1. The three divine Persons, agreeing in essence, can be distinguished only by the relation of origin. These relations of origin cannot obtain in respect of any process tending to things without, as whatever proceeded without would not be co-essential with its origin; but the process must all stay within. Now such a process, abiding within its origin, is found only in the act of understanding and will.* Hence the divine persons cannot be multiplied except in accordance with the requirements of the process of understanding and will in God. But in God there can be but one process of understanding, seeing that His act of understanding is one, simple, and perfect, whereby, understanding Himself, He understands all other things; and so there can be in God only one procession of the Word. In like manner the process of love must be one and simple, because the divine will also is one and simple, whereby in loving Himself God loves all other things. There can therefore be in God but two Persons proceeding: one by way of understanding, as the Word, or Son; the other by way of love, as the Holy Ghost: there is also one Person not proceeding, namely, the Father. There can only therefore be three Persons in the Trinity.
2. The divine Persons must be distinguished according to their mode of procession. Now the mode of personal procession can be but threefold. There may be a mode of not proceeding at all, which is proper to the Father; or of proceeding from one who does not proceed, which is proper to the Son; or of proceeding from one who does proceed, which is proper to the Holy Ghost. It is impossible therefore to assign more than three Persons.
3. If any objicient says that, the Son being perfect God, there is in Him perfect intellectual power, whereby He can produce a Word; and in like manner the Holy Ghost, being infinite goodness, which is a principle of communication,* must be able to communicate the divine nature to another divine person, he should take note that the Son is God as begotten, not as begetting; hence the power of understanding is in Him as in one proceeding as a Word, not as in one producing a Word. In like manner the Holy Ghost being God as proceeding, there is in Him infinite goodness as in a person receiving, not as in one communicating infinite goodness to another. The whole fulness of Godhead then is in the Son, numerically the same as in the Father, but with a relation of birth, as it is in the Father with a relation of active generation. If the relation of the Father were attributed to the Son, all distinction would be taken away: for the divine Persons are distinguished one from another solely by their mutual relations. And the like argument holds of the Holy Ghost.
A likeness of the divine Trinity is observable in the human mind. That mind, by actually understanding itself, conceives its 'word' in itself, which 'word' is nothing else than what is called the 'intellectual expression (intentio intellecta, cf. B. I, Chap. LIII) existing in the mind; which mind, going on to love itself, produces itself in the will as an object loved. Further it does not proceed, but is confined and complete in a circle, returning by love to its own substance, whence the process originally began by formation of the 'intellectual expression' of that substance. There is however a process going out to exterior effects, as the mind for love of itself proceeds to some action beyond itself. Thus we remark in the mind three things: the mind itself, whence the process starts within its own nature; the mind conceived in the understanding; and the mind loved in the will. And so we have seen that there is in the divine nature a God unbegotten, the Father, the origin of the entire procession of Deity; and a God begotten after the manner of a 'word' conceived in the understanding, namely, the Son; and a God proceeding by mode of love, who is the Holy Ghost: beyond Him there is no further procession within the divine nature, but only a proceeding to exterior effects. But the representation of the divine Trinity in us falls short, in regard of Father, Son and Holy Ghost being one nature, and each of them a perfect Person.* Hence there is said to be in the mind of man the 'image' of God: Let us make man to our image and likeness (Gen. i, 26). But as for the irrational creation, on account of the remoteness and obscurity of the representation as found in them, there is said to be the 'foot-print' of the Trinity, but not the 'image' (vestigium, non imago).
4.24 : That the Holy Ghost Proceeds from the Son
4.27 : Of the Incarnation of the Word according to the Tradition of Holy Scripture