27, 2; 33, 2,3) that paternity, filiation and nativity really and truly exist in God. Now, this is the difference between true "generation," whereby one proceeds from another as a son, and "making," that the maker makes something out of external matter, as a carpenter makes a bench out of wood, whereas a man begets a son from himself. Now, as a created workman makes a thing out of matter, so God makes things out of nothing, as will be shown later on (45, 1), not as if this nothing were a part of the substance of the thing made, but because the whole substance of a thing is produced by Him without anything else whatever presupposed. So, were the Son to proceed from the Father as out of nothing, then the Son would be to the Father what the thing made is to the maker, whereto, as is evident, the name of filiation would not apply except by a kind of similitude. Thus, if the Son of God proceeds from the Father out of nothing, He could not be properly and truly called the Son, whereas the contrary is stated (1 John 5:20): "That we may be in His true Son Jesus Christ." Therefore the true Son of God is not from nothing; nor is He made, but begotten.
That certain creatures made by God out of nothing are called sons of God is to be taken in a metaphorical sense, according to a certain likeness of assimilation to Him Who is the true Son. Whence, as He is the only true and natural Son of God, He is called the "only begotten," according to John 1:18, "The only begotten Son, Who is in the bosom of the Father, He hath declared Him"; and so as others are entitled sons of adoption by their similitude to Him, He is called the "first begotten," according to Romans 8:29: "Whom He foreknew He also predestinated to be made conformable to the image of His Son, that He might be the first born of many brethren." Therefore the Son of God is begotten of the substance of the Father, but not in the same way as man is born of man; for a part of the human substance in generation passes into the substance of the one begotten, whereas the divine nature cannot be parted; whence it necessarily follows that the Father in begetting the Son does not transmit any part of His nature, but communicates His whole nature to Him, the distinction only of origin remaining as explained above (Question 40, Article 2).
Reply to Objection 1. When we say that the Son was born of the Father, the preposition "of" designates a consubstantial generating principle, but not a material principle. For that which is produced from matter, is made by a change of form in that whence it is produced. But the divine essence is unchangeable, and is not susceptive of another form.
Reply to Objection 2. When we say the Son is begotten of the essence of the Father, as the Master of the Sentences explains (Sent. i, D, v), this denotes the habitude of a kind of active principle, and as he expounds, "the Son is begotten of the essence of the Father"--that is, of the Father Who is essence; and so Augustine says (De Trin. xv, 13): "When I say of the Father Who is essence, it is the same as if I said more explicitly, of the essence of the Father."
This, however, is not enough to explain the real meaning of the words. For we can say that the creature is from God Who is essence; but not that it is from the essence of God. So we may explain them otherwise, by observing that the preposition "of" [de] always denotes consubstantiality. We do not say that a house is "of" [de] the builder, since he is not the consubstantial cause. We can say, however, that something is "of" another, if this is its consubstantial principle, no matter in what way it is so, whether it be an active principle, as the son is said to be "of" the father, or a material principle, as a knife is "of" iron; or a formal principle, but in those things only in which the forms are subsisting, and not accidental to another, for we can say that an angel is "of" an intellectual nature. In this way, then, we say that the Son is begotten 'of' the essence of the Father, inasmuch as the essence of the Father, communicated by generation, subsists in the Son.
Reply to Objection 3. When we say that the Son is begotten of the essence of the Father, a term is added which saves the distinction. But when we say that the three persons are 'of' the divine essence, there is nothing expressed to warrant the distinction signified by the preposition, so there is no parity of argument.
Reply to Objection 4. When we say "Wisdom was created," this may be understood not of Wisdom which is the Son of God, but of created wisdom given by God to creatures: for it is said, "He created her [namely, Wisdom] in the Holy Ghost, and He poured her out over all His works" (Sirach 1:9-10). Nor is it inconsistent for Scripture in one text to speak of the Wisdom begotten and wisdom created, for wisdom created is a kind of participation of the uncreated Wisdom. The saying may also be referred to the created nature assumed by the Son, so that the sense be, "From the beginning and before the world was I made"--that is, I was foreseen as united to the creature. Or the mention of wisdom as both created and begotten insinuates into our minds the mode of the divine generation; for in generation what is generated receives the nature of the generator and this pertains to perfection; whereas in creation the Creator is not changed, but the creature does not receive the Creator's nature. Thus the Son is called both created and begotten, in order that from the idea of creation the immutability of the Father may be understood, and from generation the unity of nature in the Father and the Son. In this way Hilary expounds the sense of this text of Scripture (De Synod.). The other passages quoted do not refer to the Holy Ghost, but to the created spirit, sometimes called wind, sometimes air, sometimes the breath of man, sometimes also the soul, or any other invisible substance.
Source: Summa Theologica (New Advent)