Summa Th. I EN Qu.30 a.3

Article: 3 Whether the numeral terms denote anything real in God?

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.

2. Further, whatever is said of God and of creatures, belongs to God in a more eminent manner than to creatures. But the numeral terms denote something real in creatures; therefore much more so in God.

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.

I answer that The Master (Sent. i, D, 24) considers that the numeral terms do not denote anything positive in God, but have only a negative meaning. Others, however, assert the contrary.

In order to resolve this point, we may observe that all plurality is a consequence of division. Now division is twofold; one is material, and is division of the continuous; from this results number, which is a species of quantity. Number in this sense is found only in material things which have quantity. The other kind of division is called formal, and is effected by opposite or diverse forms; and this kind of division results in a multitude, which does not belong to a genus, but is transcendental in the sense in which being is divided by one and by many. This kind of multitude is found only in immaterial things.

Some, considering only that multitude which is a species of discrete quantity, and seeing that such kind of quantity has no place in God, asserted that the numeral terms do not denote anything real in God, but remove something from Him. Others, considering the same kind of multitude, said that as knowledge exists in God according to the strict sense of the word, but not in the sense of its genus (as in God there is no such thing as a quality), so number exists in God in the proper sense of number, but not in the sense of its genus, which is quantity.

But we say that numeral terms predicated of God are not derived from number, a species of quantity, for in that sense they could bear only a metaphorical sense in God, like other corporeal properties, such as length, breadth, and the like; but that they are taken from multitude in a transcendent sense. Now multitude so understood has relation to the many of which it is predicated, as "one" convertible with "being" is related to being; which kind of oneness does not add anything to being, except a negation of division, as we saw when treating of the divine unity (Question [11], Article [1]); for "one" signifies undivided being. So, of whatever we say "one," we imply its undivided reality: thus, for instance, "one" applied to man signifies the undivided nature or substance of a man. In the same way, when we speak of many things, multitude in this latter sense points to those things as being each undivided in itself.

But number, if taken as a species of quantity, denotes an accident added to being; as also does "one" which is the principle of that number. Therefore the numeral terms in God signify the things of which they are said, and beyond this they add negation only, as stated (Sent. i, D, 24); in which respect the Master was right (Sent. i, D, 24). So when we say, the essence is one, the term "one" signifies the essence undivided; and when we say the person is one, it signifies the person undivided; and when we say the persons are many, we signify those persons, and their individual undividedness; for it is of the very nature of multitude that it should be composed of units.

Reply to Objection: 1. One, as it is a transcendental, is wider and more general than substance and relation. And so likewise is multitude; hence in God it may mean both substance and relation, according to the context. Still, the very signification of such names adds a negation of division, beyond substance and relation; as was explained above.

2. Multitude, which denotes something real in creatures, is a species of quantity, and cannot be used when speaking of God: unlike transcendental multitude, which adds only indivision to those of which it is predicated. Such a kind of multitude is applicable to God.

3. "One" does not exclude multitude, but division, which logically precedes one or multitude. Multitude does not remove unity, but division from each of the individuals which compose the multitude. This was explained when we treated of the divine unity (Question [11], Article [2]).

It must be observed, nevertheless, that the opposite arguments do not sufficiently prove the point advanced. Although the idea of solitude is excluded by plurality, and the plurality of gods by unity, it does not follow that these terms express this signification alone. For blackness is excluded by whiteness; nevertheless, the term whiteness does not signify the mere exclusion of blackness.

Article: 4 Whether this term "person" can be common to the three persons?

Objection: 1. It would seem that this term "person" cannot be common to the three persons. For nothing is common to the three persons but the essence. But this term "person" does not signify the essence directly. Therefore it is not common to all three.

2. Further, the common is the opposite to the incommunicable. But the very meaning of person is that it is incommunicable; as appears from the definition given by Richard of St. Victor (Question [29], Article [3], ad 4). Therefore this term "person" is not common to all the three persons.

3. Further, if the name "person" is common to the three, it is common either really, or logically. But it is not so really; otherwise the three persons would be one person; nor again is it so logically; otherwise person would be a universal. But in God there is neither universal nor particular; neither genus nor species, as we proved above (Question [3], Article [5]). Therefore this term 'person' is not common to the three.

On the contrary Augustine says (De Trin. vii, 4) that when we ask, "Three what?" we say, "Three persons," because what a person is, is common to them.

I answer that The very mode of expression itself shows that this term "person" is common to the three when we say "three persons"; for when we say "three men" we show that "man" is common to the three. Now it is clear that this is not community of a real thing, as if one essence were common to the three; otherwise there would be only one person of the three, as also one essence.

What is meant by such a community has been variously determined by those who have examined the subject. Some have called it a community of exclusion, forasmuch as the definition of "person" contains the word "incommunicable." Others thought it to be a community of intention, as the definition of person contains the word "individual"; as we say that to be a "species" is common to horse and ox. Both of these explanations, however, are excluded by the fact that "person" is not a name of exclusion nor of intention, but the name of a reality. We must therefore resolve that even in human affairs this name "person" is common by a community of idea, not as genus or species, but as a vague individual thing. The names of genera and species, as man or animal, are given to signify the common natures themselves, but not the intentions of those common natures, signified by the terms "genus" or "species." The vague individual thing, as "some man," signifies the common nature with the determinate mode of existence of singular things---that is, something self-subsisting, as distinct from others. But the name of a designated singular thing signifies that which distinguishes the determinate thing; as the name Socrates signifies this flesh and this bone. But there is this difference---that the term "some man" signifies the nature, or the individual on the part of its nature, with the mode of existence of singular things; while this name "person" is not given to signify the individual on the part of the nature, but the subsistent reality in that nature. Now this is common in idea to the divine persons, that each of them subsists distinctly from the others in the divine nature. Thus this name "person" is common in idea to the three divine persons.

Reply to Objection: 1. This argument is founded on a real community.

2. Although person is incommunicable, yet the mode itself of incommunicable existence can be common to many.

3. Although this community is logical and not real, yet it does not follow that in God there is universal or particular, or genus, or species; both because neither in human affairs is the community of person the same as community of genus or species; and because the divine persons have one being; whereas genus and species and every other universal are predicated of many which differ in being.


We now consider what belongs to the unity or plurality in God; which gives rise to four points of inquiry:

(1) Concerning the word "Trinity";

(2) Whether we can say that the Son is other than the Father?

(3) Whether an exclusive term, which seems to exclude otherness, can be joined to an essential name in God?

(4) Whether it can be joined to a personal term?

Article: 1 Whether there is trinity in God?

Objection: 1. It would seem there is not trinity in God. For every name in God signifies substance or relation. But this name "Trinity" does not signify the substance; otherwise it would be predicated of each one of the persons: nor does it signify relation; for it does not express a name that refers to another. Therefore the word "Trinity" is not to be applied to God.

2. Further, this word "trinity" is a collective term, since it signifies multitude. But such a word does not apply to God; as the unity of a collective name is the least of unities, whereas in God there exists the greatest possible unity. Therefore this word "trinity" does not apply to God.

3. Further, every trine is threefold. But in God there is not triplicity; since triplicity is a kind of inequality. Therefore neither is there trinity in God.

4. Further, all that exists in God exists in the unity of the divine essence; because God is His own essence. Therefore, if Trinity exists in God, it exists in the unity of the divine essence; and thus in God there would be three essential unities; which is heresy.

5. Further, in all that is said of God, the concrete is predicated of the abstract; for Deity is God and paternity is the Father. But the Trinity cannot be called trine; otherwise there would be nine realities in God; which, of course, is erroneous. Therefore the word trinity is not to be applied to God.

On the contrary Athanasius says: "Unity in Trinity; and Trinity in Unity is to be revered."

I answer that The name "Trinity" in God signifies the determinate number of persons. And so the plurality of persons in God requires that we should use the word trinity; because what is indeterminately signified by plurality, is signified by trinity in a determinate manner.

Reply to Objection: 1. In its etymological sense, this word "Trinity" seems to signify the one essence of the three persons, according as trinity may mean trine-unity. But in the strict meaning of the term it rather signifies the number of persons of one essence; and on this account we cannot say that the Father is the Trinity, as He is not three persons. Yet it does not mean the relations themselves of the Persons, but rather the number of persons related to each other; and hence it is that the word in itself does not express regard to another.

2. Two things are implied in a collective term, plurality of the "supposita," and a unity of some kind of order. For "people" is a multitude of men comprehended under a certain order. In the first sense, this word "trinity" is like other collective words; but in the second sense it differs from them, because in the divine Trinity not only is there unity of order, but also with this there is unity of essence.

3. "Trinity" is taken in an absolute sense; for it signifies the threefold number of persons. "Triplicity" signifies a proportion of inequality; for it is a species of unequal proportion, according to Boethius (Arithm. i, 23). Therefore in God there is not triplicity, but Trinity.

4. In the divine Trinity is to be understood both number and the persons numbered. So when we say, "Trinity in Unity," we do not place number in the unity of the essence, as if we meant three times one; but we place the Persons numbered in the unity of nature; as the "supposita" of a nature are said to exist in that nature. On the other hand, we say "Unity in Trinity"; meaning that the nature is in its "supposita."

5. When we say, "Trinity is trine," by reason of the number implied, we signify the multiplication of that number by itself; since the word trine imports a distinction in the "supposita" of which it is spoken. Therefore it cannot be said that the Trinity is trine; otherwise it follows that, if the Trinity be trine, there would be three "supposita" of the Trinity; as when we say, "God is trine," it follows that there are three "supposita" of the Godhead.

Article: 2 Whether the Son is other than the Father?

Objection: 1. It would seem that the Son is not other than the Father. For "other" is a relative term implying diversity of substance. If, then, the Son is other than the Father, He must be different from the Father; which is contrary to what Augustine says (De Trin. vii), that when we speak of three persons, "we do not mean to imply diversity."

2. Further, whosoever are other from one another, differ in some way from one another. Therefore, if the Son is other than the Father, it follows that He differs from the Father; which is against what Ambrose says (De Fide i), that "the Father and the Son are one in Godhead; nor is there any difference in substance between them, nor any diversity."

3. Further, the term alien is taken from "alius" [other]. But the Son is not alien from the Father, for Hilary says (De Trin. vii) that "in the divine persons there is nothing diverse, nothing alien, nothing separable." Therefore the Son is not other that the Father.

4. Further, the terms "other person" and "other thing" [alius et aliud] have the same meaning, differing only in gender. So if the Son is another person from the Father, it follows that the Son is a thing apart from the Father.

On the contrary Augustine [*Fulgentius, De Fide ad Petrum i.] says: "There is one essence of the Father and Son and Holy Ghost, in which the Father is not one thing, the Son another, and the Holy Ghost another; although the Father is one person, the Son another, and the Holy Ghost another."

I answer that Since as Jerome remarks [*In substance, Ep. lvii.], a heresy arises from words wrongly used, when we speak of the Trinity we must proceed with care and with befitting modesty; because, as Augustine says (De Trin. i, 3), "nowhere is error more harmful, the quest more toilsome, the finding more fruitful." Now, in treating of the Trinity, we must beware of two opposite errors, and proceed cautiously between them---namely, the error of Arius, who placed a Trinity of substance with the Trinity of persons; and the error of Sabellius, who placed unity of person with the unity of essence.

Thus, to avoid the error of Arius we must shun the use of the terms diversity and difference in God, lest we take away the unity of essence: we may, however, use the term "distinction" on account of the relative opposition. Hence whenever we find terms of "diversity" or "difference" of Persons used in an authentic work, these terms of "diversity" or "difference" are taken to mean "distinction." But lest the simplicity and singleness of the divine essence be taken away, the terms "separation" and "division," which belong to the parts of a whole, are to be avoided: and lest quality be taken away, we avoid the use of the term "disparity": and lest we remove similitude, we avoid the terms "alien" and "discrepant." For Ambrose says (De Fide i) that "in the Father and the Son there is no discrepancy, but one Godhead": and according to Hilary, as quoted above, "in God there is nothing alien, nothing separable."

To avoid the heresy of Sabellius, we must shun the term "singularity," lest we take away the communicability of the divine essence. Hence Hilary says (De Trin. vii): "It is sacrilege to assert that the Father and the Son are separate in Godhead." We must avoid the adjective "only" [unici] lest we take away the number of persons. Hence Hilary says in the same book: "We exclude from God the idea of singularity or uniqueness." Nevertheless, we say "the only Son," for in God there is no plurality of Sons. Yet, we do not say "the only God," for the Deity is common to several. We avoid the word "confused," lest we take away from the Persons the order of their nature. Hence Ambrose says (De Fide i): "What is one is not confused; and there is no multiplicity where there is no difference." The word "solitary" is also to be avoided, lest we take away the society of the three persons; for, as Hilary says (De Trin. iv), "We confess neither a solitary nor a diverse God."

This word "other" [alius], however, in the masculine sense, means only a distinction of "suppositum"; and hence we can properly say that "the Son is other than the Father," because He is another "suppositum" of the divine nature, as He is another person and another hypostasis.

Reply to Objection: 1. "Other," being like the name of a particular thing, refers to the "suppositum"; and so, there is sufficient reason for using it, where there is a distinct substance in the sense of hypostasis or person. But diversity requires a distinct substance in the sense of essence. Thus we cannot say that the Son is diverse from the Father, although He is another.

2. "Difference" implies distinction of form. There is one form in God, as appears from the text, "Who, when He was in the form of God" (Ph 2,6). Therefore the term "difference" does not properly apply to God, as appears from the authority quoted. Yet, Damascene (De Fide Orth. i, 5) employs the term "difference" in the divine persons, as meaning that the relative property is signified by way of form. Hence he says that the hypostases do not differ from each other in substance, but according to determinate properties. But "difference" is taken for "distinction," as above stated.

3. The term "alien" means what is extraneous and dissimilar; which is not expressed by the term "other" [alius]; and therefore we say that the Son is "other" than the Father, but not that He is anything "alien."

4. The neuter gender is formless; whereas the masculine is formed and distinct; and so is the feminine. So the common essence is properly and aptly expressed by the neuter gender, but by the masculine and feminine is expressed the determined subject in the common nature. Hence also in human affairs, if we ask, Who is this man? we answer, Socrates, which is the name of the "suppositum"; whereas, if we ask, What is he? we reply, A rational and mortal animal. So, because in God distinction is by the persons, and not by the essence, we say that the Father is other than the Son, but not something else; while conversely we say that they are one thing, but not one person.

Article: 3 Whether the exclusive word "alone" should be added to the essential term in God?

Objection: 1. It would seem that the exclusive word "alone" [solus] is not to be added to an essential term in God. For, according to the Philosopher (Elench. ii, 3), "He is alone who is not with another." But God is with the angels and the souls of the saints. Therefore we cannot say that God is alone.

2. Further, whatever is joined to the essential term in God can be predicated of every person "per se," and of all the persons together; for, as we can properly say that God is wise, we can say the Father is a wise God; and the Trinity is a wise God. But Augustine says (De Trin. vi, 9): "We must consider the opinion that the Father is not true God alone." Therefore God cannot be said to be alone.

3. Further if this expression "alone" is joined to an essential term, it would be so joined as regards either the personal predicate or the essential predicate. But it cannot be the former, as it is false to say, "God alone is Father," since man also is a father; nor, again, can it be applied as regards the latter, for, if this saying were true, "God alone creates," it would follow that the "Father alone creates," as whatever is said of God can be said of the Father; and it would be false, as the Son also creates. Therefore this expression "alone" cannot be joined to an essential term in God.

On the contrary It is said, "To the King of ages, immortal, invisible, the only God" (1Tm 1,17).

I answer that This term "alone" can be taken as a categorematical term, or as a syncategorematical term. A categorematical term is one which ascribes absolutely its meaning to a given "suppositum"; as, for instance, "white" to man, as when we say a "white man." If the term "alone" is taken in this sense, it cannot in any way be joined to any term in God; for it would mean solitude in the term to which it is joined; and it would follow that God was solitary, against what is above stated (Article [2]). A syncategorematical term imports the order of the predicate to the subject; as this expression "every one" or "no one"; and likewise the term "alone," as excluding every other "suppositum" from the predicate. Thus, when we say, "Socrates alone writes," we do not mean that Socrates is solitary, but that he has no companion in writing, though many others may be with him. In this way nothing prevents the term "alone" being joined to any essential term in God, as excluding the predicate from all things but God; as if we said "God alone is eternal," because nothing but God is eternal.

Reply to Objection: 1. Although the angels and the souls of the saints are always with God, nevertheless, if plurality of persons did not exist in God, He would be alone or solitary. For solitude is not removed by association with anything that is extraneous in nature; thus anyone is said to be alone in a garden, though many plants and animals are with him in the garden. Likewise, God would be alone or solitary, though angels and men were with Him, supposing that several persons were not within Him. Therefore the society of angels and of souls does not take away absolute solitude from God; much less does it remove respective solitude, in reference to a predicate.

2. This expression "alone," properly speaking, does not affect the predicate, which is taken formally, for it refers to the "suppositum," as excluding any other suppositum from the one which it qualifies. But the adverb "only," being exclusive, can be applied either to subject or predicate. For we can say, "Only Socrates"---that is, no one else---"runs: and Socrates runs only"---that is, he does nothing else. Hence it is not properly said that the Father is God alone, or the Trinity is God alone, unless some implied meaning be assumed in the predicate, as, for instance, "The Trinity is God Who alone is God." In that sense it can be true to say that the Father is that God Who alone is God, if the relative be referred to the predicate, and not to the "suppositum." So, when Augustine says that the Father is not God alone, but that the Trinity is God alone, he speaks expositively, as he might explain the words, "To the King of ages, invisible, the only God," as applying not to the Father, but to the Trinity alone.

3. In both ways can the term "alone" be joined to an essential term. For this proposition, "God alone is Father," can mean two things, because the word "Father" can signify the person of the Father; and then it is true; for no man is that person: or it can signify that relation only; and thus it is false, because the relation of paternity is found also in others, though not in a univocal sense. Likewise it is true to say God alone creates; nor, does it follow, "therefore the Father alone creates," because, as logicians say, an exclusive diction so fixes the term to which it is joined that what is said exclusively of that term cannot be said exclusively of an individual contained in that term: for instance, from the premiss, "Man alone is a mortal rational animal," we cannot conclude, "therefore Socrates alone is such."

Article: 4 Whether an exclusive diction can be joined to the personal term?

Objection: 1. It would seem that an exclusive diction can be joined to the personal term, even though the predicate is common. For our Lord speaking to the Father, said: "That they may know Thee, the only true God" (Jn 17,3). Therefore the Father alone is true God.

2. Further, He said: "No one knows the Son but the Father" (Mt 11,27); which means that the Father alone knows the Son. But to know the Son is common (to the persons). Therefore the same conclusion follows.

3. Further, an exclusive diction does not exclude what enters into the concept of the term to which it is joined. Hence it does not exclude the part, nor the universal; for it does not follow that if we say "Socrates alone is white," that therefore "his hand is not white," or that "man is not white." But one person is in the concept of another; as the Father is in the concept of the Son; and conversely. Therefore, when we say, The Father alone is God, we do not exclude the Son, nor the Holy Ghost; so that such a mode of speaking is true.

4. Further, the Church sings: "Thou alone art Most High, O Jesus Christ."

On the contrary This proposition "The Father alone is God" includes two assertions---namely, that the Father is God, and that no other besides the Father is God. But this second proposition is false, for the Son is another from the Father, and He is God. Therefore this is false, The Father alone is God; and the same of the like sayings.

I answer that When we say, "The Father alone is God," such a proposition can be taken in several senses. If "alone" means solitude in the Father, it is false in a categorematical sense; but if taken in a syncategorematical sense it can again be understood in several ways. For if it exclude (all others) from the form of the subject, it is true, the sense being "the Father alone is God"---that is, "He who with no other is the Father, is God." In this way Augustine expounds when he says (De Trin. vi, 6): "We say the Father alone, not because He is separate from the Son, or from the Holy Ghost, but because they are not the Father together with Him." This, however, is not the usual way of speaking, unless we understand another implication, as though we said "He who alone is called the Father is God." But in the strict sense the exclusion affects the predicate. And thus the proposition is false if it excludes another in the masculine sense; but true if it excludes it in the neuter sense; because the Son is another person than the Father, but not another thing; and the same applies to the Holy Ghost. But because this diction "alone," properly speaking, refers to the subject, it tends to exclude another Person rather than other things. Hence such a way of speaking is not to be taken too literally, but it should be piously expounded, whenever we find it in an authentic work.

Reply to Objection: 1. When we say, "Thee the only true God," we do not understand it as referring to the person of the Father, but to the whole Trinity, as Augustine expounds (De Trin. vi, 9). Or, if understood of the person of the Father, the other persons are not excluded by reason of the unity of essence; in so far as the word "only" excludes another thing, as above explained.

The same Reply can be given to OBJ 2. For an essential term applied to the Father does not exclude the Son or the Holy Ghost, by reason of the unity of essence. Hence we must understand that in the text quoted the term "no one" [*Nemo = non-homo, i.e. no man] is not the same as "no man," which the word itself would seem to signify (for the person of the Father could not be excepted), but is taken according to the usual way of speaking in a distributive sense, to mean any rational nature.

3. The exclusive diction does not exclude what enters into the concept of the term to which it is adjoined, if they do not differ in "suppositum," as part and universal. But the Son differs in "suppositum" from the Father; and so there is no parity.

4. We do not say absolutely that the Son alone is Most High; but that He alone is Most High "with the Holy Ghost, in the glory of God the Father."


We proceed to inquire concerning the knowledge of the divine persons; and this involves four points of inquiry:

(1) Whether the divine persons can be known by natural reason?

(2) Whether notions are to be attributed to the divine persons?

(3) The number of the notions?

(4) Whether we may lawfully have various contrary opinions of these notions?

Article: 1 Whether the trinity of the divine persons can be known by natural reason?

Objection: 1. It would seem that the trinity of the divine persons can be known by natural reason. For philosophers came to the knowledge of God not otherwise than by natural reason. Now we find that they said many things about the trinity of persons, for Aristotle says (De Coelo et Mundo i, 2): "Through this number"---namely, three---"we bring ourselves to acknowledge the greatness of one God, surpassing all things created." And Augustine says (Confess. vii, 9): "I have read in their works, not in so many words, but enforced by many and various reasons, that in the beginning was the Word, and the Word was with God, and the Word was God," and so on; in which passage the distinction of persons is laid down. We read, moreover, in a gloss on Rm 1 Ex 8 that the magicians of Pharaoh failed in the third sign---that is, as regards knowledge of a third person---i.e. of the Holy Ghost ---and thus it is clear that they knew at least two persons. Likewise Trismegistus says: "The monad begot a monad, and reflected upon itself its own heat." By which words the generation of the Son and procession of the Holy Ghost seem to be indicated. Therefore knowledge of the divine persons can be obtained by natural reason.

2. Further, Richard St. Victor says (De Trin. i, 4): "I believe without doubt that probable and even necessary arguments can be found for any explanation of the truth." So even to prove the Trinity some have brought forward a reason from the infinite goodness of God, who communicates Himself infinitely in the procession of the divine persons; while some are moved by the consideration that "no good thing can be joyfully possessed without partnership." Augustine proceeds (De Trin. x, 4; x, 11,12) to prove the trinity of persons by the procession of the word and of love in our own mind; and we have followed him in this (Question [27], Articles [1],3). Therefore the trinity of persons can be known by natural reason.

3. Further, it seems to be superfluous to teach what cannot be known by natural reason. But it ought not to be said that the divine tradition of the Trinity is superfluous. Therefore the trinity of persons can be known by natural reason.

On the contrary Hilary says (De Trin. i), "Let no man think to reach the sacred mystery of generation by his own mind." And Ambrose says (De Fide ii, 5), "It is impossible to know the secret of generation. The mind fails, the voice is silent." But the trinity of the divine persons is distinguished by origin of generation and procession (Question [30], Article [2]). Since, therefore, man cannot know, and with his understanding grasp that for which no necessary reason can be given, it follows that the trinity of persons cannot be known by reason.

I answer that It is impossible to attain to the knowledge of the Trinity by natural reason. For, as above explained (Question [12], Articles [4],12), man cannot obtain the knowledge of God by natural reason except from creatures. Now creatures lead us to the knowledge of God, as effects do to their cause. Accordingly, by natural reason we can know of God that only which of necessity belongs to Him as the principle of things, and we have cited this fundamental principle in treating of God as above (Question [12], Article [12]). Now, the creative power of God is common to the whole Trinity; and hence it belongs to the unity of the essence, and not to the distinction of the persons. Therefore, by natural reason we can know what belongs to the unity of the essence, but not what belongs to the distinction of the persons. Whoever, then, tries to prove the trinity of persons by natural reason, derogates from faith in two ways. Firstly, as regards the dignity of faith itself, which consists in its being concerned with invisible things, that exceed human reason; wherefore the Apostle says that "faith is of things that appear not" (He 11,1), and the same Apostle says also, "We speak wisdom among the perfect, but not the wisdom of this world, nor of the princes of this world; but we speak the wisdom of God in a mystery which is hidden" (1Co 2,6-7). Secondly, as regards the utility of drawing others to the faith. For when anyone in the endeavor to prove the faith brings forward reasons which are not cogent, he falls under the ridicule of the unbelievers: since they suppose that we stand upon such reasons, and that we believe on such grounds.

Therefore, we must not attempt to prove what is of faith, except by authority alone, to those who receive the authority; while as regards others it suffices to prove that what faith teaches is not impossible. Hence it is said by Dionysius (Div. Nom. ii): "Whoever wholly resists the word, is far off from our philosophy; whereas if he regards the truth of the word"---i.e. "the sacred word, we too follow this rule."

Reply to Objection: 1. The philosophers did not know the mystery of the trinity of the divine persons by its proper attributes, such as paternity, filiation, and procession, according to the Apostle's words, "We speak the wisdom of God which none of the princes of the world"---i.e. the philosophers---"knew" (1Co 2,6). Nevertheless, they knew some of the essential attributes appropriated to the persons, as power to the Father, wisdom to the Son, goodness to the Holy Ghost; as will later on appear. So, when Aristotle said, "By this number," etc., we must not take it as if he affirmed a threefold number in God, but that he wished to say that the ancients used the threefold number in their sacrifices and prayers on account of some perfection residing in the number three. In the Platonic books also we find, "In the beginning was the word," not as meaning the Person begotten in God, but as meaning the ideal type whereby God made all things, and which is appropriated to the Son. And although they knew these were appropriated to the three persons, yet they are said to have failed in the third sign---that is, in the knowledge of the third person, because they deviated from the goodness appropriated to the Holy Ghost, in that knowing God "they did not glorify Him as God" (Rm 1); or, because the Platonists asserted the existence of one Primal Being whom they also declared to be the father of the universe, they consequently maintained the existence of another substance beneath him, which they called "mind" or the "paternal intellect," containing the idea of all things, as Macrobius relates (Som. Scip. iv). They did not, however, assert the existence of a third separate substance which might correspond to the Holy Ghost. So also we do not assert that the Father and the Son differ in substance, which was the error of Origen and Arius, who in this followed the Platonists. When Trismegistus says, "Monad begot monad," etc., this does not refer to the generation of the Son, or to the procession of the Holy Ghost, but to the production of the world. For one God produced one world by reason of His love for Himself.

2. Reason may be employed in two ways to establish a point: firstly, for the purpose of furnishing sufficient proof of some principle, as in natural science, where sufficient proof can be brought to show that the movement of the heavens is always of uniform velocity. Reason is employed in another way, not as furnishing a sufficient proof of a principle, but as confirming an already established principle, by showing the congruity of its results, as in astrology the theory of eccentrics and epicycles is considered as established, because thereby the sensible appearances of the heavenly movements can be explained; not, however, as if this proof were sufficient, forasmuch as some other theory might explain them. In the first way, we can prove that God is one; and the like. In the second way, reasons avail to prove the Trinity; as, when assumed to be true, such reasons confirm it. We must not, however, think that the trinity of persons is adequately proved by such reasons. This becomes evident when we consider each point; for the infinite goodness of God is manifested also in creation, because to produce from nothing is an act of infinite power. For if God communicates Himself by His infinite goodness, it is not necessary that an infinite effect should proceed from God: but that according to its own mode and capacity it should receive the divine goodness. Likewise, when it is said that joyous possession of good requires partnership, this holds in the case of one not having perfect goodness: hence it needs to share some other's good, in order to have the goodness of complete happiness. Nor is the image in our mind an adequate proof in the case of God, forasmuch as the intellect is not in God and ourselves univocally. Hence, Augustine says (Tract. xxvii. in Joan.) that by faith we arrive at knowledge, and not conversely.

3. There are two reason why the knowledge of the divine persons was necessary for us. It was necessary for the right idea of creation. The fact of saying that God made all things by His Word excludes the error of those who say that God produced things by necessity. When we say that in Him there is a procession of love, we show that God produced creatures not because He needed them, nor because of any other extrinsic reason, but on account of the love of His own goodness. So Moses, when he had said, "In the beginning God created heaven and earth," subjoined, "God said, Let there be light," to manifest the divine Word; and then said, "God saw the light that it was good," to show proof of the divine love. The same is also found in the other works of creation. In another way, and chiefly, that we may think rightly concerning the salvation of the human race, accomplished by the Incarnate Son, and by the gift of the Holy Ghost.

Summa Th. I EN Qu.30 a.3