Thomas Aquinas

“ For we nowhere find that several things proceed from one without order except in those which differ only by their matter; as for instance one smith produces many knives distinct from each other materially, with no order to each other; whereas in things in which there is not only a material distinction we always find that some order exists in the multitude produced. Hence also in the order of creatures produced, the beauty of the divine wisdom is displayed. So if from the one Person of the Father, two persons proceed, the Son and the Holy Ghost, there must be some order between them. ”

“ It would seem that the Father and the Son are not one principle of the Holy Ghost. For the Holy Ghost does not proceed from the Father and the Son as they are one; not as they are one in nature, for the Holy Ghost would in that way proceed from Himself, as He is one in nature with Them; nor again inasmuch as they are united in any one property, for it is clear that one property cannot belong to two subjects. Therefore the Holy Ghost proceeds from the Father and the Son as distinct from one another. Therefore the Father and the Son are not one principle of the Holy Ghost. ”

“ For the Father also is a spirit, and the Son is a spirit; and the Father is holy, and the Son is holy." Secondly, from the proper signification of the name. For the name spirit in things corporeal seems to signify impulse and motion; for we call the breath and the wind by the term spirit. Now it is a property of love to move and impel the will of the lover towards the object loved. Further, holiness is attributed to whatever is ordered to God. Therefore because the divine person proceeds by way of the love whereby God is loved, that person is most properly named "The Holy Ghost." ”