Summa Theologica, by St. Thomas Aquinas, [1947], at sacred-texts.com
We now consider the persons singly; and first, the Person of the Father, concerning Whom there are four points of inquiry:
(1) Whether the Father is the Principle?
(2) Whether the person of the Father is properly signified by this name "Father"?
(3) Whether "Father" in God is said personally before it is said essentially?
(4) Whether it belongs to the Father alone to be unbegotten?
Objection 1: It would seem that the Father cannot be called the principle of the Son, or of the Holy Ghost. For principle and cause are the same, according to the Philosopher (Metaph. iv). But we do not say that the Father is the cause of the Son. Therefore we must not say that He is the principle of the Son.
Objection 2: 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.
Objection 3: Further, the word principle is taken from priority. But in God there is no "before" and "after," as Athanasius says. Therefore in speaking of God we ought not to used the term principle.
On the contrary, Augustine says (De Trin. iv, 20), "The Father is the Principle of the whole Deity."
I answer that, The word "principle" signifies only that whence another proceeds: since anything whence something proceeds in any way we call a principle; and conversely. As the Father then is the one whence another proceeds, it follows that the Father is a principle.
Reply to Objection 1: The Greeks use the words "cause" and "principle" indifferently, when speaking of God; whereas the Latin Doctors do not use the word "cause," but only "principle." The reason is because "principle" is a wider term than "cause"; as "cause" is more common than "element." For the first term of a thing, as also the first part, is called the principle, but not the cause. Now the wider a term is, the more suitable it is to use as regards God (Q[13], A[11]), because the more special terms are, the more they determine the mode adapted to the creature. Hence this term "cause" seems to mean diversity of substance, and dependence of one from another; which is not implied in the word "principle." For in all kinds of causes there is always to be found between the cause and the effect a distance of perfection or of power: whereas we use the term "principle" even in things which have no such difference, but have only a certain order to each other; as when we say that a point is the principle of a line; or also when we say that the first part of a line is the principle of a line.
Reply to Objection 2: It is the custom with the Greeks to say that the Son and the Holy Ghost are principled. This is not, however, the custom with our Doctors; because, although we attribute to the Father something of authority by reason of His being the principle, still we do not attribute any kind of subjection or inferiority to the Son, or to the Holy Ghost, to avoid any occasion of error. In this way, Hilary says (De Trin. ix): "By authority of the Giver, the Father is the greater; nevertheless the Son is not less to Whom oneness of nature is give."
Reply to Objection 3: Although this word principle, as regards its derivation, seems to be taken from priority, still it does not signify priority, but origin. For what a term signifies, and the reason why it was imposed, are not the same thing, as stated above (Q[13], A[8]).
Objection 1: It would seem that this name "Father" is not properly the name of a divine person. For the name "Father" signifies relation. Moreover "person" is an individual substance. Therefore this name "Father" is not properly a name signifying a Person.
Objection 2: Further, a begetter is more common than father; for every father begets; but it is not so conversely. But a more common term is more properly applied to God, as stated above (Q[13], A[11]). Therefore the more proper name of the divine person is begetter and genitor than Father.
Objection 3: Further, a metaphorical term cannot be the proper name of anyone. But the word is by us metaphorically called begotten, or offspring; and consequently, he of whom is the word, is metaphorically called father. Therefore the principle of the Word in God is not properly called Father.
Objection 4: Further, everything which is said properly of God, is said of God first before creatures. But generation appears to apply to creatures before God; because generation seems to be truer when the one who proceeds is distinct from the one whence it proceeds, not only by relation but also by essence. Therefore the name "Father" taken from generation does not seem to be the proper name of any divine person.
On the contrary, It is said (Ps. 88:27): "He shall cry out to me: Thou art my Father."
I answer that, The proper name of any person signifies that whereby the person is distinguished from all other persons. For as body and soul belong to the nature of man, so to the concept of this particular man belong this particular soul and this particular body; and by these is this particular man distinguished from all other men. Now it is paternity which distinguishes the person of the Father from all other persons. Hence this name "Father," whereby paternity is signified, is the proper name of the person of the Father.
Reply to Objection 1: 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, as above explained (Q[29], A[4]), this name "person" in God signifies a relation subsisting in the divine nature.
Reply to Objection 2: According to the Philosopher (De Anima ii, text 49), a thing is denominated chiefly by its perfection, and by its end. 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.
Reply to Objection 4: The terms "generation" and "paternity" like the other terms properly applied to God, are said of God before creatures as regards the thing signified, but not as regards the mode of signification. Hence also the Apostle says, "I bend my knee to the Father of my Lord Jesus Christ, from whom all paternity in heaven and on earth is named" (Eph. 3:14). This is explained thus. It is manifest that generation receives its species from the term which is the form of the thing generated; and the nearer it is to the form of the generator, the truer and more perfect is the generation; as univocal generation is more perfect than non-univocal, for it belongs to the essence of a generator to generate what is like itself in form. Hence the very fact that in the divine generation the form of the Begetter and Begotten is numerically the same, whereas in creatures it is not numerically, but only specifically, the same, shows that generation, and consequently paternity, is applied to God before creatures. Hence the very fact that in God a distinction exists of the Begotten from the Begetter as regards relation only, belongs to the truth of the divine generation and paternity.
Objection 1: It would seem that this name "Father" is not applied to God, firstly as a personal name. For in the intellect the common precedes the particular. But this name "Father" as a personal name, belongs to the person of the Father; and taken in an essential sense it is common to the whole Trinity; for we say "Our Father" to the whole Trinity. Therefore "Father" comes first as an essential name before its personal sense.
Objection 2: Further, in things of which the concept is the same there is no priority of predication. But paternity and filiation seem to be of the same nature, according as a divine person is Father of the Son, and the whole Trinity is our Father, or the creature's; since, according to Basil (Hom. xv, De Fide), to receive is common to the creature and to the Son. Therefore "Father" in God is not taken as an essential name before it is taken personally.
Objection 3: Further, it is not possible to compare things which have not a common concept. But the Son is compared to the creature by reason of filiation or generation, according to Col. 1:15: "Who is the image of the invisible God, the first-born of every creature." Therefore paternity taken in a personal sense is not prior to, but has the same concept as, paternity taken essentially.
On the contrary, The eternal comes before the temporal. But God is the Father of the Son from eternity; while He is the Father of the creature in time. Therefore paternity in God is taken in a personal sense as regards the Son, before it is so taken as regards the creature.
I answer that, A name is applied to that wherein is perfectly contained its whole signification, before it is applied to that which only partially contains it; for the latter bears the name by reason of a kind of similitude to that which answers perfectly to the signification of the name; since all imperfect things are taken from perfect things. Hence this name "lion" is applied first to the animal containing the whole nature of a lion, and which is properly so called, before it is applied to a man who shows something of a lion's nature, as courage, or strength, or the like; and of whom it is said by way of similitude.
Now it is manifest from the foregoing (Q[27], A[2]; Q[28], A[4]), that the perfect idea of paternity and filiation is to be found in God the Father, and in God the Son, because one is the nature and glory of the Father and the Son. 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? or who begot the drops of dew?" Of some, namely, the rational creature (He is the Father), by reason of the likeness of His image, according to Dt. 32:6: "Is He not thy Father, who possessed, and made, and created thee?" And of others He is the Father by similitude of grace, and these are also called adoptive sons, as ordained to the heritage of eternal glory by the gift of grace which they have received, according toRom. 8:16, 17: "The Spirit Himself gives testimony to our spirit that we are the sons of God; and if sons, heirs also." 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.
Reply to Objection 1: Common terms taken absolutely, in the order of our intelligence, come before proper terms; because they are included in the understanding of proper terms; but not conversely. For in the concept of the person of the Father, God is understood; but not conversely. But common terms which import relation to the creature come after proper terms which import personal relations; because the person proceeding in God proceeds as the principle of the production of creatures. For as the word conceived in the mind of the artist is first understood to proceed from the artist before the thing designed, which is produced in likeness to the word conceived in the artist's mind; so the Son proceeds from the Father before the creature, to which the name of filiation is applied as it participates in the likeness of the Son, as is clear from the words of Rom. 8:29: "Whom He foreknew and predestined to be made conformable to the image of His Son."
Reply to Objection 2: To "receive" is said to be common to the creature and to the Son not in a univocal sense, but according to a certain remote similitude whereby He is called the First Born of creatures. Hence the authority quoted subjoins: "That He may be the First Born among many brethren," after saying that some were conformed to the image of the Son of God. But the Son of God possesses a position of singularity above others, in having by nature what He receives, as Basil also declares (Hom. xv De Fide); hence He is called the only begotten (Jn. 1:18): "The only begotten Who is in the bosom of the Father, He hath declared unto us."
From this appears the Reply to the Third Objection.
Objection 1: It would seem that it is not proper to the Father to be unbegotten. For every property supposes something in that of which it is the property. But "unbegotten" supposes nothing in the Father; it only removes something. Therefore it does not signify a property of the Father.
Objection 2: Further, Unbegotten is taken either in a privative, or in a negative sense. If in a negative sense, then whatever is not begotten can be called unbegotten. But the Holy Ghost is not begotten; neither is the divine essence. Therefore to be unbegotten belongs also to the essence; thus it is not proper to the Father. But if it be taken in a privative sense, as every privation signifies imperfection in the thing which is the subject of privation, it follows that the Person of the Father is imperfect; which cannot be.
Objection 3: Further, in God, "unbegotten" does not signify relation, for it is not used relatively. Therefore it signifies substance; therefore unbegotten and begotten differ in substance. But the Son, Who is begotten, does not differ from the Father in substance. Therefore the Father ought not to be called unbegotten.
Objection 4: Further, property means what belongs to one alone. Since, then, there are more than one in God proceeding from another, there is nothing to prevent several not receiving their being from another. Therefore the Father is not alone unbegotten.
Objection 5: Further, as the Father is the principle of the person begotten, so is He of the person proceeding. So if by reason of his opposition to the person begotten, it is proper to the Father to be unbegotten it follows that it is proper to Him also to be unproceeding.
On the contrary, Hilary says (De Trin. iv): "One is from one ---that is, the Begotten is from the Unbegotten---namely, by the property in each one respectively of innascibility and origin."
I answer that, As in creatures there exist a first and a secondary principle, so also in the divine Persons, in Whom there is no before or after, is formed the principle not from a principle, Who is the Father; and the principle from a principle, Who is the Son.
Now in things created a first principle is known in two ways; in one way as the first "principle," by reason of its having a relation to what proceeds from itself; in another way, inasmuch as it is a "first" principle by reason of its not being from another. Thus therefore the Father is known both by paternity and by common spiration, as regards the persons proceeding from Himself. But as the principle, not from a principle He is known by the fact that He is not from another; and this belongs to the property of innascibility, signified by this word "begotten."
Reply to Objection 1: Some there are who say that innascibility, signified by the word "unbegotten," as a property of the Father, is not a negative term only, but either that it means both these things together---namely, that the Father is from no one, and that He is the principle of others; or that it imports universal authority, or also His plenitude as the source of all. 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; as, for instance, a point is defined as what has no part.
Reply to Objection 2: "Unbegotten" is taken sometimes in a negative sense only, and in that sense Jerome says that "the Holy Ghost is unbegotten," that is, He is not begotten. Otherwise "unbegotten" may be taken in a kind of privation sense, but not as implying any imperfection. For privation can be taken in many ways; in one way when a thing has not what is naturally belongs to another, even though it is not of its own nature to have it; as, for instance, if a stone be called a dead thing, as wanting life, which naturally belongs to some other things. 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. In this sense the term "unbegotten" can be applied also to the Holy Ghost. Hence to consider it as a term proper to the Father alone, it must be further understood that the name "unbegotten" belongs to a divine person as the principle of another person; so that it be understood to imply negation in the genus of principle taken personally in God. Or that there be understood in the term "unbegotten" that He is not in any way derived from another; and not only that He is not from another by way only of generation. In this sense the term "unbegotten" does not belong at all to the Holy Ghost, Who is from another by procession, as a subsisting person; nor does it belong to the divine essence, of which it may be said that it is in the Son or in the Holy Ghost from another---namely, from the Father.
Reply to Objection 3: According to Damascene (De Fide Orth. ii, 9), "unbegotten" in one sense signifies the same as "uncreated"; and thus it applies to the substance, for thereby does the created substance differ from the uncreated. In another sense it signifies what is not begotten, and in this sense it is a relative term; just as negation is reduced to the genus of affirmation, as "not man" is reduced to the genus of substance, and "not white" to the genus of quality. Hence, since "begotten" implies relation in God, "unbegotten" belongs also to relation. Thus it does not follow that the Father unbegotten is substantially distinguished from the Son begotten; but only by relation; that is, as the relation of Son is denied of the Father.
Reply to Objection 4: In every genus there must be something first; so in the divine nature there must be some one principle which is not from another, and which we call "unbegotten." To admit two innascibles is to suppose the existence of two Gods, and two divine natures. 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.
Reply to Objection 5: The property of the Father, whereby He is not from another, is more clearly signified by the removal of the nativity of the Son, than by the removal of the procession of the Holy Ghost; both because the procession of the Holy Ghost has no special name, as stated above (Q[27], A[4], ad 3), and because also in the order of nature it presupposes the generation of the Son. 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.