Introduction and Preliminary Considerations
The “two natures, one person” orthodoxy forged by the Council of Chalcedon seems immediately relevant to the profession and content of the original truths of the Christian faith. However, the more ontologically nuanced and exacting precisions of later centuries may not present themselves to modern ears as evidently continuous with the mindset of the first followers of Christ. In the face of the questions raised about the unity of Christ and his wills and operations, the Second and Third Councils of Constantinople, the Second Council of Nicaea, and the later Greek fathers and schoolmen offer much in the way of developing the full implications of the basic biblical truth that the “Word became flesh.”Footnote 1 Receiving the later contributions as organic developments of biblical faith and the earlier creeds is not a task that modern theologians have found easy to accomplish, or even desirable to undertake. Pondering this challenge, then-Cardinal Joseph Ratzinger makes the following observation:
It is common enough for the theological textbooks to pay scant attention to the theological development which followed Chalcedon. In many ways one is left with the impression that dogmatic Christology comes to a stop with a certain parallelism of the two natures in Christ…. In fact, however, the affirmation of the true humanity and the true divinity in Christ can only retain its meaning if the mode of the unity of both is clarified. The Council defined this unity by speaking of the “one Person” in Christ, but it was a formula which remained to be explored in its implications. For the unity of divinity and humanity in Christ which brings “salvation” to man is not just a juxtaposition but a mutual indwelling.Footnote 2
Ratzinger goes on to document how in the centuries following Chalcedon, individuals like Maximus the Confessor and councils like the Third of Constantinople (680–681) developed the metaphysical “oneness” of Christ’s personhood without “amputating” one of the natures or leaving them in a parallel dualism.Footnote 3 The ongoing debate over the meaning and implications of the fourth article of Thomas Aquinas’s disputed question De unione verbi incarnati lies at the heart of this sustained, centuries-long, post-Chalcedonian development of the mode of the union of Christ’s two natures in the single subsistent Person of the Word.Footnote 4
The De unione stands somewhat as a special test case in contemporary theology for how Aquinas fits in the tapestry of post-Chalcedonian christology because some scholars argue that he offers something novel (and better) in one line of one article of this work than what is found in the rest of his corpus.Footnote 5 The intrigue around this one line in the fourth article of De unione verbi incarnati can be summarized as follows: In what is now held to be a rather late work,Footnote 6 many scholars have argued that Thomas breaks with his otherwise consistent position that Christ is one and unified in the order of being—that there is one esse, or being, in Christ—and affirms a second esse, or being, that is contributed to by Christ’s human nature.Footnote 7 Some of these scholars argue that this change in formulation saves Thomas’s christology from the error of Monophysitism.Footnote 8
To provide just one example of his standard, single esse formulation, in the Compendium of theologiae, Thomas lays out his position as follows: “If, therefore, we consider Christ as a complete suppositum having two natures, there will be only one being (unum esse), just as there is but one suppositum.”Footnote 9 Variations of this unum esse formulation are standard throughout his corpus, except in the one line in the fourth article of De unione. In this line, Thomas says that although the being of Christ’s human nature “is not accidental being—because man is not accidentally predicated of the Son of God, as was said above—it is nevertheless not the principal being of its suppositum, but a subordinated [secundarium] being [esse].”Footnote 10
As a consequence of this unique formulation, many contemporary theologians have received the De unione as a kind of late-career recognition by Thomas that the single esse position of the rest of his corpus was problematic, unnecessary, and in need of revision.
In the remainder of this paper, the claim that the unique formulation found in De unione article four is a reversal of the single esse position will be challenged. The conclusion reached is that a reading of all five articles of the De unione as a carefully structured argument reveals a thought-out single esse understanding of the Incarnate Word. What is ultimately at stake in this dispute is the truth of the hypostatic nature of the union between God and man in Christ. As Thomas underscores, the positing of a second esse in Christ not only conflicts with the conciliar tradition that he appropriates, but it would also compromise the very truth of the hypostatic union itself.
Two Preliminary Aids
As a primer to Thomas’s thinking in the five articles of the De unione, two items need to be reviewed so that the reader can fully appreciate the landscape of the discussion.
First, from the perspective of christological orthodoxy, considerations about the unity of Christ in the order of esse must be appreciated in relation to the teachings of the councils of the first millennium. The great fathers and councils that navigated the challenges of Arianism, Nestorianism, Monophysitism, and later heresies related to Christ’s operations established more than just the grammatical parameters of orthodoxy.Footnote 11 The pivotal Council of Chalcedon in 451, for example, which developed the anti-Nestorian teaching of Ephesus (431) against the Monophysites, qualified the grammar of orthodoxy with a firm doctrine of Christ’s unity. Immediately following the profession of the unconfused and unmixed union of the two natures in the one Christ, the Chalcedon declaration added:
At no point was the difference between the natures taken away through the union, but rather the property of both natures is preserved and comes together into a single person (prosopon/personam) and a single subsistent being (hypostasin/subsistentiam); he is not parted or divided into two persons, but is one and the same only-begotten Son, God, Word, Lord Jesus Christ.Footnote 12
Further clarification on Chalcedon’s teaching that Christ was a “single subsistent being” was a priority of the teaching of Constantinople II (553) against erroneous attempts to recognize a second subsistence in Christ. This point is deeply connected to the question of Christ’s esse that is best understood as a development of the single subsistence doctrine. In Anathema five, the Council teaches:
If anyone understands by the single subsistence of our lord Jesus Christ that it covers the meaning of many subsistences, and by this argument tries to introduce into the mystery of Christ two subsistences or two persons, and having brought in two persons then talks of one person only in respect of dignity … and if he does not acknowledge that the Word of God is united with human flesh by subsistence, and that on account of this there is only one subsistence or one person, and that the holy synod of Chalcedon thus made a formal statement of belief in the single subsistence of our lord Jesus Christ: let him be anathema.Footnote 13
Anathema four of the same council also shows how Christ’s unity is preserved despite the presence of two real natures: “The holy Church of God … states her belief in a union between the Word of God and human flesh which is by synthesis (secundum compositionem), that is by a union of subsistence (quod est secundum subsistentiam). In the mystery of Christ the union of synthesis (per compositionem) not only conserves without confusing the elements that come together but also allows no division.”Footnote 14 This teaching on the composite mode of existence that the Word has as true God and true man according to a single subsistence is, of course, much more than a grammar of orthodoxy.
It is this quest to articulate Christ’s unity in the centuries following Chalcedon that fueled the continued speculations of late-patristic and early-Byzantine figures such as Leontius of Byzantium, Maximus the Confessor, and John of Damascus.Footnote 15 For example, in his famous fifth Ambiguum to Thomas, which is a defense of Dionysius against the charge of monoenergism, Maximus the Confessor articulates Christ’s divinity and humanity as two modes of a single unified existence and not two numeric existences. “[The Word] . . . [h]aving united His transcendent mode of existence,” Maximus explains, “with the principle of His human nature, so that the ongoing existence of that nature might be confirmed by the newness of the mode of existence, not suffering any change at the level of its inner principle, and thereby make known His power that is beyond infinity, recognized through the generation of opposites.”Footnote 16 This post-Chalcedonian development on Christ’s unity left a deep imprint on Thomas’s thinking about Christ.Footnote 17 Aquinas’s knowledge of the Greek patrimony of Christian theology, read through Latin translations, in the words of Gilles Emery, “visibly makes its mark in the structure [of his] Christology.”Footnote 18 Although Aquinas is often viewed as a quintessential Latin schoolman, his use of Greek patristic sources, Emery explains, “designates Thomas as a pioneer: He was the first Latin Scholastic,” for example, “truly to exploit Constantinople II in Christology and exegesis. His knowledge of the Third Council of Constantinople is no less evident.”Footnote 19 Disputes over Thomas’s position on Christ’s unity of being need to take a fuller account of his unique reception of these influences and later Greek developments.Footnote 20 Corey Barnes notes that Thomas’s assimilation of these new sources and ideas about Christ’s wills and operations shifts the focus of his dyothelite christology away from accenting the perfection of Christ’s human nature to “elevating as central a proper understanding of the hypostatic union.”Footnote 21 This Greek-inspired development, which focused his thought on how to account for Christ’s unity, sets Aquinas’s various articulations within a broader framework than the scholasticism of the thirteenth century.
Because the metaphysics of esse considers a thing from the perspective of its existence qua existence as the highest perfection that makes each thing to be and not from a particular aspect of its being, such as the substance underlying a thing, it is important not to conflate esse with subsistence. The claim of this paper is not that the medieval metaphysics of esse and the doctrine of Christ’s single subsistence are exactly the same. The metaphysics of esse is a higher and more terminal understanding of a subsistent entity. In a question meant to parse the received definitions on the commonality and difference between theological terms like person, hypostasis, and suppositum, Thomas clarifies the meaning of subsistence:
[Substance] 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.Footnote 22
The question, therefore, of Christ’s per se existence (subsistence) and the singularity of his esse must be intimately related. Without a second subsistence, what would a second esse be the perfection of in the order of existence? While not identical with the medieval metaphysics of esse, Maximus the Confessor’s distinction, as noted above, between Christ’s unity of existence and dual (divine and human) “modes” of that one existence presses the same point that Aquinas is attempting to spell out. Indeed, some criticisms of Thomas’s single esse position would equally rule out the single subsistence doctrine of Chalcedon and its later developments, as would the addition of a second esse. Footnote 23
As a second primer, some reference to the christological framework established by Peter Lombard is crucial to understanding what Thomas and the other schoolmen were trying to pinpoint in their speculations about Christ’s esse and unity. Lombard dutifully received the christology of the first millennium and endeavored to give a scholastic account of Christ’s manner of existence.Footnote 24 Thomas makes direct reference to Lombard’s famous three opinions about the hypostatic union in De unione, so Lombard’s work is not far from his mind. Footnote 25 Lombard frames the speculative challenge posed by the incarnation with the following question: “Whether a person or nature took on a person or nature, and whether God’s nature became flesh.”Footnote 26 While Lombard’s own position on the orthodox doctrine of the incarnation was something disputed even by his own disciples in the twelfth century, Thomas is clear in the De unione (and the Summa theologiae) that the Second Opinion, the “subsistence theory,”Footnote 27 is not merely an opinion but the faith of the Catholic Church.Footnote 28 A less appreciated but equally important point that Lombard leaves for subsequent generations to clarify is the status of Christ’s human nature. Having laid out the “opinions” of the past generations on the hypostatic union, Lombard raises what later is referred to as the problem of “christological nihilianism.” This problem serves as a catalyst for the speculations about Christ’s esse.Footnote 29 It is called “nihilianism” because of Lombard’s formulation of the problem: “Whether Christ, according to his being a man, is a person or anything. It is also usual for some to ask whether Christ, according to his being a man, is a person, or even is anything.”Footnote 30 Lombard’s parsing unveils the possible answers to this mercurial question: either Christ is a second person as man, which would negate the union, or he is a person as a man but not a numerically second one, which would make him, as a man, Lombard reasons, a person of the Trinity, which would then make him, as a man, God.Footnote 31 So, if he is not a person according to his being a man, then, Lombard wonders whether, as a man, he is anything at all. In short, Lombard understood that in the order of being, without positing two persons, which vacates the union, it is difficult to account for both the divinity and the humanity of Christ with just one person and being for each.Footnote 32
The Greek patrimony of Christ’s composite mode of existence as a single subsistence and Lombard’s struggle to account for the human nature of Christ are two loci that spurred on medieval theologians like Thomas Aquinas to wrestle with the question of Christ’s unity in the order of being.Footnote 33
Reasons for a Single Esse Reading of the De unione verbi incarnati
The Argumentative Progression of the Articles Does not Allow a “Second Esse” Reading of Article Four
Many of the scholars who refer to article four to claim that Thomas abandoned a single esse position cling to his use of the word “secundarium” without any reference to other articles of the disputed question.Footnote 34 In fact, each preceding point leading up to article four and the one article that follows it hem the consideration into very narrow parameters rooted in the doctrine of the single composite subsistence of the Word as God and man. A sketch of the key hinges in this progression demonstrates why the second esse arguments betray Thomas’s actual position.
In the first article, Thomas treats the mode of the hypostatic union in a fashion that is consistent with his other works, with his preference for the subsistence theory among the Three Opinions, and with the influence of the Greek fathers and early councils on his thought. Considering seventeen speculative objections that assert that a union between God and man in Christ must result in a conflation of the natures into something new or be only accidental, Thomas develops the distinction between person and nature to clarify precisely how the Incarnate Word is truly God and man. Having established that nature is the specific difference that makes something the certain kind of being that it is, and that this person is a concretely subsisting suppositum of a rational nature, Thomas draws the following conclusion: “nothing prevents some things that are not united in a nature from being united in a person, for an individual substance of a rational nature can have something that does not belong to the nature of the species: this is united to it personally, not naturally.”Footnote 35
Because it is possible for things that are not united in a nature to be united in a person, Thomas clarifies that in Christ “the non-composite divine person subsists in two natures.”Footnote 36 He then makes a conclusion about Christ’s being, or esse, that must be kept in mind when reading the fourth article: “the being [esse] of the person of the Word Incarnate is one from the perspective of the person subsisting, but not from the perspective of the nature.”Footnote 37
Having concluded in article one that the union of the two natures is according to the single subsistence of the person of the Word, Thomas then asks in the second article whether there is hypostatic (personal) unity in Christ, and, if so, how the duality of natures are maintained. The objections that Thomas considers raise two perennial concerns: first, that anything that is one by hypostasis cannot be identified as two things without duplicating the hypostases according to each nature; and, second, that the infinite being of the hypostasis of the Word cannot be the hypostasis of Christ’s created human nature. So, the objections press the problem raised by Lombard to their logical conclusions: Christ’s human nature, as something real and not nothing, it seems, is a hypostasis. As a result, there must be two esses and no substantial union, which is the Nestorian heresy; or Christ’s human nature takes on the status of some accidental property in relation to the Word, which would compromise the integrity of his human mode of existence.
Thomas dismisses any attempt to reduce Christ’s human nature to an accident. However, since the nature is complete, possessing a rational soul and body, what, if anything, would prevent it from being a suppositum and hypostasis, thereby making it a numerically second esse? Thomas answers this challenge in the following way:
So, then, because the human nature in Christ does not subsist separately through itself but exists in another, that is, in the hypostasis of the Word of God—not indeed as an accident in a subject, nor properly as a part in a whole, but by means of an ineffable assumption—on that account, the human nature in Christ can indeed be called something individual or particular or singular; but nevertheless, as it is not a person, Christ’s human nature cannot be called either a hypostasis or a suppositum. Hence it remains that in Christ there is but one hypostasis or suppositum, namely, that of the Divine Word.Footnote 38
The work that Thomas does in this passage can be easily passed over without a full appreciation of its significance. Because Christ’s human nature is whole and complete, it enjoys the status of individuality and singularity in the order of substance. On the other hand, what enables Thomas’s account to avoid the Nestorian error of vitiating the union is that in the order of being the individual and singular nature, while being complete, is not a hypostasis or suppositum. This point is crucial for how one reads the formulation of article four. Thomas’s explicit reference to the condemnation by the Second Council of Constantinople of any attempt to add a second subsistence or person to the mystery of Christ in the body of this article shows a self-awareness of how his account of Christ’s unity stands in continuity with the post-Chalcedon patrimony.
In the third article, Thomas ties the ripening implications of these principles together in preparation for the direct account of Christ’s esse. The careful linguistic formulation of the third article can be difficult to penetrate for modern readers. The question that Thomas asks at the start of the article is “whether Christ is one in the neuter or two” (unum neutraliter vel duo). What Thomas is aiming for with this construction follows upon the conclusions of the previous articles. Since he is not two persons, Christ cannot be said to be alius et alius (one person and another), but, as true God and true man, could he be said to be aliud et aliud (one reality and something else)? By reference to the neuter category, Thomas opens the way for a consideration of how Christ’s hypostatic unity is related to his numeric unity (after Chalcedon, some had tried to say that Christ was one person but two things, each with its own separate subsistence). All of the fourteen objections that Thomas considers press the point of Christ’s duality of natures, offering variations of the following conclusion: “Therefore Christ is one reality and something else (aliud et aliud), and accordingly he is two.”Footnote 39
His terse response to these objections in the first sed contra is arresting and seemingly unorthodox: “Christ is not two persons nor two hypostases nor two supposita…. Christ is also not two natures since human nature is not predicated of Christ. Therefore Christ is not two.”Footnote 40 What could Thomas mean by asserting that Christ is not two natures? The specificity of the question is important for understanding his point. The one Christ certainly has two natures, but it is the one person of the Word who is both God and man. So, Christ is not two natures in the sense of being two different realities in the order of being: the union of the two natures in the one subsisting person constitutes the one Christ.
Any being can be considered one thing or many things, depending on whether or not the consideration is made from the perspective of its substantial unity or accidental or composite diversity. Thomas appeals to the distinction between something considered secundum quid (in a certain respect) or simpliciter (absolutely speaking) to demonstrate how this is true. Recognizing that a particular person is tall, dark, handsome, and skinny is to acknowledge many true things about them secundum quid. However, it does not make them many things simpliciter. Even in the order of substance, Thomas notes, following Aristotle, that two aspects are included: the “suppositum, which is not [a] predicate of something else, and [the] form or nature of the species, which is [a] predicate of the suppositum.”Footnote 41 In the case of Christ, as true particular man, the human nature is predicated of the suppositum of the Word, but not according to a per se (subsistence) or absolute standing as a person or suppositum of that particular nature but as the nature assumed by the suppositum. So Thomas concludes that “Christ can in some way be called one because he is one by the suppositum, and he can in some way be called many, or two, because he has two natures.”Footnote 42 This means that “if one certain suppositum has many substantial natures,” as is the case with Christ, “it will be one simpliciter, and many in a certain respect.”Footnote 43 Thomas clearly forges this conclusion as a first clarification of the esse question in Christ. Whatever is two or multiple in Christ is recognized secundum quid and not in the order of being.
Thus, when Thomas arrives at the famous fourth article of the disputed question, which asks “whether in Christ there is only one being (esse),” he very evidently considers the question to be answered already. So much so that, in the opening line of the body of the article, he says, “It should be said that the argument of this question is, in a certain sense, the same as that of the previous question, because something is said to be one and a being on the same grounds.”Footnote 44 Likewise, the sed contra, from whence the title of this paper is derived, invokes the distinction of the previous article to conclude that Christ is not two in the order of esse: “Everything that is one simpliciter is one according to being. But Christ is one simpliciter, as was said above. In Christ, therefore, there is one being [esse].”Footnote 45
What, then, does Thomas mean by his unique use of the word secundarium if it is not meant to indicate a second esse? It is clear that he means to qualify the truth of Christ’s human nature according to the secundum quid, or composite duality, that follows upon the Word’s possession of two existent natures.Footnote 46 In fact, he says as much in the line preceding the famous passage: “there is another being of this suppositum,” he explains, “not insofar as this other being is eternal, but insofar as the suppositum was made man temporally.”Footnote 47 Why not conclude, as many have tried to claim,Footnote 48 that the secundarium entity, the human nature of Christ, contributes esse in a numerically second sense? The words of the sentence that follow his reference to the secundarium entity make the second esse conclusion impossible by clarifying that esse cannot be recognized without a corresponding suppositum: “If, however, there were two supposita in Christ, then each suppositum would have its own principal being of itself. And thus there would be in Christ a twofold being simpliciter.”Footnote 49 Given that this is not the case, the subordinate or secondary esse mentioned clearly pertains to Christ’s secundum quid duality and does not contribute esse in a numerically second manner, because there are not two supposita in Christ.Footnote 50 John Froula’s assessment seems to capture most strongly the sense of Thomas’s intention:
Christ does have a human life and principle of his assumed created nature that is not the divine esse. There is a secondary, or subordinate, esse of the human nature of Christ that is other than the divine esse and not the divine esse, that is, the act by which Christ is human. It is esse not in the supposital sense, but in a legitimate analogical use of the word.Footnote 51
What Christ’s human nature does contribute as numerically second is specified by Thomas in the fifth and final article of the disputed question, namely, the truth of his operational duality. “It should be said,” Thomas recognizes, “that Christ is one simplicter on account of the suppositum. Nevertheless, there are two natures in him: and therefore Christ is one agent, but there are two actions in him.”Footnote 52 The human nature truly acts in a human mode; however, the truth of the human nature and its act do not constitute a second agent of action, but a second mode of being by which the one agent acts.Footnote 53 Corey Barnes recognizes that Thomas’s specification of Christ’s unity simpliciter in the order of esse and his secundum quid duality “prepares for consideration of Christ’s wills and operations.”Footnote 54 If Christ had more than one esse, his human actions would not enjoy a theandric character.
A Brief Note on Thomas’s Use of Secundarium in the “De unione”
There is a final contextual thought that indicates Thomas’s single esse mindset in this disputed question. It is understandable why readers are induced to read Thomas’s use of the word secundarium in a numerical sense, because of its semantic affinity to the number two and natural cognates from the root “second.” Translating secundarium as “secondary,” which is not inaccurate, leads the mind to think of something numerically or quantitatively second. The fact is, however, that the comparison made in De unione article four is not between that which is primus and that which is secundus, between numerically first and second things. Rather, the contrast is between that which is “predicated” of the Word Incarnate according to the suppositum and that which is predicated according to the created nature. Comparing what is principale and secundarium in Christ is a way of affirming the modes of being that the Word has through his composite natures.Footnote 55
Few scholars who have interpreted De unione article four as a break from his unum esse position have tested their readings on other places in his corpus where Thomas uses the distinction between principale and secundarium—a tool that he turns to not infrequently to clarify diverse aspects of a composite reality.Footnote 56
Thomas’s most concentrated use of this distinction is found in the section of the Prima secundae of the Summa theologiae devoted to the New Law or Gospel of Grace (qq. 106–114). It is used in this sequence as his preferred tool for clarifying the spiritual and material aspects of the New Law. In the first article (a. 106, a. 1) of this treatise, where Thomas asks whether the New Law is a “written law,” he answers: “the new law is chiefly (principaliter) the grace itself of the Holy Spirit, which is given to those who believe in Christ.” Footnote 57 Thomas teaches that the other elements of the New Law, which are not the grace of the Holy Spirit but are integrally related to it, are
of secondary [secundaria] importance, so to speak, in the New Law; and the faithful need to be instructed concerning them, both by word and writings, both as to what they should believe and what they should do. Consequently we must say that the New Law is in the first place [principaliter] a law that is inscribed on our hearts, but that secondarily [secundario] it is a written law.Footnote 58
Likewise, in a subsequent article on the same question, Thomas makes use of the same distinction to establish the unity of the New Law while also affirming its composite nature: “There is a two-fold element in the Law of the Gospel,” Thomas teaches, “there is the chief element [principaliter], viz., the grace of the Holy Spirit bestowed inwardly…. The other element of the Evangelical Law is secondary [secundario]: namely, the things of faith, and those commandments which direct human affections and actions.”Footnote 59
Thomas returns to this distinction numerous times throughout this sequence of questions to accentuate the primacy of the grace of the Holy Spirit in relation to the material components of the New Law.Footnote 60 It is clear from this usage that Thomas in no way intends to identify those secondary (or subordinate) aspects of the New Law that are related to the grace of the Holy Spirit as a numerically “second” law. Rather, Thomas invokes this comparison to accentuate the relationship of the two aspects, spiritual and material, of the New Law as one law with an ordering to the primary reality of the grace of the Holy Spirit.
From this perspective, the position of the De unione is hardly as novel as it might appear when studied in christology without reference to other instances of the distinction between that which is principale and secundarium to underscore the ultimate unity of composite realities.Footnote 61 Given the order of dependency or subordination of the secundaria to whatever is principal, many of the confusions surrounding article four could be avoided if secundarium were not presented by means of the potentially misleading cognates like “second,” but with ordered comparatives like primary or principal and subordinate.Footnote 62
Conclusion
What advantage is to be gained by wrestling with these complicated issues that putatively belong to a bygone age? These post-Chalcedonian attempts to articulate Christ’s unity are not merely academic, scholastic, or even thomistic. They pertain to the union of God and man in Christ as affirmed in the most basic sources of the Christian faith. As Thomas Joseph White observes, key passages in the New Testament about Christ
point us toward [a] deeper ontological mystery. How is it that God the Son and Word subsists as a human being, having a human nature, even while he retains the prerogatives of his divine identity and nature? Christ is able to cure the sick, raise the dead, and even forgive sins. Christ is also subject to human suffering, death, and resurrection from the dead. The subject who acts is one, but he acts always both as God and as man, simultaneously able to do what only God can do, and able to suffer what only a human being can suffer. To approach this mystery in its depth is to approach the heart of New Testament teaching. But this approach can only be one grounded in a distinctively metaphysical mode of Christological reflection. Footnote 63
Like the Greek authors after Chalcedon, Aquinas sought to illumine Christ’s unity and duality in terms of their metaphysical implications. By his appropriation and development of the post-Chalcedon tradition, Thomas understood that Jesus did not need a second esse to authenticate the truth of his humanity. What he assumed did not give him a new, second being; it gave him a new human mode of being, through which he loved and suffered in the single subsistent existence of the Word. Thomas’s formulation of Christ’s unity in the De unione is unique, but it is precisely the uniqueness of this formulation, so vexing to commentators, that underscores his ultimate commitment to Christ’s ontological unity: the secundarium of this enigmatic work is not another esse but the created mode of existence that Christ acted through as a human being.
