I answer that, The Son was not begotten from nothing, but from the Father's substance. For it was explained above (Question [27], Article [2]; Question [33], Articles [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 (Question [45], Article [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 Jn. 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 Jn. 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 Rm. 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]).