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 Obj. 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 Obj. 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 Obj. 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). _______________________
SECOND ARTICLE [I, Q. 33, Art. 2]
Whether This Name "Father" Is Properly the Name of a Divine Person?
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.
Obj. 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.
Obj. 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.