Summa Th. I EN Qu.28 a.4
Having premised what have appeared necessary notions concerning the processions and the relations, we must now approach the subject of the persons.
First, we shall consider the persons absolutely, and then comparatively as regards each other. We must consider the persons absolutely first in common; and then singly.
The general consideration of the persons seemingly involves four points: (1) The signification of this word "person"; (2) the number of the persons; (3) what is involved in the number of persons, or is opposed thereto; as diversity, and similitude, and the like; and (4) what belongs to our knowledge of the persons.
Four subjects of inquiry are comprised in the first point:(1) The definition of "person."
(2) The comparison of person to essence, subsistence, and hypostasis.
(3) Whether the name of person is becoming to God?
(4) What does it signify in Him?
Objection: 1. It would seem that the definition of person given by Boethius (De Duab. Nat.) is insufficient---that is, "a person is an individual substance of a rational nature." For nothing singular can be subject to definition. But "person" signifies something singular. Therefore person is improperly defined.
2. Further, substance as placed above in the definition of person, is either first substance, or second substance. If it is the former, the word "individual" is superfluous, because first substance is individual substance; if it stands for second substance, the word "individual" is false, for there is contradiction of terms; since second substances are the "genera" or "species." Therefore this definition is incorrect.
3. Further, an intentional term must not be included in the definition of a thing. For to define a man as "a species of animal" would not be a correct definition; since man is the name of a thing, and "species" is a name of an intention. Therefore, since person is the name of a thing (for it signifies a substance of a rational nature), the word "individual" which is an intentional name comes improperly into the definition.
4. Further, "Nature is the principle of motion and rest, in those things in which it is essentially, and not accidentally," as Aristotle says (Phys. ii). But person exists in things immovable, as in God, and in the angels. Therefore the word "nature" ought not to enter into the definition of person, but the word should rather be "essence."
5. Further, the separated soul is an individual substance of the rational nature; but it is not a person. Therefore person is not properly defined as above.
I answer that Although the universal and particular exist in every genus, nevertheless, in a certain special way, the individual belongs to the genus of substance. For substance is individualized by itself; whereas the accidents are individualized by the subject, which is the substance; since this particular whiteness is called "this," because it exists in this particular subject. And so it is reasonable that the individuals of the genus substance should have a special name of their own; for they are called "hypostases," or first substances.
Further still, in a more special and perfect way, the particular and the individual are found in the rational substances which have dominion over their own actions; and which are not only made to act, like others; but which can act of themselves; for actions belong to singulars. Therefore also the individuals of the rational nature have a special name even among other substances; and this name is "person."
Thus the term "individual substance" is placed in the definition of person, as signifying the singular in the genus of substance; and the term "rational nature" is added, as signifying the singular in rational substances.
Reply to Objection: 1. Although this or that singular may not be definable, yet what belongs to the general idea of singularity can be defined; and so the Philosopher (De Praedic., cap. De substantia) gives a definition of first substance; and in this way Boethius defines person.
2. In the opinion of some, the term "substance" in the definition of person stands for first substance, which is the hypostasis; nor is the term "individual" superfluously added, forasmuch as by the name of hypostasis or first substance the idea of universality and of part is excluded. For we do not say that man in general is an hypostasis, nor that the hand is since it is only a part. But where "individual" is added, the idea of assumptibility is excluded from person; for the human nature in Christ is not a person, since it is assumed by a greater---that is, by the Word of God. It is, however, better to say that substance is here taken in a general sense, as divided into first and second, and when "individual" is added, it is restricted to first substance.
3. Substantial differences being unknown to us, or at least unnamed by us, it is sometimes necessary to use accidental differences in the place of substantial; as, for example, we may say that fire is a simple, hot, and dry body: for proper accidents are the effects of substantial forms, and make them known. Likewise, terms expressive of intention can be used in defining realities if used to signify things which are unnamed. And so the term "individual" is placed in the definition of person to signify the mode of subsistence which belongs to particular substances.
4. According to the Philosopher (Metaph. v, 5), the word "nature" was first used to signify the generation of living things, which is called nativity. And because this kind of generation comes from an intrinsic principle, this term is extended to signify the intrinsic principle of any kind of movement. In this sense he defines "nature" (Phys. ii, 3). And since this kind of principle is either formal or material, both matter and form are commonly called nature. And as the essence of anything is completed by the form; so the essence of anything, signified by the definition, is commonly called nature. And here nature is taken in that sense. Hence Boethius says (De Duab. Nat.) that, "nature is the specific difference giving its form to each thing," for the specific difference completes the definition, and is derived from the special form of a thing. So in the definition of "person," which means the singular in a determined "genus," it is more correct to use the term "nature" than "essence," because the latter is taken from being, which is most common.
5. The soul is a part of the human species; and so, although it may exist in a separate state, yet since it ever retains its nature of unibility, it cannot be called an individual substance, which is the hypostasis or first substance, as neither can the hand nor any other part of man; thus neither the definition nor the name of person belongs to it.
Objection: 1. It would seem that "person" is the same as "hypostasis," "subsistence," and "essence." For Boethius says (De Duab. Nat.) that "the Greeks called the individual substance of the rational nature by the name hypostasis." But this with us signifies "person." Therefore "person" is altogether the same as "hypostasis."
2. Further, as we say there are three persons in God, so we say there are three subsistences in God; which implies that "person" and "subsistence" have the same meaning. Therefore "person" and "subsistence" mean the same.
3. Further, Boethius says (Com. Praed.) that the Greek (ousia), which means essence, signifies a being composed of matter and form. Now that which is composed of matter and form is the individual substance called "hypostasis" and "person." Therefore all the aforesaid names seem to have the same meaning.
4. On the contrary, Boethius says (De Duab. Nat.) that genera and species only subsist; whereas individuals are not only subsistent, but also substand. But subsistences are so called from subsisting, as substance or hypostasis is so called from substanding. Therefore, since genera and species are not hypostases or persons, these are not the same as subsistences.
5. Further, Boethius says (Com. Praed.) that matter is called hypostasis, and form is called (ousiosis)---that is, subsistence. But neither form nor matter can be called person. Therefore person differs from the others.
I answer that According to the Philosopher (Metaph. v), substance is twofold. In one sense it means the quiddity of a thing, signified by its definition, and thus we say that the definition means the substance of a thing; in which sense substance is called by the Greeks (ousia), what we may call "essence." In another sense substance means a subject or "suppositum," which subsists in the genus of substance. To this, taken in a general sense, can be applied a name expressive of an intention; and thus it is called "suppositum." It is also called by three names signifying a reality---that is, "a thing of nature," "subsistence," and "hypostasis," according to a threefold consideration of the substance thus named. For, as it exists in itself and not in another, it is called "subsistence"; as we say that those things subsist which exist in themselves, and not in another. As it underlies some common nature, it is called "a thing of nature"; as, for instance, this particular man is a human natural thing. As it underlies the accidents, it is called "hypostasis," or "substance." What these three names signify in common to the whole genus of substances, this name "person" signifies in the genus of rational substances.
Reply to Objection: 1. Among the Greeks the term "hypostasis," taken in the strict interpretation of the word, signifies any individual of the genus substance; but in the usual way of speaking, it means the individual of the rational nature, by reason of the excellence of that nature.
2. As we say "three persons" plurally in God, and "three subsistences," so the Greeks say "three hypostases." But because the word "substance," which, properly speaking, corresponds in meaning to "hypostasis," is used among us in an equivocal sense, since it sometimes means essence, and sometimes means hypostasis, in order to avoid any occasion of error, it was thought preferable to use "subsistence" for hypostasis, rather than "substance."
3. Strictly speaking, the essence is what is expressed by the definition. Now, the definition comprises the principles of the species, but not the individual principles. Hence in things composed of matter and form, the essence signifies not only the form, nor only the matter, but what is composed of matter and the common form, as the principles of the species. But what is composed of this matter and this form has the nature of hypostasis and person. For soul, flesh, and bone belong to the nature of man; whereas this soul, this flesh and this bone belong to the nature of this man. Therefore hypostasis and person add the individual principles to the idea of essence; nor are these identified with the essence in things composed of matter and form, as we said above when treating of divine simplicity (Question , Article ).
4. Boethius says that genera and species subsist, inasmuch as it belongs to some individual things to subsist, from the fact that they belong to genera and species comprised in the predicament of substance, but not because the species and genera themselves subsist; except in the opinion of Plato, who asserted that the species of things subsisted separately from singular things. To substand, however, belongs to the same individual things in relation to the accidents, which are outside the essence of genera and species.
5. The individual composed of matter and form substands in relation to accident from the very nature of matter. Hence Boethius says (De Trin.): "A simple form cannot be a subject." Its self-subsistence is derived from the nature of its form, which does not supervene to the things subsisting, but gives actual existence to the matter and makes it subsist as an individual. On this account, therefore, he ascribes hypostasis to matter, and (ousiosis), or subsistence, to the form, because the matter is the principle of substanding, and form is the principle of subsisting.
Objection: 1. It would seem that the name "person" should not be said of God. For Dionysius says (Div. Nom.): "No one should ever dare to say or think anything of the supersubstantial and hidden Divinity, beyond what has been divinely expressed to us by the oracles." But the name "person" is not expressed to us in the Old or New Testament. Therefore "person" is not to be applied to God.
2. Further, Boethius says (De Duab. Nat.): "The word person seems to be taken from those persons who represented men in comedies and tragedies. For person comes from sounding through [personando], since a greater volume of sound is produced through the cavity in the mask. These "persons" or masks the Greeks called (prosopa), as they were placed on the face and covered the features before the eyes." This, however, can apply to God only in a metaphorical sense. Therefore the word "person" is only applied to God metaphorically.
3. Further, every person is a hypostasis. But the word "hypostasis" does not apply to God, since, as Boethius says (De Duab. Nat.), it signifies what is the subject of accidents, which do not exist in God. Jerome also says (Ep ad Damas.) that, "in this word hypostasis, poison lurks in honey." Therefore the word "person" should not be said of God.
4. Further, if a definition is denied of anything, the thing defined is also denied of it. But the definition of "person," as given above, does not apply to God. Both because reason implies a discursive knowledge, which does not apply to God, as we proved above (Question , Article ); and thus God cannot be said to have "a rational nature." And also because God cannot be called an individual substance, since the principle of individuation is matter; while God is immaterial: nor is He the subject of accidents, so as to be called a substance. Therefore the word "person" ought not to be attributed to God.
On the contrary In the Creed of Athanasius we say: "One is the person of the Father, another of the Son, another of the Holy Ghost."
I answer that "Person" signifies what is most perfect in all nature---that is, a subsistent individual of a rational nature. Hence, since everything that is perfect must be attributed to God, forasmuch as His essence contains every perfection, this name "person" is fittingly applied to God; not, however, as it is applied to creatures, but in a more excellent way; as other names also, which, while giving them to creatures, we attribute to God; as we showed above when treating of the names of God (Question , Article ).
Reply to Objection: 1. Although the word "person" is not found applied to God in Scripture, either in the Old or New Testament, nevertheless what the word signifies is found to be affirmed of God in many places of Scripture; as that He is the supreme self-subsisting being, and the most perfectly intelligent being. If we could speak of God only in the very terms themselves of Scripture, it would follow that no one could speak about God in any but the original language of the Old or New Testament. The urgency of confuting heretics made it necessary to find new words to express the ancient faith about God. Nor is such a kind of novelty to be shunned; since it is by no means profane, for it does not lead us astray from the sense of Scripture. The Apostle warns us to avoid "profane novelties of words" (1Tm 6,20).
2. Although this name "person" may not belong to God as regards the origin of the term, nevertheless it excellently belongs to God in its objective meaning. For as famous men were represented in comedies and tragedies, the name "person" was given to signify those who held high dignity. Hence, those who held high rank in the Church came to be called "persons." Thence by some the definition of person is given as "hypostasis distinct by reason of dignity." And because subsistence in a rational nature is of high dignity, therefore every individual of the rational nature is called a "person." Now the dignity of the divine nature excels every other dignity; and thus the name "person" pre-eminently belongs to God.
3. The word "hypostasis" does not apply to God as regards its source of origin, since He does not underlie accidents; but it applies to Him in its objective sense, for it is imposed to signify the subsistence. Jerome said that "poison lurks in this word," forasmuch as before it was fully understood by the Latins, the heretics used this term to deceive the simple, to make people profess many essences as they profess several hypostases, inasmuch as the word "substance," which corresponds to hypostasis in Greek, is commonly taken amongst us to mean essence.
4. It may be said that God has a rational "nature," if reason be taken to mean, not discursive thought, but in a general sense, an intelligent nature. But God cannot be called an "individual" in the sense that His individuality comes from matter; but only in the sense which implies incommunicability. "Substance" can be applied to God in the sense of signifying self-subsistence. There are some, however, who say that the definition of Boethius, quoted above (Article ), is not a definition of person in the sense we use when speaking of persons in God. Therefore Richard of St. Victor amends this definition by adding that "Person" in God is "the incommunicable existence of the divine nature."
Objection: 1. It would seem that this word "person," as applied to God, does not signify relation, but substance. For Augustine says (De Trin. vii, 6): "When we speak of the person of the Father, we mean nothing else but the substance of the Father, for person is said in regard to Himself, and not in regard to the Son."
2. Further, the interrogation "What?" refers to essence. But, as Augustine says: "When we say there are three who bear witness in heaven, the Father, the Word, and the Holy Ghost, and it is asked, Three what? the answer is, Three persons." Therefore person signifies essence.
3. According to the Philosopher (Metaph. iv), the meaning of a word is its definition. But the definition of "person" is this: "The individual substance of the rational nature," as above stated. Therefore "person" signifies substance.
4. Further, person in men and angels does not signify relation, but something absolute. Therefore, if in God it signified relation, it would bear an equivocal meaning in God, in man, and in angels.
On the contrary Boethius says (De Trin.) that "every word that refers to the persons signifies relation." But no word belongs to person more strictly than the very word "person" itself. Therefore this word "person" signifies relation.
I answer that A difficulty arises concerning the meaning of this word "person" in God, from the fact that it is predicated plurally of the Three in contrast to the nature of the names belonging to the essence; nor does it in itself refer to another, as do the words which express relation.
Hence some have thought that this word "person" of itself expresses absolutely the divine essence; as this name "God" and this word "Wise"; but that to meet heretical attack, it was ordained by conciliar decree that it was to be taken in a relative sense, and especially in the plural, or with the addition of a distinguishing adjective; as when we say, "Three persons," or, "one is the person of the Father, another of the Son," etc. Used, however, in the singular, it may be either absolute or relative. But this does not seem to be a satisfactory explanation; for, if this word "person," by force of its own signification, expresses the divine essence only, it follows that forasmuch as we speak of "three persons," so far from the heretics being silenced, they had still more reason to argue. Seeing this, others maintained that this word "person" in God signifies both the essence and the relation. Some of these said that it signifies directly the essence, and relation indirectly, forasmuch as "person" means as it were "by itself one" [per se una]; and unity belongs to the essence. And what is "by itself" implies relation indirectly; for the Father is understood to exist "by Himself," as relatively distinct from the Son. Others, however, said, on the contrary, that it signifies relation directly; and essence indirectly; forasmuch as in the definition of "person" the term nature is mentioned indirectly; and these come nearer to the truth.
To determine the question, we must consider that something may be included in the meaning of a less common term, which is not included in the more common term; as "rational" is included in the meaning of "man," and not in the meaning of "animal." So that it is one thing to ask the meaning of the word animal, and another to ask its meaning when the animal in question is man. Also, it is one thing to ask the meaning of this word "person" in general; and another to ask the meaning of "person" as applied to God. For "person" in general signifies the individual substance of a rational figure. The individual in itself is undivided, but is distinct from others. Therefore "person" in any nature signifies what is distinct in that nature: thus in human nature it signifies this flesh, these bones, and this soul, which are the individuating principles of a man, and which, though not belonging to "person" in general, nevertheless do belong to the meaning of a particular human person.
Now distinction in God is only by relation of origin, as stated above (Question , Articles ,3), while relation in God is not as an accident in a subject, but is the divine essence itself; and so it is subsistent, for the divine essence subsists. Therefore, as the Godhead is God so the divine paternity is God the Father, Who is a divine person. Therefore a divine person signifies a relation as subsisting. And this is to signify relation by way of substance, and such a relation is a hypostasis subsisting in the divine nature, although in truth that which subsists in the divine nature is the divine nature itself. Thus it is true to say that the name "person" signifies relation directly, and the essence indirectly; not, however, the relation as such, but as expressed by way of a hypostasis. So likewise it signifies directly the essence, and indirectly the relation, inasmuch as the essence is the same as the hypostasis: while in God the hypostasis is expressed as distinct by the relation: and thus relation, as such, enters into the notion of the person indirectly. Thus we can say that this signification of the word "person" was not clearly perceived before it was attacked by heretics. Hence, this word "person" was used just as any other absolute term. But afterwards it was applied to express relation, as it lent itself to that signification, so that this word "person" means relation not only by use and custom, according to the first opinion, but also by force of its own proper signification.
Reply to Objection: 1. This word "person" is said in respect to itself, not to another; forasmuch as it signifies relation not as such, but by way of a substance---which is a hypostasis. In that sense Augustine says that it signifies the essence, inasmuch as in God essence is the same as the hypostasis, because in God what He is, and whereby He is are the same.
2. The term "what" refers sometimes to the nature expressed by the definition, as when we ask; What is man? and we answer: A mortal rational animal. Sometimes it refers to the "suppositum," as when we ask, What swims in the sea? and answer, A fish. So to those who ask, Three what? we answer, Three persons.
3. In God the individual---i.e. distinct and incommunicable substance---includes the idea of relation, as above explained.
4. The different sense of the less common term does not produce equivocation in the more common. Although a horse and an ass have their own proper definitions, nevertheless they agree univocally in animal, because the common definition of animal applies to both. So it does not follow that, although relation is contained in the signification of divine person, but not in that of an angelic or of a human person, the word "person" is used in an equivocal sense. Though neither is it applied univocally, since nothing can be said univocally of God and creatures (Question , Article ).
We are now led to consider the plurality of the persons: about which there are four points of inquiry:
(1) Whether there are several persons in God?
(2) How many are they?
(3) What the numeral terms signify in God?
(4) The community of the term "person."
Objection: 1. It would seem that there are not several persons in God. For person is "the individual substance of a rational nature." If then there are several persons in God, there must be several substances; which appears to be heretical.
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 (Question , Article ). Therefore there cannot be several persons in God.
3. Further, Boethius says of God (De Trin. i), that "this is truly one which has no number." But plurality implies number. Therefore there are not several persons in God.
4. Further, where number is, there is whole and part. Thus, if in God there exist a number of persons, there must be whole and part in God; which is inconsistent with the divine simplicity.
On the contrary Athanasius says: "One is the person of the Father, another of the Son, another of the Holy Ghost." Therefore the Father, and the Son, and the Holy Ghost are several persons.
I answer that It follows from what precedes that there are several persons in God. For it was shown above (Question , Article ) that this word "person" signifies in God a relation as subsisting in the divine nature. It was also established (Question , Article ) 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 to Objection: 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.
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 (Question , Article ). Hence the plurality of persons in God.
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.
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 (Question , Articles , 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 (Article ). But there are four relations in God as stated above (Question , Article ), paternity, filiation, common spiration, and procession. Therefore there are four persons in God.
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.
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.
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.
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" (1Jn 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 (Question , Articles ,4). 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.
Reply to Objection: 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.
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 (Question , Article ), 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.
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 (Question , Articles ,4).
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.
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 (Question , Articles ,4), and the same is not measured by the same.
Summa Th. I EN Qu.28 a.4