Summa Theologica, Part I (Prima Pars) From the Complete...

Obj. 2: Further, Plurality of absolute properties does not make a distinction of persons, either in God, or in ourselves. Much less therefore is this effected by a plurality of relations. But in God there is no plurality but of relations (Q. 28, A. 3). Therefore there cannot be several persons in God.

I answer that, It follows from what precedes that there are several persons in God. For it was shown above (Q. 29, A. 4) that this word "person" signifies in God a relation as subsisting in the divine nature. It was also established (Q. 28, A. 1) that there are several real relations in God; and hence it follows that there are also several realities subsistent in the divine nature; which means that there are several persons in God.

Reply Obj. 1: The definition of "person" includes "substance," not as meaning the essence, but the suppositum which is made clear by the addition of the term "individual." To signify the substance thus understood, the Greeks use the name "hypostasis." So, as we say, "Three persons," they say "Three hypostases." We are not, however, accustomed to say Three substances, lest we be understood to mean three essences or natures, by reason of the equivocal signification of the term.

Reply Obj. 2: The absolute properties in God, such as goodness and wisdom, are not mutually opposed; and hence, neither are they really distinguished from each other. Therefore, although they subsist, nevertheless they are not several subsistent realities—that is, several persons. But the absolute properties in creatures do not subsist, although they are really distinguished from each other, as whiteness and sweetness; on the other hand, the relative properties in God subsist, and are really distinguished from each other (Q. 28, A. 3). Hence the plurality of persons in God.

Reply Obj. 3: The supreme unity and simplicity of God exclude every kind of plurality of absolute things, but not plurality of relations. Because relations are predicated relatively, and thus the relations do not import composition in that of which they are predicated, as Boethius teaches in the same book.

Reply Obj. 4: Number is twofold, simple or absolute, as two and three and four; and number as existing in things numbered, as two men and two horses. 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. But if number be taken as it is in the things numbered, in that sense as existing in creatures, one is part of two, and two of three, as one man is part of two men, and two of three; but this does not apply to God, because the Father is of the same magnitude as the whole Trinity, as we shall show further on (Q. 42, AA. 1, 4). _______________________

Objection 1: It would seem that there are more than three persons in God. For the plurality of persons in God arises from the plurality of the relative properties as stated above (A. 1). But there are four relations in God as stated above (Q. 28, A. 4), paternity, filiation, common spiration, and procession. Therefore there are four persons in God.

Obj. 2: The nature of God does not differ from His will more than from His intellect. But in God, one person proceeds from the will, as love; and another proceeds from His nature, as Son. Therefore another proceeds from His intellect, as Word, besides the one Who proceeds from His nature, as Son; thus again it follows that there are not only three persons in God.

Obj. 3: Further, the more 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 an infinite number of ways. Therefore there are an infinite number of persons in God.

Obj. 4: Further, it is from the infinite goodness of the Father that He communicates Himself infinitely in the production of a divine person. But also in the Holy Ghost is infinite goodness. Therefore the Holy Ghost produces a divine person; and that person another; and so to infinity.

Obj. 5: Further, everything within a determinate number is measured, for number is a measure. But the divine persons are immense, as we say in the Creed of Athanasius: "The Father is immense, the Son is immense, the Holy Ghost is immense." Therefore the persons are not contained within the number three.

On the contrary, It is said: "There are three who bear witness in heaven, the Father, the Word, and the Holy Ghost" (1 John 5:7). To those who ask, "Three what?" we answer, with Augustine (De Trin. vii, 4), "Three persons." Therefore there are but three persons in God.

I answer that, As was explained above, there can be only three persons in God. For it was shown above that the several persons are the several subsisting relations really distinct from each other. But a real distinction between the divine relations can come only from relative opposition. Therefore two opposite relations must needs refer to two persons: and if any relations are not opposite they must needs belong to the same person. Since then paternity and filiation are opposite relations, they belong necessarily to two persons. Therefore the subsisting paternity is the person of the Father; and the subsisting filiation is the person of the Son. The other two relations are not opposed to each other; therefore these two cannot belong to one person: hence either one of them must belong to both of the aforesaid persons; or one must belong to one person, and the other to the other. Now, procession cannot belong to the Father and the Son, or to either of them; for thus it would follows that the procession of the intellect, which in God is generation, wherefrom paternity and filiation are derived, would issue from the procession of love, whence spiration and procession are derived, if the person generating and the person generated proceeded from the person spirating; and this is against what was laid down above (Q. 27, AA. 3, 4).

We must consequently 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.

Reply Obj. 1: Although there are four relations in God, one of them, spiration, is not separated from the person of the Father and of the Son, but belongs to both; thus, although it is a relation, it is not called a property, because it does not belong to only one person; nor is it a personal relation—i.e. constituting a person. The three relations—paternity, filiation, and procession—are called personal properties, constituting as it were the persons; for paternity is the person of the Father, filiation is the person of the Son, procession is the person of the Holy Ghost proceeding.

Reply Obj. 2: That which proceeds by way of intelligence, as word, proceeds according to similitude, as also that which proceeds by way of nature; thus, as above explained (Q. 27, A. 3), the procession of the divine Word is the very same as generation by way of nature. But love, as such, does not proceed as the 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 Obj. 3: As man is more perfect than other animals, he has more intrinsic operations than other animals, because his perfection is something composite. Hence the angels, who are more perfect and more simple, have fewer intrinsic operations than man, for they have no imagination, or feeling, or the like. In God there exists only one real operation—that is, His essence. How there are in Him two processions was above explained (Q. 27, AA. 1, 4).

Reply Obj. 4: This argument would prove if the Holy Ghost possessed another goodness apart from the goodness of the Father; for then if the Father produced a divine person by His goodness, the Holy Ghost also would do so. But the Father and the Holy Ghost have one and the same goodness. Nor is there any distinction between them except by the personal relations. So goodness belongs to the Holy Ghost, as derived from another; and it belongs to the Father, as the principle of its communication to another. The opposition of relation does not allow the relation of the Holy Ghost to be joined with the relation of principle of another divine person; because He Himself proceeds from the other persons who are in God.

Reply Obj. 5: A determinate number, if taken as a simple number, existing in the mind only, is measured by one. But when we speak of a number of things as applied to the persons in God, the notion of measure has no place, because the magnitude of the three persons is the same (Q. 42, AA. 1, 4), and the same is not measured by the same. _______________________

Objection 1: It would seem that the numeral terms denote something real in God. For the divine unity is the divine essence. But every number is unity repeated. Therefore every numeral term in God signifies the essence; and therefore it denotes something real in God.

Obj. 3: Further, if the numeral terms do not denote anything real in God, and are introduced simply in a negative and removing sense, as plurality is employed to remove unity, and unity to remove plurality; it follows that a vicious circle results, confusing the mind and obscuring the truth; and this ought not to be. Therefore it must be said that the numeral terms denote something real in God.

On the contrary, Hilary says (De Trin. iv): "If we admit companionship"—that is, plurality—"we exclude the idea of oneness and of solitude;" and Ambrose says (De Fide i): "When we say one God, unity excludes plurality of gods, and does not imply quantity in God." Hence we see that these terms are applied to God in order to remove something; and not to denote anything positive.