Thomas Aquinas

“ But it cannot be the former, as it is false to say, "God alone is Father," since man also is a father; nor, again, can it be applied as regards the latter, for, if this saying were true, "God alone creates," it would follow that the "Father alone creates," as whatever is said of God can be said of the Father; and it would be false, as the Son also creates. Therefore this expression "alone" cannot be joined to an essential term in God. ”

“ I answer that, This term "alone" can be taken as a categorematical term, or as a syncategorematical term. A categorematical term is one which ascribes absolutely its meaning to a given "suppositum"; as, for instance, "white" to man, as when we say a "white man." If the term "alone" is taken in this sense, it cannot in any way be joined to any term in God; for it would mean solitude in the term to which it is joined; and it would follow that God was solitary, against what is above stated (A [2] ) . A syncategorematical term imports the order of the predicate to the subject ”

“ I answer that, The name "Trinity" in God signifies the determinate number of persons. And so the plurality of persons in God requires that we should use the word trinity; because what is indeterminately signified by plurality, is signified by trinity in a determinate manner.
Reply to Objection 1: In its etymological sense, this word "Trinity" seems to signify the one essence of the three persons, according as trinity may mean trine-unity. But in the strict meaning of the term it rather signifies the number of persons of one essence ”

“ The neuter gender is formless; whereas the masculine is formed and distinct; and so is the feminine. So the common essence is properly and aptly expressed by the neuter gender, but by the masculine and feminine is expressed the determined subject in the common nature. ”

“ But because this diction "alone," properly speaking, refers to the subject, it tends to exclude another Person rather than other things. Hence such a way of speaking is not to be taken too literally, but it should be piously expounded, whenever we find it in an authentic work. ”