Thomas Aquinas

“ In another sense, privation is so called when something has not what naturally belongs to some members of its genus; as for instance when a mole is called blind. In a third sense privation means the absence of what something ought to have; in which sense, privation imports an imperfection. In this sense, "unbegotten" is not attributed to the Father as a privation, but it may be so attributed in the second sense, meaning that a certain person of the divine nature is not begotten, while some person of the same nature is begotten. ”

“ Now generation signifies something in process of being made, whereas paternity signifies the complement of generation; and therefore the name "Father" is more expressive as regards the divine person than genitor or begettor.
Reply to Objection 3: In human nature the word is not a subsistence, and hence is not properly called begotten or son. But the divine Word is something subsistent in the divine nature; and hence He is properly and not metaphorically called Son, and His principle is called Father. ”

“ Lastly, He is the Father of others by similitude of glory, forasmuch as they have obtained possession of the heritage of glory, according to Rom. 5:2: "We glory in the hope of the glory of the sons of God." Therefore it is plain that "paternity" is applied to God first, as importing regard of one Person to another Person, before it imports the regard of God to creatures. ”

“ Further, a principle is so called in relation to the thing principled. So if the Father is the principle of the Son, it follows that the Son is a person principled, and is therefore created; which appears false. ”

“ Among us relation is not a subsisting person. So this name "father" among us does not signify a person, but the relation of a person. In God, however, it is not so, as some wrongly thought; for in God the relation signified by the name "Father" is a subsisting person. ”

“ Hence Hilary says (De Synod.) : "As there is one God, so there cannot be two innascibles." And this especially because, did two innascibles exist, one would not be from the other, and they would not be distinguished by relative opposition: therefore they would be distinguished from each other by diversity of nature. ”

“ Hence, it being denied of the Father that He is begotten, although He is the principle of generation, it follows, as a consequence, that He does not proceed by the procession of the Holy Ghost, because the Holy Ghost is not the principle of generation, but proceeds from the person begotten. ”

“ This, however, does not seem true, because thus innascibility would not be a property distinct from paternity and spiration; but would include them as the proper is included in the common. For source and authority signify in God nothing but the principle of origin. We must therefore say with Augustine (De Trin. v, 7) that "unbegotten" imports the negation of passive generation. For he says that "unbegotten" has the same meaning as "not a son." Nor does it follow that "unbegotten" is not the proper notion of the Father; for primary and simple things are notified by negations ”

“ But in the creature, filiation is found in relation to God, not in a perfect manner, since the Creator and the creature have not the same nature; but by way of a certain likeness, which is the more perfect the nearer we approach to the true idea of filiation. For God is called the Father of some creatures, by reason only of a trace, for instance of irrational creatures, according to Job 38:28: "Who is the father of the rain? ”