Skip to main content Accessibility help
Hostname: page-component-768dbb666b-tcprc Total loading time: 0.746 Render date: 2023-02-05T05:43:05.459Z Has data issue: true Feature Flags: { "useRatesEcommerce": false } hasContentIssue true

Thomas Aquinas on Christ’s Unity: Revisiting the De Unione Debate

Published online by Cambridge University Press:  10 November 2021

Roger W. Nutt*
Ave Maria University;


The claim that article four of Thomas Aquinas’s De unione verbi incarnati is a reversal of his consistently held single esse position is challenged in this paper. The article argues that reading all five articles of the De unione as a single-structured argument discloses a single esse understanding of the Incarnate Word. The very nature of the radically hypostatic union between God and man in Christ is at stake in this dispute. According to Thomas, positing a second esse in Christ not only contradicts the tradition, especially of the Christian East, that he appropriates, but it would also compromise the reality of the hypostatic union itself.

Research Article
© The Author(s), 2021. Published by Cambridge University Press on behalf of the President and Fellows of Harvard College

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)


1 Thomas Joseph White argues: “In some real sense it is true to say: ignorance of ontology is ignorance of Christ. The understanding of the Bible offered by the fathers and scholastics, then, is not something that can be justified as one possible form of reading among others (defensively, as against a post-critical anthropological turn in modern philosophy). Rather, it is the only reading that attains objectively to the deepest truth of the New Testament: a truth concerning the identity of Christ as the God-man” (Thomas Joseph White, The Incarnate Lord: A Thomistic Study in Christology [Washington, DC: The Catholic University of America Press, 2015] 8).

2 Joseph Cardinal Ratzinger, Behold the Pierced One: An Approach to Spiritual Christology (trans. Graham Harrison; San Francisco: Ignatius Press, 1986) 37–38.

3 Ibid.

4 In Latin-French, Latin-German, and Latin-English, respectively, there are editions of Thomas Aquinas’s De unione that include extensive notes and theological commentary. See Question disputée. L’union du Verbe incarné (De unione Verbi incarnati) (Textes philosophiques; ed. Marie-Hélène Deloffre; Paris: Vrin, 2000). See also Quaestio disputata “De unione Verbi incarnati” (“Über die Union des fleischgewordenen Wortes”) (ed. and trans. Klaus Obenauer; Stuttgart-Bad Cannstatt: Frommann-Holzboog, 2011), which includes a much-needed critical Latin text prepared by Walter Senner, O.P., Barbara Bartocci, and Klaus Obenauer. And see De unione verbi incarnati (trans. and intro. Roger W. Nutt; Leuven: Peeters, 2015), which reproduces (with permission) Obenauer’s critical Latin text.

5 For a very helpful historical treatment of the developments in christology after Chalcedon that touches on the teachings of many of the most important schoolmen and the speculative issues surrounding Aquinas’s De unione, see Corey L. Barnes, Christ’s Two Wills in Scholastic Thought: The Christology of Aquinas and Its Historical Contexts (Toronto: Pontifical Institute of Mediaeval Studies, 2012).

6 For a summary of the debates over the dating of De unione, see Nutt, introduction to Thomas Aquinas, De unione verbi incarnati, 6–9. On Obenauer’s treatment of the dating of the De unione, see Quaestio disputata (ed. Obenauer), 169. For Sr. Deloffre’s discussion, see Question disputée (ed. Deloffre), 24–25. For a discussion and defense of the authenticity of the De unione, see Franz Pelster, S.J., “La quaestio disputata de saint Thomas De unione Verbi incarnati,” Archives de philosophie 3 (1925–26) 198–245. Pelster’s work is viewed as resolving the question of authenticity.

7 For a very helpful summary of the issues surrounding the De unione, see David Tamisiea, “St. Thomas on the One Esse of Christ,” Angelicum 88 (2011) 383–402. There, Tamisiea also summarizes what Thomas means by esse: “that which causes a thing to exist in reality, and is only attributed to real things contained within the categories of being identified by Aristotle” (385).

8 Thomas G. Weinandy, for example, states: “I believe that Aquinas implicitly held two esses from the start (and so was never a Monophysite), but only explicitly stated this position on the one occasion in the De Unione Verbi Incarnati” (Thomas G. Weinandy, “Aquinas: God Is Man: The Marvel of the Incarnation,” in Aquinas on Doctrine: A Critical Introduction [ed. Thomas Weinandy, Daniel Keating, and John Yocum; London: T&T Clark, 2004] 67–89, at 80). See also Jean Galot, S.J., The Person of Christ, Covenant between God and Man: A Theological Insight (trans. M. Angeline Bouchard; Chicago: Franciscan Herald Press, 1984). Galot argues that Aquinas cannot be confidently invoked by advocates of a single esse understanding of Christ because, in article four of the De unione, “St. Thomas clearly declares that ‘the esse of the human nature is not the esse of the divine nature’; and that in addition to the eternal esse of the eternal Person there is an esse that belongs to the human nature, not a principal but a secondary to be” (Galot, The Person of Christ, 17). Finally, medievalist Richard Cross has frequently critiqued Aquinas’s single esse position; see Richard Cross, The Metaphysics of the Incarnation (Oxford: Oxford University Press, 2002); idem, Duns Scotus (Oxford: Oxford University Press, 1999) esp. 114–15; and idem, “Aquinas on Nature, Hypostasis, and the Metaphysics of the Incarnation,” The Thomist 60 (1996) 171–202. What differentiates the De unione from Thomas’s other accounts, Cross explains, is “his abandoning the claim that the human nature is a truth-maker precisely in virtue of its sharing in the esse of the suppositum” (idem, The Metaphysics of the Incarnation, 64). The reference to an esse secundarium in Christ means that Thomas holds, at least in this one case, Cross argues, that Christ’s human nature “communicate[s] esse to its suppositum” (ibid., 63). For a response to Cross’s reading of Aquinas, see James Reichman, S.J., “Aquinas, Scotus, and the Christological Mystery: Why Christ is Not a Human Person,” The Thomist 71 (2007) 451–74.

9 Thomas Aquinas, Compendium theologiae, cap. 212: “Manifestum est enim quod partes divisae singulae proprium esse habent, secundum autem quod in toto considerantur, non habent suum esse, sed omnes sunt per esse totius. Si ergo consideremus ipsum Christum ut quoddam integrum suppositum duarum naturarum, eius erit unum tantum esse, sicut et unum suppositum.” The other parallel passages where Aquinas also treats Christ’s esse are: Quodlibet 9, art. 3; Summa theologiae, III, q. 17, art. 2; Scriptum super Sententias, III, dist. 6, q. 2, art. 2.

10 Aquinas, De unione verbi incarnati (trans. and intro. Nutt) 135.

11 As early as the thirty-seventh article of the Athanasian Creed, which Aquinas quotes authoritatively in the De unione, analogical reasoning to affirm Christ’s unity is used: “just as one man is a rational soul and flesh, just so the one Christ is God and man.” See Heinrich Denzinger, Compendium of Creeds, Definitions, and Declarations on Matters of Faith and Morals; Latin-English (ed. Peter Hünermann; 43rd ed.; San Francisco: Ignatius Press, 2012) § 76.

12 Decrees of the Ecumenical Councils (ed. Norman P. Tanner, S.J.; 2 vols.; Washington, DC: Georgetown University Press, 1990) 1:86–87.

13 Ibid., 1:116.

14 Ibid., 1:115.

15 For a summary of the influence of the later Greek fathers on Aquinas’s christology and the influence of John of Damascus on the De unione, see, Nutt, introduction to Thomas Aquinas, De unione verbi incarnati, 23–41. For a related treatment of the contributions of Leontius, see Brian Daley, S.J., “A Richer Union: Leontius of Byzantium and the Relationship of Human and Divine in Christ,” StPatr 24 (1993) 239–65.

16 Maximus the Confessor, On Difficulties in the Church Fathers: The Ambigua (2 vols.; Dumbarton Oaks Medieval Library 28; Cambridge, MA: Harvard University Press, 2014) 1:45.

17 For a helpful treatment of the unique way that Dionysius influenced and informed Aquinas’s christology, see Andrew Hofer, O.P., “Dionysian Elements in Thomas Aquinas’s Christology: A Case for the Authority and Ambiguity of Pseudo-Dionysius,” The Thomist 72 (2008) 409–42.

18 Gilles Emery, “A Note of St. Thomas and the Eastern Fathers,” in idem, Trinity, Church, and the Human Person: Thomistic Essays (Naples, FL.: Sapientia Press, 2007), 193–207, at 194. Emery also points out that in several key theological passages of Aquinas’s work, citations from the Greek fathers often double those from their Latin counterparts. Emery notes, further, that Aquinas’s understanding of the “structure” of the hypostatic union is “fundamentally” Greek (at 202), and that his use of the term “instrument” (organum) to explain the causal merit of Christ’s humanity is particularly indebted to the Greek fathers.

19 James Weisheipl points out: “Thomas d’Aquino was the first Latin Scholastic writer to utilize verbatim the acts of the first five ecumenical councils of the Church, namely in the Catena aurea … and in the Summa theologiae” (James Weisheipl, Friar Thomas D’Aquino: His Life, Thought, and Works [Washington, DC: The Catholic University of America Press, 1983] 164). See also Martin Morard, “Thomas d’Aquin lecteur des conciles,” Archivum Franciscanum Historicum 98 (2005) 211–365.

20 See Thomas Aquinas and the Greek Fathers (ed. Michael Dauphinais, Andrew Hofer, and Roger Nutt; Ave Maria, FL: Sapientia Press, 2019), passim, esp. Khaled Anatolios, “The Ontological Grammar of Salvation and the Salvific Work of Christ in Athanasius and Aquinas,” 89–109.

21 Barnes, Christ’s Two Wills, 123.

22 Summa theologiae, I, q. 29, art. 2. Taken from Thomas Aquinas, Summa theologiae (ed. John Mortensen and Enrique Alarcon; trans. Laurence Shapcote, O.P.; 60 vols.; Lander, WY: The Aquinas Institute, 2012) 13:310 (emphasis added).

23 See Aaron Riches, “After Chalcedon: The Oneness of Christ and the Dyothelite Mediation of His Theandric Unity,” Modern Theology 24 (2008) 199–224.

24 For a presentation of the theological and philosophical issues in the century prior to Aquinas that includes a thorough and original analysis of Peter Lombard’s christology, see Lauge Olaf Nielsen, Theology and Philosophy in the Twelfth Century: A Study of Gilbert Porreta’s Thinking and the Theological Expositions of the Doctrine of the Incarnation during the Period 1130–1180 (Leiden: E.J. Brill, 1982).

25 See, for example, the corpus of De unione, a. 2.

26 Peter Lombard, Sentences, book III, dist. 5, ch. 1 (trans. Giulio Silano; Toronto: Pontifical Institute of Mediaeval Studies, 2008) 17–18. For Nielsen’s original conclusion about Lombard’s preference for the Third Opinion, see Nielsen, Theology and Philosophy in the Twelfth Century, 264.

27 The adherents of the Subsistence Theory, Lombard explains, “profess this Christ to be only one person; however, that person was simple only before the incarnation, but in the incarnation he was made into a person composed of divinity and humanity…. And so the person [of God] which before was simple and existed only in one nature, then subsists in and from two natures” (Lombard, Sentences, book III, dist. 6, ch. 3, 26).

28 See, for example, Summa theologiae, III, q. 2, art. 6. For a discussion of the “three opinions” in relation to developments within Aquinas’s christology over the course of his career, see Michael Raschko, “Aquinas’s Theology of the Incarnation in Light of Lombard’s Subsistence Theory,” The Thomist 65 (2001) 409–39.

29 See Philipp W. Rosemann, Peter Lombard (New York: Oxford University Press, 2004) 131.

30 Lombard, Sentences, book III, dist. 10, ch. 1, 41.

31 Ibid.

32 For a helpful treatment of this issue that contextualizes Thomas’s teaching, see Stephen F. Brown, “Thomas Aquinas and His Contemporaries on the Unique Existence in Christ,” in Christ among the Medieval Dominicans: Representations of Christ in the Texts and Images of the Order of Preachers (ed. Kent Emery and Joseph Wawrykow; Notre Dame: University of Notre Dame Press, 1998) 220–37.

33 In Super Sententiis, lib. 3, dist. 6, q. 3, art. 2, ad 1, Thomas catalogs some attempts to account for Christ’s human nature as an accident of the Word. Interestingly he refers to these as accounts of “antiqui” that are inaccurate. From his early writings, Thomas considered the question of Christ’s unity and the status of his human nature to be a long-standing problem and not simply one of 13th-cent. speculations. “Ad primum ergo dicendum, quod humana natura in Christo habet aliquam similitudinem cum accidente, et praecipue cum habitu, quantum ad tria. Primo, quia advenit personae divinae post esse completum, sicut habitus, et omnia alia accidentia. Secundo, quia est in se substantia, et advenit alteri, sicut vestis homini. Tertio, quia melioratur ex unione ad verbum, et non mutat verbum; sicut vestis formatur secundum formam vestientis, et non mutat vestientem. Unde antiqui dixerunt, quod vergit in accidens; et quidam propter hoc addiderunt, quod degenerat in accidens: quod tamen non ita proprie dicitur; quia natura humana in Christo non degenerat, immo magis nobilitatur.” Sr. Deloffre catalogs the different ways in which the other major authors of the 13th cent., with the issues raised by Lombard as a point of reference, framed Christ’s human mode of existence in the order of being. These formulations included, among others: esse simpliciter and esse personale (used by Alexander of Hales, Albert the Great, and Bonaventure) and esse hypostasis (Albert the Great and Alexander of Hales). Conversely, many of these same authors, especially the Franciscans, employed the phrase esse humanum to speak of Christ’s human nature. See Aquinas, Question disputée (ed. Deloffre), 45–50. A problem arose for these authors, however, regarding Christ’s unity. How can the unity of Christ be articulated in terms of esse simpliciter “without eliminating the reality of the human nature?” (ibid., 46). To address this problem, Bonaventure spoke of the categorical status of Christ’s human nature in relation to the divine esse of the Word as “inclining toward an accident” (vergit in accidens) (ibid., 47; Bonaventure formulates this position in the commentary on Lombard’s Sentences, book III, dist. 6, art. 1, q. 3.).

34 See n. 8 above for a list of authors and works.

35 Aquinas, De unione verbi incarnati, art. 1, 93.

36 Ibid., ad 6, 99.

37 Ibid., ad 10, 101.

38 Ibid., art. 2, 113.

39 Ibid., art. 3, arg. 6, 120–21. Thomas maintains this same point in Summa theologiae, III, q. 17, art. 1, ad 2: “Ad secundum dicendum quod, cum dicitur, Christus est aliud et aliud locutio est exponenda ut sit sensus, habens aliam et aliam naturam. Et hoc modo exponit Augustinus in libro contra Felicianum, ubi, cum dixisset, in mediatore Dei et hominum aliud Dei filius, aliud hominis filius subdit, aliud, inquam, pro discretione substantiae, non alius, pro unitate personae. Et Gregorius Nazianzenus, in epistola ad Chelidonium, si oportet compendiose dicere, aliud quidem et aliud ea ex quibus salvator est, siquidem non idem est invisibile visibili, et quod absque tempore ei quod sub tempore. Non autem alius et alius absit. Haec enim ambo unum.”

40 Aquinas, De unione verbi incarnati, Sed contra, 123.

41 Ibid., art. 3, 125.

42 Ibid., 127.

43 Ibid.

44 Ibid., art. 4, 133.

45 Ibid., Sed contra, 133.

46 John Emery explains Aquinas’s intention as follows: “By means of this hapax legomenon, Aquinas does not posit a new distinct act of being in Christ; instead, he attempts to account for the creaturely character of his human nature, whose participated degree of being is different from the divine nature’s limitless being” (John Emery, “A Christology of Communication: Christ’s Charity according to Thomas Aquinas” [PhD diss., University of Fribourg, 2017] 87).

47 Aquinas, De unione verbi incarnati, art. 4, 135.

48 See n. 8 above. In addition to the authors and positions outlined in n. 8, for a consideration of Aquinas’s ontology of the hypostatic union in relation to the work of Karl Rahner, Friedrich Schleiermacher, John Hick, Jacques Dupuis, and Jon Sobrino, see White, The Incarnate Lord, 91–111.

49 Aquinas, De unione verbi incarnati, art. 4, 135.

50 One of the great commentators on Aquinas, Dominic Bañez (1528–1604), recognizes that an esse cannot be posited without a supposital reality: “The constitutive mode of a supposit is really distinct from that supposit as one thing from another…. All the more distinct then is esse from essence, for esse does not come to essence except through suppositality” (Dominic Bañez, The Primacy of Existence in Thomas Aquinas [trans. Benjamin S. Llamzon; Chicago: Henry Regnery Company, 1966] 49 [emphasis added]). For a helpful exploration of this issue, see Thomas M. Osborne, Jr., “Which Essence Is Brought into Being by the Existential Act?” The Thomist 81 (2017) 471–505.

51 John Froula, “Esse Secundarium: An Analogical Term Meaning that by which Christ Is Human,” The Thomist 78 (2014) 557–80, at 580. The article is also extremely helpful for its summary of a number of different readings of article four of the De unione. See also Victor Salas, “Thomas Aquinas on Christ’s esse: A Metaphysics of the Incarnation,” The Thomist 70 (2006) 577–603; and, Corey L. Barnes, “Albert the Great and Thomas Aquinas on Person, Hypostasis, and Hypostatic Union,” The Thomist 72 (2008) 107–46, at 144.

52 Aquinas, De unione verbi incarnati, art. 5, ad 14, 145. Romanus Cessario clarifies this point in Aquinas’s thought by distinguishing the person and the natures as “effective subject” and “possessive subjects.” “As a personal unity,” Cessario argues, “Christ enjoys only one effective subject, the eternal Logos. But besides the effective principle of unity which Christ receives through his uncreated personhood, he also enjoys two possessive subjects, since each nature does what remains proper to it” (Romanus Cessario, The Godly Image: Christ and Salvation in Catholic Thought from Anselm to Aquinas [Petersham: St. Bede’s Publications, 1990] 134).

53 For a very tentative, but somewhat compatible, reading of De unione, article 4, see Michael Gorman, Aquinas on the Metaphysics of the Hypostatic Union (Cambridge: Cambridge University Press, 2017) esp. 161.

54 Barnes, Christ’s Two Wills, 244.

55 This is why I have translated secundarium as “subordinate” in this article in my own translation of the De unione. Secundarius denotes a reality of a secondary or subordinate order and not a numeric continuum of discrete realities. Roy Deferrari offers the following English choices for secundarius, a, um: “coming in second place, subordinate, secondary, the opposite of prinicipalis” (Roy J. Defarrari, A Latin-English Lexicon of Saint Thomas Aquinas [Fitzwilliam, NH: Loreto Publications, 1948] 1006).

56 In addition to the examples provided below, here are two other texts that demonstrate Thomas’s recourse to the distinction between principale and secundarium: Summa theologiae, II–II, q. 17, art. 4, co: “In genere autem utriusque causae invenitur principale et secundarium. Principalis enim finis est finis ultimus; secundarius autem finis est bonum quod est ad finem. Similiter principalis causa agens est primum agens; secundaria vero causa efficiens est agens secundarium instrumentale.” And, De veritate, q. 23, art. 1, ad 3: “Ad tertium dicendum, quod voluntas est alicuius dupliciter: uno modo principaliter, et alio modo secundario. Principaliter quidem voluntas est finis, qui est ratio volendi omnia alia; secundario autem est eorum quae sunt ad finem, quae propter finem volumus.”

57 Summa theologiae, I–II, q. 106, art. 1. Taken from Aquinas, Summa theologiae (ed. Mortensen and Alarcon) 16:408.

58 Ibid.

59 Ibid., art. 2, 409.

60 Thomas also uses this distinction to distinguish the primary and subordinate ends of a composite action and the primary and secondary agents of a composite motion. In both cases the secondary component depends on the primary aspect. See, for example, Summa contra gentiles, book III, ch. 109, no. 5: “ut scilicet secundarius finis a principali dependeat, sicut secundarium agens a principali dependet” (Leonine ed. 14:341).

61 Jason L. A. West (in “Aquinas on the Metaphysics of Esse in Christ,” The Thomist 66 [2002] 231–50) argues that there is no sense in which the esse secundarium of the De unione can be read without contradicting Aquinas’s metaphysics and consistent arguments against the christological heresies. West’s metaphysics are correct, but his reading of the De unione is not. If Aquinas were distinguishing between two numerically distinct esses in one subsistent being, his position in the De unione would contradict his principles. However, that is not the case. The subordinate, or secundarium, esse is not some metaphysically incoherent attempt to affirm a half-esse but an affirmation that the human nature of Christ is an existent particular but not a per se subsistence or person.

62 On translating the De unione in light of these controversies see, Nutt, introduction to Aquinas, De unione verbi incarnati, 58–59.

63 White, The Incarnate Lord, 21.

Save article to Kindle

To save this article to your Kindle, first ensure is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about saving to your Kindle.

Note you can select to save to either the or variations. ‘’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.

Find out more about the Kindle Personal Document Service.

Thomas Aquinas on Christ’s Unity: Revisiting the De Unione Debate
Available formats

Save article to Dropbox

To save this article to your Dropbox account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you used this feature, you will be asked to authorise Cambridge Core to connect with your Dropbox account. Find out more about saving content to Dropbox.

Thomas Aquinas on Christ’s Unity: Revisiting the De Unione Debate
Available formats

Save article to Google Drive

To save this article to your Google Drive account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you used this feature, you will be asked to authorise Cambridge Core to connect with your Google Drive account. Find out more about saving content to Google Drive.

Thomas Aquinas on Christ’s Unity: Revisiting the De Unione Debate
Available formats

Reply to: Submit a response

Please enter your response.

Your details

Please enter a valid email address.

Conflicting interests

Do you have any conflicting interests? *