Thomas Aquinas

“ So, if number in God is taken absolutely or abstractedly, there is nothing to prevent whole and part from being in Him, and thus number in Him is only in our way of understanding; forasmuch as number regarded apart from things numbered exists only in the intellect. ”

“ We must frequently admit that spiration belongs to the person of the Father, and to the person of the Son, forasmuch as it has no relative opposition either to paternity or to filiation; and consequently that procession belongs to the other person who is called the person of the Holy Ghost, who proceeds by way of love, as above explained, Therefore only three persons exist in God, the Father, the Son, and the Holy Ghost. ”

“ Further, the mroe perfect a creature is, the more interior operations it has; as a man has understanding and will beyond other animals. But God infinitely excels every creature. Therefore in God not only is there a person proceeding from the will, and another from the intellect, but also in a infinite number of ways. Therefore there are an infinite number of persons in God. ”

“ Although person is incommunicable, yet the mode itself of incommunicable existence can be common to many.
Reply to Objection 3: Although this community is logical and not real, yet it does not follow that in God there is universal or particular, or genus, or species; both because neither in human affairs is the community of person the same as community of genus or species; and because the divine persons have one being; whereas genus and species and every other universal are predicated of many which differ in being. ”

“ I answer that, The very mode of expression itself shows that this term "person" is common to the three when we say "three persons"; for when we say "three men" we show that "man" is common to the three. ”

“ But love, as such, does not proceed as teh similitude of that whence it proceeds; although in God love is co-essential as being divine; and therefore the procession of love is not called generation in God.
Reply to Objection 3: As man is more perfect that other animals, he has more intrinsic operations than other animals, because his perfection is something composite. ”