"Holy Spirit," taken as a term, is the name used of a Divine Person, proceeding by Love; a mode of Procession which has no proper name; hence the Relations which it implies are unnamed also, but we express ourselves according to the custom of language and of Holy Scripture. And there appears to be a twofold suitability in doing so: first, from the very community of that which is called Holy Spirit, for the Father and the Son are likewise Spirit and Holy; and secondly, from the proper signification of the word, for spirit in corporeal things is seen to imply impulse and motion. It is, indeed, of the nature of love that it moves and impels the will of the lover to the loved; while holiness is a special attribute of things belonging to God; therefore, since a Divine Person proceeds by way of Love, where God is loved the Person proceeding is suitably called Holy Spirit.
And the Holy Spirit proceeds necessarily from the Son, from Whom He would not otherwise be personally distinguished; for the Divine Persons are not distinguished from one another by anything Absolute, but only by opposing Relations of Origin, according as they are the Principle or from the Principle. If, therefore, the Holy Spirit had not this Relation of Origin from the Son, the Son and the Holy Spirit would not be Personally distinguished, and there would not be Trinity of Persons. Thus the Son proceeds by way of Intellect, as Word; and the Holy Spirit by way of Will, as Love; and Love proceeds necessarily from the Word, for we do not love anything except it be apprehended by the mind. It is clear, for these reasons, that the Holy Ghost proceeds from the Son. Moreover, order requires it; for we never find several proceeding from one without order, except in the case of things which are materially different; in others there is always an order. Therefore, since the Son and the Holy Spirit both proceed from the Father, there must be some order, nor can any other be assigned than that of Nature; for as one proceeds from the other, the Holy Spirit must proceed from the Son, none having ever been called the Son of the Holy Ghost.
Moreover, the Holy Ghost proceeds from the Father through the Son; for that which distinguishes the Son from the Father is His Procession; and as the word "through" denotes a medium deriving from its principle the faculty of producing, it may be said that the Father breathes the Holy Spirit through the Son. Thus the Father and the Son are One Principle (or Source) of the Holy Spirit, as they are One in everything except where distinguished from each other by relative opposition. In this they are not relatively opposed; therefore, as the Father and the Son are One in virtue of the Unity signified by the name God, so they are One Source of the Holy Spirit in virtue of the Unity expressed by the word Principle.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni