Speaking of the God-Man:
The Communicatio Idiomatum in St. Thomas Aquinas
By Hilaire K. Troyer de Romero, STAGS Master (Cand.)
Copyright of Ite ad Thomam © 2012
In ST III.16, St. Thomas Aquinas discusses the question of what may be said about Christ. We find this treatment in the Summa situated well after Aquinas’ thorough explanation of the Hypostatic Union because what may be said of Christ follows directly from the reality of the Hypostatic Union. More specifically, the answer lies in the concept known as the communicatio idiomatum (or ‘communication of properties’). This concept refers to an ontological, and not just logical, reality in Christ. It can be defined as “the mutual exchange of divine and human properties in virtue of the Hypostatic Union.” As a logical reality, it is sometimes referred to as the ‘predication of properties’. Sacred Scripture is replete with practical examples of this concept. Misunderstandings of the communicatio idiomatum have been at the heart of numerous Christological heresies such as Nestorianism and Monophysitism. In this paper, I shall explain the principal elements of the communicatio idiomatum as presented by St. Thomas in ST III.16, beginning with its direct connection to the Hypostatic Union, and then a brief overview of the general rules that can be gleaned from St. Thomas’ treatment of this issue.
I. Communicatio idiomatum and the Hypostatic Union
In the first article of question 16, St. Thomas begins his treatment by examining a fundamental Christological statement, “God is man.” Here Aquinas establishes the immediate connection between the communicatio idiomatum and the Hypostatic Union. He says,
Hence, supposing the truth of the Catholic belief, that the true Divine Nature is united with true Human Nature not only in person, but also in suppositum or hypostasis; we say that this proposition is true and proper, ‘God is man’ –not only by the truth of its terms, that is, because Christ is true God and true man, but by the truth of the predication. For a word signifying the common nature in the concrete may stand for all contained in the common nature, as this word ‘man’ may stand for any individual man. And thus this word ‘God,’ from its very mode of signification, may stand for the Person of the Son of God, as was said in I.39.4.
The “Catholic belief” referred to here is the Hypostatic Union. For the sake of clarity I shall review a few key terms found in the text above. A suppostium or hypostasis is a bearer of properties, that is, any primary, or individual, substance that subsists in itself. Aquinas refers to the suppositum or hypostasis as a “subsistence,” rather than as a mere substance, to express the connotation of “subsisting in itself.” A person is a particular kind of hypostasis or suppositum, namely, an individual substance of a rational nature. If we speak of an ordinary man, such as Aristotle, the suppositum or hypostasis is Aristotle himself; and that hypostasis has one human nature. The suppositum is the “who” (the quis est), namely Aristotle, and the nature is the “what” (quid est), namely a man. The two are not identical: Aristotle subsists in the human nature but we do not say that the nature is Aristotle, since this would be the same thing as saying that “Aristotle is humanity,” which is patently false. In the case of Christ, the suppositum or hypostasis is the quis est, the Divine Second Person, who subsists in the two natures, which are united substantially.
The communicatio idiomatum is the logical consequence of the perichoresis, or unity of the two natures in Christ, that results from the Hypostatic Union. That is, there is a natural interchange of properties between the two, because of the substantial union whereby the divine nature penetrates the Human Nature (without changing it) within the one Divine Personality or hypostasis that we call the Logos.
Returning to the text at hand, the proposition “God is man” is a true statement and not merely in an analogical or metaphorical sense, in the way many heresies have interpreted it, precisely because of this Union of One Person and two natures. In other words, when the subject of a sentence refers to the Second Person of the Trinity, it is possible to predicate of that subject the concrete properties of either nature, regardless of whether the term in the subject of the sentence connotes the divine nature or the human nature of Christ. Since the Person of the Son of God for Whom this word ‘God’ stands, is a suppositum of human nature this word “man” may be predicated truly and properly of this word ‘God’. Just as it is possible to say that “Socrates is man,” it is possible to say that “God is man,” when by “God” we mean “The Second Person.” In the statement “God is man,” the term ‘man’ is being predicated of the Divine Person, the Son of God as of the suppositum, and not of the divine nature itself. This suppositum has both human and divine natures and hence both human and divine properties. The inverse statement, namely, “Man is God,” which Aquinas examines in article 2 of question 16, is also true because the term “man” can refer to any hypostasis that has a human nature. So it can refer to the Second Person as the subject who, as I mentioned above, has both human and divine natures. The statement would be false, of course, if the term ‘Man’ were to refer to all men, or any man other than Christ, such as Socrates, because then it would refer to a suppositum that possesses human nature only.
Throughout question 16, Aquinas examines particular statements regarding Christ and asks whether or not they are true. The result of his discussion of each statement is an overall framework, or a set of rules, that govern the communicatio idiomatum. Below I address the primary rules that can be gleaned from question 16, which follow as logical consequences of the particulars of the Hypostatic Union.
A. Only Concrete Terms of Concrete Subjects May Be Predicated of Either Nature
The most fundamental of these rules arises from the distinction between concrete and abstract terms, as St. Thomas explains in Article 1. Concrete terms are those that refer to a property as it exists in a subject (man, carnal, animate, etc.), whereas abstract terms refer to properties in se, apart from a subject (divinity, humanity, truth, etc.). This means that the concrete term will signify the suppositum and abstract terms will signify the nature apart from the suppositum
Aquinas, in his reply to Objection 2 in Article 1, says,
[I]n the mystery of the Incarnation the Natures, being distinct, are not predicated one of the other, in the abstract. For the divine nature is not the human nature. But because they agree in suppositum, they are predicated of each other in the concrete.
Fr. Garrigou-Lagrange formulates this rule thus:
[C]oncrete words of concrete subjects, both of natures and properties, generally speaking, can of themselves be predicated of either [nature]; but abstract words of abstract subjects cannot of themselves formally be predicated of either.
In other words, because concrete terms directly signify the suppositum and only indirectly the nature, they may be predicated of both the divine and human natures of Christ. Abstract terms, however, may only be predicated of both natures when they refer to divine properties, because only in the divine nature is there an identity between its properties and the nature itself. For example, we may say that “Christ is Truth,” because the Second Person by virtue of His Divinity is indeed Truth itself. But we cannot say that “Christ is humanity” because in this case, the abstract term “humanity” is not identical with the divine suppositum.
B. Reduplication Limits the Concrete Term to One Nature
Another general rule is that we must exercise great care when there is an instance of reduplication such as in the statement, “Christ, as man is a creature.” In such instances the reduplication limits the concrete term to one particular nature so that what is predicated must be true of that nature by itself. For example, in article 10, Aquinas examines the truth of the statement, “Christ, as man, is a creature.” Here he affirms the statement because the reduplication “as man” means the concrete term “creature” refers to the human nature specifically, which is indeed created.
It must however be borne in mind that the term covered by the reduplication signifies the nature rather than the suppositum, since it is added as a predicate, which is taken formally, for it is the same to say “Christ as Man” and to say “Christ as He is a Man.” Hence this is to be granted rather than denied: “Christ as Man is a creature.” But if something further be added whereby [the term covered by the reduplication] is attracted to the suppositum, this proposition is to be denied rather than granted, for instance were one to say: “Christ as ‘this’ Man is a creature.”
Following the same guideline, in article 11, Aquinas points out that the statement “Christ, as man, is God” would be false, because the reduplication limits the concrete term “God” to the human nature alone which, though possessed by God in the Second Person, is not identical to God. In some instances, the reduplication adds clarification, as in the first example found in article 10, and in others it makes an otherwise true statement false—as seen with the example from article 11.
C. Essential Properties of the Divine Nature Must Not be Predicated denominatively of Christ
A third rule is that adjectival names, in the concrete, that are derived cannot always be accurately predicated of Christ. For example, we cannot call Christ “lordly.”  Aquinas says in ST III.16.3:
Now ‘God’ and ‘Lord’ are predicated essentially of the Son of God; and hence they ought not to be predicated denominatively, since this is derogatory to the truth of the union. Hence, since we say ‘lordly’ denominatively from ‘lord’, it cannot truly and properly be said that this Man is lordly, but rather that He is Lord. 
In other words, to call Christ ‘lordly’ would imply that He is not God because, to use the word ‘lordly’ with respect to Christ, either (a) is not fitting to Him who is Lord of Lords and therefore derogatory, and fails to show a true belief in the Hypostatic Union, or (b) is indicative of mere god-likeness, which is heretical, as the Church has already decreed many times since its condemnation of Arianism.
D. Phrases that Sound Heretical Should be Avoided
Although the communicatio idiomatum does provide us with a great amount of freedom with regard to what we say about Christ, St. Thomas warns us against those statements that may be technically true but that can easily be interpreted in a heretical way. For example, if we were to say, “Christ is a creature,” we run the risk of sounding like the Arians, who believed Christ was completely created and, therefore, less than the Father. If what we mean to say is that Christ’s human nature is created, it would be better expressed with a clarification, such as that which Aquinas suggests in article 8: “And hence we must not say absolutely that Christ is a ‘creature’ or ‘less than the Father’; but with a qualification, viz. ‘in His human nature’." In all cases, our use of language concerning Christ must be faithful to the reality of the Hypostatic Union. Those predicates that appear ambiguous should always be clarified so as to avoid heretical interpretations. It is always of the utmost importance that the language we use display logical congruence with the Hypostatic Union and the divinity of Christ. Aquinas says, “As Jerome [Gloss, Ord. in Hosea 2:16] says, ‘words spoken amiss lead to heresy’; hence with us and heretics the very words ought not to be in common, lest we seem to countenance their error.”
In sum, the communicatio idiomatum is simply a consequence of the Hypostatic Union. Because the two natures in Christ are united in the one suppositum, the Son of God, we may predicate what belongs to one nature of the other in the concrete. Yet the two natures are not intermingled or confused, so we must always ensure that the rules discussed above are always observed, so as not to imply a lack of unity or a confusion of the natures in Christ. Joseph Pohle summarizes the communicatio idiomatum as a logical reality thus:
Formulated in logical terms, the ontological law underlying the communicatio idiomatum gives us the following rule of predication: “Whatever is predicated of the Divine Person of Christ according to His Divine Nature, can and must be predicated of the same Divine Person also in His human nature, and vice versa; but the predicates proper to the Divine Nature must not be assigned to the human nature, and vice versa.” The first part of this rule is based on upon the unity of the one Divine Person in two natures; the second, upon the fact that the two natures co-exist separately and in-confused in one Person.
It is on the basis of this principle that the Church was able to develop her Christological doctrine and combat the heresies that have so relentlessly assailed her. St. Cyril of Alexandria in his third letter to Nestorius, expresses this in a profound way:
For we do not divide up the words of our Saviour in the gospels among two hypostases or persons. For the one and only Christ is not dual, even though he be considered to be from two distinct realities, brought together into an unbreakable union. In the same sort of way a human being, though he be composed of soul and body, is considered to be not dual, but rather one out of two. Therefore, in thinking rightly, we refer both the human and divine expressions to the same person. For when he speaks about himself in a divine manner as “he that sees me sees the Father,” and “I and the Father are one,” we think of his divine and unspeakable nature, according to which he is one with his own Father through identity of nature and is the “image and impress and brightness of his glory.” But when, not dishonouring the measure of his humanity, he says to the Jews: “But now you seek to kill me, a man who has spoken the truth to you,” again no less than before, we recognise that he who, because of his equality and likeness to God the Father is God the Word, is also within the limits of his humanity. For if it is necessary to believe that being God by nature he became flesh, that is man ensouled with a rational soul.... All the expressions, therefore, that occur in the gospels are to be referred to one person, the one enfleshed hypostasis of the Word. For there is one Lord Jesus Christ, according to the Scriptures.
Council of Ephesus.
Garrigou-Lagrange, Reginald. De Christo Salvatore. Turin: R. Berruti, 1948. <http://www.ewtn.com/library/theology/christ.htm>
Ott, Ludwig. Fundamentals of Catholic Dogma. St. Louis: B. Herder, 1964.
Pohle, Joseph; Arthur Preuss (trans., ed.). Christology: A Dogmatic Treatise on the Incarnation. St. Louis: B. Herder, 1916.
Tanquerey, Adolphe. A Manual of Dogmatic Theology. Translated by John J. Byrnes. New York: Desclee Company, 1969.
The Holy Bible (Douay- Rheims Version). London: Baronius Press, 2003.
Thomas Aquinas, Saint. Summa Theologica. Notre Dame, IN: Ave Maria Press, 1981.
 Cf. John 8:58; Acts 3:15, 2:8; Romans 8:32, 6:3; I Cor. 2:8;
 Summa theologiae (henceforth, ST) III.16.1.
 Cf. ST I.29.2c: “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.”
 Cf. ST I.I.29 Aquinas sometimes uses the term hypostasis to include the concept of “rational nature.” It seems that the terms suppositum, hypostasis, and person, are used interchangeably.
 Cf. ST III.2.2c.
 This by the grace of union that imparts the Personal Being of the Word on the human nature of Christ. Cf. Adolphe Tanquerey, A Manual of Dogmatic Theology, Vol. 2, p. 28.
 Cf. Pohle & Preuss, p. 184: “Though practically identical with Perichoresis, the Communication of Idioms may more appropriately be regarded as an effect thereof.”
 St. Thomas references the errors of Photinus and Nestorius specifically in ST III.16.1.
 Adolphe Tanquerey, A Manual of Dogmatic Theology, Vol. 2, p. 28.
 Cf. Ludwig Ott, Fundamentals of Catholic Dogma, p. 161: “communication idiomatum fit in concreto, non in abstracto.”
 ST III.16.1. Cf. article 5: “Now concrete words stand for the hypostasis of the nature; and hence of concrete words we may predicate indifferently what belongs to either nature--whether the word of which they are predicated refers to one nature, as the word “Christ,” by which is signified “both the Godhead anointing and the manhood anointed”; or to the Divine Nature alone, as this word “God” or “the Son of God”; or to the manhood alone, as this word “Man” or “Jesus.” Hence Pope Leo says (Ep. ad Palaest., cxxiv): “It is of no consequence from what substance we name Christ; because since the unity of person remains inseparably, one and the same is altogether Son of Man by His flesh, and altogether Son of God by the Godhead which He has with the Father.”
 Garrigou-Lagrange, Christ the Saviour, Ch. 18, Subsection: “The Consequences Of The Union As Regards Those Things That Belong To Christ In Himself”.
 Ibid.: “It must be observed concerning this communication that concrete names, such as God, man, in opposition to abstract names, such as Godhead, humanity, signify directly the suppositum, and indirectly the nature. For “God,” signifies the suppositum that has the divinity, and “man” signifies the suppositum that has the humanity. If, therefore, the suppositum is the same for the two natures, then it is true to say: “God is man,” although it is false to say: “The Godhead is the humanity.” Thus we shall see that the generally accepted rule, namely, concrete words of concrete subjects, both of natures and properties, generally speaking, can of themselves be predicated of either; but abstract words of abstract subjects cannot of themselves formally be predicated of either. Thus we shall see that we cannot say the Godhead is the humanity or that God is the humanity, or that the humanity is God. Therefore we must take great care to distinguish between abstract terms and concrete terms. The abstract term signifies the nature separated from the subject, for example, humanity. The concrete term signifies the nature as existing in the subject, for example, man. Hence this distinction between concrete and abstract term is of great importance in distinguishing between the nature and the suppositum, since the nature is an essential part of the suppositum. There is the same distinction between “being” as a noun and “being” as a participle, or between the reality and the real itself.”
 Ludwig Ott, Fundamentals of Catholic Dogma, p. 161.
 ST III.16.10.c.
 Adolphe Tanquerey, A Manual of Dogmatic Theology, Vol. 2, p. 28.
 ST III.16.3; cf., Pohle & Preuss, pp. 188-189; Adolphe Tanquerey, A Manual of Dogmatic Theology, Vol. 2, p. 28.
 ST III.16.8c.
 Pohle & Preuss, p. 186.
 Council of Ephesus, “Letter of Cyril to Nestorius” (Cum salvator noster).