Note: I don't guarantee the accuracy of this translation, so please compare it to the original before using it for any important purpose. There's a French translation available too, by the way. If you do happen to find any errors in the following, please let me know. Thanks.

Introductory letter from Thomas Aquinas to Pope Urban

I have diligently read the little book that your excellency, Holy Father Pope Urban, has sent me and I have found in it very many useful arguments in support of our faith. But I think its good fruits may be lost to many, on account of certain passages from the Holy Fathers contained in it which may appear to be doubtful in their orthodoxy, and for this reason they may act as fuel for error and give occasion to calumnious arguments; and so, in order to remove every ambiguity from the authorities contained in the aforesaid book, so that the most pure fruit of the true faith might be drawn from it, I have decided, first, to explain those things which may be cause for doubt in the aforesaid authorities, and then to explain how to use them to teach and defend the truth of the Catholic faith. I believe that there are two reasons why modern people may find things they consider doubtful in the sayings of the ancient saints.

First, because when errors concerning the faith originated, it encouraged the holy doctors of the Church to teach with more circumspection those things that belong to the faith, in order to eliminate those errors; for example, it is clear that the holy doctors who lived before the heresy of Arius did not speak as expressly of the unity of the divine essence as did doctors of later times; and the same is true for other heresies, and this can be seen not only in the various doctors, but especially in the writings of the great doctor Augustine. For in the books he wrote after the appearance of the Pelagian heresy, he spoke more cautiously of power of free will than in the books he wrote before the birth of the aforesaid heresy: in these, when he defended free will against the Manichaeans, he made some points which the Pelagians used in defense of their errors, which attacked divine grace. So it is not surprising if modern doctors of the faith, living after the birth of the various errors, spoke so cautiously or more rigorously concerning the doctrine of the faith, in order to avoid every heresy. For this reason, if there are things in the sayings of the ancient doctors which are found to be said with not as much caution as modern doctors would use, these are not to be condemned or rejected, nor are they to be stretched beyond what was intended, but instead they should be reverendly explained.

Second, because many terms which sound good in the Greek languages may not sound good in Latin, and because of this the same truth of the faith may be expressed differently in Latin and in Greek. For among the Greeks it is said correctly, in accordance with the Catholic faith, that the Father and the Son and the Holy Spirit are three hypostases; among the Latins, however, it does not sound correct if one were to say that they are three substances, even though the word "hypostasis" has the same meaning among the Greeks as "substance" has among the Latins. For, among the Latins, substance is more commonly used to mean essence, which both we and the Greeks confess to be one in the divine persons. Because of this, when the Greeks say three hypostases, we say three persons, as Augustine teaches in book vii on the Trinity. And there is no doubt that the same is true in many other cases. For this reason, it is the duty of a good translator to transfer from one language to another the truths of the Catholic faith, keeping the meaning while changing the manner of speaking according to the particular properties of the languages in question. For it is clear that if something written in literary Latin were translated word for word into more common Latin, the resulting text would be improper. This is even more true when things in one language are translated into another word for word, and so it is no surprise if some doubt results from doing this.

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PART I

How it should be understood when it is said that the Son comes from the Father as an effect from a cause.

There can be doubt among some concerning what is said in various places by these authorities, that the Father is the cause of the Son, or that the Father or the Son is the cause of the Holy Spirit. And indeed we can see this, first, in the words of Athanasius, which he is reported to have spoken by him at the Nicene Council: Whosoever maintains that the Son is from the Father, maintains this as a word is from the heart, as light is from the sun, as a river is from a spring, and as every effect is from a cause. But he who insults or denies the effect, he in fact also denies the cause. So speaks the caused and begotten Son: whoever rejects me, rejects he who sent me. And elsewhere: The Spirit is not without principle, that is without beginning or cause: rather, he shows himself to be true God, although his principle is not in time, but in the cause of his true origin. Likewise, Basil says: The Holy Spirit, sent by God himself, has himself for a cause. And likewise Theodoritus on the epistle to the Hebrews: The cause of the Son is the Father. But among Latins but it is not customary to say that the Father is the cause of the Son or of the Holy Spirit, but solely the principle or originator. And this for three reasons.

First, because the Father cannot be understood as the cause of the Son in the sense of being a formal or material or final cause, but solely as an originating cause, which is the efficient cause. But we always find that such a cause differs in essence from that which it causes. And for this reason, lest the Son be understood to be of another essence than the Father, it is not our custom to say that the Father is the cause of the Son; instead, we use names that signify a consubstantial origin, such as fount, head, and others names of this kind.

Second, because, among us, a cause corresponds to an effect: which is why we do not say the Father is a cause, lest someone understand by this that the son were made. For among philosophers God is called the first cause; and every effect is included by them in the universality of creatures: and for this reason if the Son is said to have a cause, he could be understood to be included in the universality of creatures [and thus he could be understood to have been created].

Third, because man should hesitate to speak of the divine in a manner that is different from that of Sacred Scripture. But Sacred Scripture calls the Father the principle of the Son, as is clear in John 1:1: In the beginning [principio] was the Word. But nowhere is the Father called the cause, or the son the effect. And so, since the word "cause" says more than "principle", we do not presume to call the Father the cause or the Son the effect. No word pertains to origin [in the divine persons], if indeed we can speak of origin with regard to the divine, as much as "principle" does. For, since those things that are in God are incomprehensible and cannot be defined by us, it is more suitable for us to use, with respect to God, general words rather than precise ones: and so the most proper name for God is the one that appears in Exodus 3: "he who is", which is very general. But since "cause" is more general than "element", so is "principle" more general than "cause": for a point is the principle (beginning) of a line, but not its cause. And for this reason the word "principle" is very fittingly used of the divine.

However, it must be understood that the aforesaid saints, who use the names "cause" and "effect" for the divine persons, did not intend to imply that the divine persons are different in nature or that the Son is a creature. In doing so, they meant solely to explain the origin of the persons, as when we use the word "principle". For this reason, Gregory of Nyssa says: However, in saying "cause" and "effect", we do not refer to nature. Nor do we, by these names, offer a description of the essence or nature; but to the extent that there is a difference, we indicate it; for example, when we say that the Son is not unbegotten, or that the Father is not generated from another. Likewise, Basil says: I say that the Holy Spirit is unbegotten, that he does not have a father; and that he is not a creature; for he is not created; but has God for a cause, whose spirit he truly is, and from whom he proceeds.

How it should be understood when it is said that the Son is second, after the Father, and the Holy Spirit is third.

According to the authority of the aforesaid doctors, the Son is second from the Father, and the Holy Spirit is third from the same. For Athanasius says in his sermon to Serapion: The Holy Spirit is third from the Father; but from the Son he is second. And Basil says: In dignity and in order, the Spirit is second from the Son. However, this may appear false to some. For in the divine persons there is no order except for the order of nature, according to which, as Augustine says, one is not prior to the other, but one is from the other. There is indeed no mode of priority according to which the Father can be said to be prior to the Son. Neither is he prior in time, for with the Son he is eternal; nor is he prior in nature, for the Father and the Son are of a single nature; nor in dignity, for the Father and the Son are equals; nor in intellect, for they are not distinguished from each other except in their relations [as persons], while they are the same according to the relations of the intellect, since one is the intellect of the other. And so it is clear that, properly speaking, the Son cannot be said second from the Father, nor can the Holy Spirit be said to be third from the Father. Therefore, the aforesaid doctors are saying that the Son is second and the Holy Spirit third according to numerical order, which is clear from the same Basil, who says: We receive the Holy Spirit from the Father and the Son, third in number and glorified by the Son of God, who said when delivering the order of the baptism of salvation: 'Go, baptize all peoples in name Father and of the Son and of the Holy Spirit.' And Epiphanius says: The Spirit of God from the Father and from the Son is third in appellation.

However, what Basil says, that the Spirit is second from the Son in dignity, seems to be a more serious problem: for he seems to establish levels of dignity in the Trinity, whereas there is equal dignity among the three persons. This may be said not of natural dignity, but of personal dignity; such as when we say that a person is a distinct hypostasis by the dignity pertaining to that person. It is in this sense that Hilary says [#11] that the Father is greater than the Son on account of the authority of origin; however, the Son is not less than the Father, on account of of the unity of substance.

How it should be understood when it is said that the Holy Spirit is the third light.

However, there seems to be even more error in what is suggested by the words of Saint Epiphanius, bishop of Cyprus, when he says: The Holy Spirit is the spirit of truth, the third light from the Father and the Son. Where there is unity, there is no first or third order. However, the Father and the Son and the Holy Spirit are one light, as one God. So just as it cannot be said in a Catholic sense that the Holy Spirit is the third God from the Father and the Son, in the same way it cannot be said that he is the third light. However, he is called the third person on account of the plurality of persons. So when he says that the Holy Spirit is the third light, it follows that there are three lights; which the same person later on expressly adds, saying: All others lights are denoted by position or by composition or by appellation; but they are not similar to these three [divine] lights. It can be said that light implies a certain origin: for light is that which shines from some other light, and also that from which another light can shine forth. And this second definition of the word "light" can be applied to the personal properties [of the persons of the Trinity] due to the diffusive property, although, according to its very nature, it pertains to the essence of the light. And the aforesaid father, with this in mind, spoke of the divine Persons in terms of the "third light" and the "three lights"; although we must be sure not draw false conclusions from this, but rather simply confess that the Father and the Son and the Holy Spirit are one light.

How it should be understood when it is said that essence is begotten in the Son, and spirated in the Holy Spirit.

We find in the sayings of the aforesaid fathers that essence is begotten in the Son, and spirated in the Holy Spirit. For Athanasius says in the third sermon on the acts of the Nicene Council, speaking of the person of the Son: From my essence I dispense to mankind your Spirit begotten from thee; and a little later: From your essence, that in me you have begotten, I give them the Holy Spirit. And similarly in the epistle to Serapion: The Father retaining his essence in himself, begot it wholly in the Son in an indescribable manner. And again: As the Father has life in himself, that is a living and spirating nature, he has thus given the Son to also have life in himself, that is he has begotten in the Son the same nature that spirates the living spirit. And below this he says, The Father and the Son have one divinity, by its nature spirating one Holy Spirit. From these words we would conclude that divine nature is begotten in the Son, and is spirated in the Father and the Son. Likewise Cyril in his book the Thesaurus Against Heretics: Virtue, uncreated and begotten in the Son, is in every way the nature of the Son and the Father. And again: Father has given life to the Son, that is he begets his natural life in the Son. Likewise Basil: The Son the Father has given us God essentially begotten from God, having in himself, through generation, all the essence of the Father. Likewise Athanasius says in the epistle to Serapion that the divine essence is spirated in the Holy Spirit, saying that The Holy Spirit is the true and natural image of the Son through the essence spirated in him, in every way the same.

But this manner of speaking is false: and the sacred Council of the Lateran condemned the teaching of Joachim, who presumed to use this manner of speaking to defend himself against master Peter Lombard. In fact, the aforesaid master shows in the fifth distinction in the first book of the Sentences, which he wrote, that a common essence neither generates, nor is begotten, nor proceeds. This is true because in the divine [Persons] there is something that is common and indistinct, and something that is distinct and not common [to the three divine Persons]. So that which is distinct in the divine [Persons] cannot be attributed to what is common and indistinct, but solely to what is distinct. But there is no distinction between the divine [Persons] other than the fact that one generates and another is begotten and another proceeds. So to generate or to be begotten or to proceed cannot be attributed to the divine essence, which is common to and wholly indistinct in the three Persons. What is distinct in the divine [Persons] is the person or the hypostasis, or the suppositum of divine nature, that is, that which has the divine nature. And for this reason that which signifies or can stand for a Person aptly receives the name of generation or procession, as the names Father and Son and Holy Spirit signify distinct persons, and the word person signifies hypostasis in general. For this reason it is suitable to say that the Father generates the Son, and that the Son is begotten from the Father, and that the Holy Spirit proceeds from the Father and the Son; and similarly that a person generates or spirates another person, or is generated or spirated from another person. However, the word "God", because it signifies a common essence in a concrete mode (that is, signifies having divinity), can stand for "person", depending on how it is used; and for this reason sayings of the following kind are acceptable: God generates God, and God is begotten or proceeds from God. However, the word "essence", and "divinity", and similar abstract terms, cannot stand for "person", no matter how they are used, or whether they are intended to signify or stand for a person. And for this reason terms such as those just mentioned cannot properly be used for what is proper to persons, such as when essence is called generating or generated; although certain of these names are more applicable to the Persons, in so far as they signify the principles of acts that are proper to the Persons; such as light, wisdom, goodness, and others of this kind. And for this reason, those terms which are proper to the Persons are less unsuitably spoken; such as when it is said that the Son is light of light, or wisdom of wisdom; but essence of essence is more unsuitable to say. But although the manner of expression may be different, as when one says "God" and "divinity", nonetheless the thing represented is the same: and so, on account of the identity of the thing in question, one may say "God is the divinity" or "the divine person", or "the Father is the divine essence". It is in this sense that the saints sometimes used one term in the place of another, such as when they say that "the divine essence generates", because the Father, who is the divine essence, generates; and "essence is from essence", because the Son, who is essence, is from the Father, who is the same divine essence. And thus explains Cyril in his book the Thesaurus: The Father, in generating the Son from his own living life and from his truly existing essence, as from the true source, naturally gives him his natural life and essence. And so when it is said that the Father has begotten his nature in the Son, this expresses that by generation he has given the Son his nature, as we see from the aforementioned words of Cyril.

How it should be understood when it is said that Jesus is the Son of the Father's essence.

From this it is clear what is meant when the same Cyril in the same book says: So how would Jesus, the Son of the essence of the Father, be a creature? He is not called the Son of the Father's essence, as if he were begotten from the Father's essence, but as if receiving the Father's essence through generation. And in this way are explained all similar expressions that one may find; such as when it is said that the Son or the Spirit essentially proceed, in so far as in proceeding they receive the essence from the Father.

How it should be understood when it is said that what naturally belongs to the Father, naturally belongs to the Son.

There may be doubt concerning what Cyril says in the same book, the Thesaurus: All that is proper to the Father by its nature is also proper to the Son. Either this is understood to refer to essential attributes, which are proper neither to the Father nor to the Son individually, but common to both; or to the persons, and thus what is proper to the Father, is not proper to the Son, such as innascibility and paternity, which belong in no way to the Son, but solely to the Father. However, it is clear from the foregoing that what is being spoken of by Cyril are the essential attributes. Indeed, he puts forward that whatsoever by its nature is said to be in the Father is also in the Son, such as life, truth, light and things of that kind. However, these are said to be proper to the Father not in relation to the Son, and nor to the Son in relation to the Father, but to both in relation to creatures, to which, in comparison to God, the aforesaid things do not properly befit; or proper, not in the sense of befitting one [Person] alone, but in the sense that they properly and truly befit the Person in himself, [as a member of the Trinity].

How it should be understood when it is said that the perfection of the Father does not lack anything from either the Son or the Holy Spirit.

Likewise there may be doubt concerning what Athanasius says in the epistle to Serapion, that the Father, by himself and in himself lacking nothing, exists as full and perfect God, and for his perfection needs nothing from the Son or the Holy Spirit. That the Father lacks nothing, there is no doubt: and the same is true of the Son or the Holy Spirit. Properly speaking, one is lacking when, considered in oneself, there is something missing from one's perfection: this can be said neither of the Father, nor of the Son, nor of the Holy Spirit. However, the Father is not perfect unless he has the Son, because he cannot be the Father without the Son, nor would he be perfect God if he did not have the Word, or if he did not have the spiramen of life, as the same Athanasius says in the third sermon from the acts of the Nicene Council, replying to the Arians, who denied that the Son and the Holy Spirit are coessential with the Father: they say the nature of the Father is sterile and unfruitful, he who gave all things a similar innate and propagative nature. And they make the Father mute and without Word, he who gave to all rational creatures the faculty of speaking. And they say the Father himself is dead, and free from the nature of the living, insofar as they deny that the Holy Spirit is coessential with the Father. And so it is clear that God the Father is not perfect unless he has the Son and the Holy Spirit. The same Athanasius says in the epistle to Serapion that the Father not could create a creature except through the Word, and in deifying creatures he cannot communicate himself except through the same Word: and, similarly, neither can the Son exist without in the Holy Spirit. Therefore it is common to the Father and to the Son and to the Holy Spirit that none of them are lacking. Likewise it is common to each of them that without the other two they cannot be perfect God. Athanasius is right to use this reason to show that the Father's perfection does not require anything from the Son and the Holy Spirit, because his perfection does not come from another; however, the Son and the Holy Spirit have their perfection from the Father. For this reason the same Athanasius says in the epistle to Serapion: it is not because of the Son or of the Holy Spirit that the Father exists as perfect and blessed God. Neither from above does he have what he is, nor from below does he have what he has, that is, he has nothing from the Son or the Holy Spirit.

How it should be understood when the Holy Spirit is called unbegotten.

Likewise it seems that there may be doubt concerning what Gregory of Nazianzus says in the sermon on the Epiphany (ch. xii) [see also ch. viii], that the Holy Spirit proceeds, and is not begotten like the Son, being between the Unbegotten and the Begotten. But it does not seem that the Holy Spirit can be said to be unbegotten. For Hilary says in the book on the councils that if one says that there are two unbegotten, it would mean there are two Gods. And Athanasius says in the epistle to Serapion that the Holy Spirit is not unbegotten, because the Catholic Church congregated at Nicaea rightly and faithfully attributes being unoriginate and unbegotten to God the Father alone, and solely of the Father is this to be believed and preached, as commanded to the entire world under penalty of anathema. But there are two ways to say that one is unbegotten. The first way is when one lacks a principle, and this befits the Father alone, as is clear from what Athanasius says. The other way is when one is not begotten while nonetheless having a principle, and it is in this sense that not only Gregory of Nazianzus in the aforementioned words, but also Jerome in the prescriptions against heretics, says that the Holy Spirit is unbegotten.

How it should be understood when the Holy Spirit is said to be the middle between the Father and the Son.

Likewise, in the words of the aforesaid Gregory of Nazianzus, there may be doubt when it is said that the Holy Spirit is between the Unbegotten and the Begotten, that is between the Father and the Son, when he is more often called the third, or the third Person in the Trinity, as said above. But he is not said to be in the middle according to the order of enumeration, which corresponds to the order of origin, since in this sense the Son is between the Father and the Holy Spirit; but he is said to be in the middle as if he were the common link between the two others: for he is the mutual love between the Father and the Son. And Epiphanius expresses this same thought when he says in his book on the Trinity that the Holy Spirit is in the middle, between the Father and the Son.

How it should be understood when it is said that the Holy Spirit is the image of the Son.

Likewise in many places it is said by these authorities that the Holy Spirit is the image of the Son, for example Athanasius in the third sermon of the Nicene Council says that the Holy Spirit is called the deifying and vivifying image of the Father and the Son, and it is true, he is the image of the Son, containing him essentially in himself in every way, representing him by nature, as the Son is the image of the Father; and in the epistle to Serapion: the Holy Spirit contains the very Son in himself by his nature, as his true and natural image. Likewise Basil: the Holy Spirit is called finger, spiramen, unction, breath, mind of Christ, procession, production, mission, emanation, effusion, vapor, brightness, image, character, true God; and again: the Holy Spirit, the third from the Father and the Son, exists as the true and natural image of the Father and of the Son, by his nature representing both of them to us.

However, among the Latins it is not customary to say that the Holy Spirit is the image of the Father or of the Son. For Augustine says in book vi on the Trinity that only the Son is known as the Word, and that it is as in image that he is called the Word, and that the Son alone is the image of the Father, because he is the Son. And Richard of St. Victor in his book on the Trinity explains why the Holy Spirit cannot be called an image, as the Son is; it is, of course, because, though he is the same as the Father, as well as the Son, in nature, nonetheless he does not participate with him in certain relative properties, as the Son does with the Father in the active spiration of the Holy Spirit. Some say that on account of this the Holy Spirit cannot be called an image, because he would be the image of two, namely the Father and the Son: but one cannot be the image of two. And in the authority of the Sacred Scriptures, which no one can surpass when speaking of the divine, it is expressly stated that the Son is the image of the Father: for it says in Colossians 1:13: he has translated us into the kingdom of the Son of his love, in whom we have remission of sins, who is the image of the invisible God; and in Hebrews 1:3, it is said of the Son: who being the brightness of his glory and the figure of his substance.

But we must be aware that the Greek saints cite two authorities from the Sacred Scriptures, which seem to say that the Holy Spirit is the image of the Son. For Romans 8:29 says: whom he foreknew, he also predestinated to be made conformable to the image of his Son. But it would seem that the image of the Son is nothing other than the Holy Spirit. Likewise 1 Corinthians 15:49 says: as we have borne the image of the earthly, let us bear also the image of the heavenly, that is, the image of Christ; by that image is understood the Holy Spirit, although in these authorities the Holy Spirit is not expressly called an image. For we can understand from this that men become conformed to the image of the Son, or that they bear the image of Christ insofar as these same holy men, through the gift of grace, in order that they might be similar to Christ, are perfected, according to the apostle in 2 Corinthians 3:18: we all, beholding the glory of the Lord with open face, are transformed into the same image from glory to glory, as by the Spirit of the Lord. It does not say that this image is the spirit of Christ, but something from the Spirit of God that exists in us. But because it would be presumptuous to contradict the clear authority of so many doctors, we may indeed say that the Holy Spirit is the image of the Father and of the Son, understanding by image nothing other than that which has its being from another and bears its likeness. If, however, by image we understand something that is draws its existence from another, and producing, from the very cause of its origin, the likeness of the cause of its existence, in so far as it comes from the other, such as the Son begotten, or the Word conceived, then in this case only the Son can be said to be an image: for it is of the nature of the Son to represent the likeness of the Father in every way; and similarly it is of the nature of the Word to be a likeness of that which is expressed by the Word, from whomever the Word comes; but is not of the nature of the spirit or of love to be a likeness of that which holds all that it is. But this is true only in the Spirit of God on account of the unity and simplicity of the divine essence, from which it follows that whatsoever is in God, is God. The essence of the image is not destroyed, because there are in the Holy Spirit certain personal properties that do not befit the Father, because the likeness and the equality of the divine persons does not depend on the properties of the persons, but solely on the essential attributes. And we should not say that there is inequality or dissimilitude according to the different personal properties in the divine persons, as Augustine says in his book against Maximinus. And when it is said that the Son is begotten from the Father, we do not mean to say that there is inequality in substance, but rather in the order of nature. This is not contrary to the Holy Spirit proceeding from the other two persons. For he proceeds from the two in so far as they are one, for the Father and the Son are a single principle of the Holy Spirit.