St. Thomas on Peter Lombard's Error of Positing Two Hypostases/Supposits in Christ

St. Thomas on Peter Lombard's Error of Positing Two Hypostases/Supposits in Christ

In the Tertia Pars of the Summa Theologiae, St. Thomas seems to be particularly concerned about the Nestorian-like error of some of his contemporaries and recent predecessors---among them Peter Lomabard---who held that there was one Person in Christ, but two hypostases or supposits. 

He introduces us to this error in ST IIIa, q. 2, a. 6 explicitly: 

“But some more recent masters, thinking to avoid these heresies, through ignorance fell into them. For some conceded one person in Christ, but maintained two hypostases, or two supposita... And this is the first opinion set down by the Master (Sent. iii, D, 6). But others desirous of keeping the unity of person, held that the soul of Christ was not united to the body, but that these two were mutually separate... And this is the third opinion which the Master sets down (Sent. iii, D, 6). But both of these opinions fall into the heresy of Nestorius...”  

He addresses this issue several times afterwards, and considers the logical consequences of holding that in Christ there are two hypostases/supposits (even if only one person).  For example, ST IIIa, q. 16, a. 7, he tells us that: 

“if [in Christ] there were a different hypostasis of God and man, so that ‘to be God’ was predicated of the man, and, conversely... then with equal reason might it be said that Man was made God, i.e. joined to God, and that God was made Man, i.e. joined to man.”

Let us backtrack a bit.  In the Prima Pars, St. Thomas addresses the question of how there are "persons" in God, and there he unpacks for us and defends Boethius' definition of person, as "an individual substance of a rational nature" (De Duabus Naturis).  There St. Thomas says that within the genus "substance" we find hypostases or supposits, which are individual substances.  And in turn, within the genus of hypostasis/supposit, we find persons, which are nothing but rational hypostases.  So from general to particular we have: substances > hypostases/supposits > persons.  In St. Thomas' words:

"Although the universal and particular exist in every genus, nevertheless, in a certain special way, the individual belongs to the genus of substance. For substance is individualized by itself; whereas the accidents are individualized by the subject, which is the substance; since this particular whiteness is called "this," because it exists in this particular subject. And so it is reasonable that the individuals of the genus substance should have a special name of their own; for they are called "hypostases," or first substances.

Further still, in a more special and perfect way, the particular and the individual are found in the rational substances which have dominion over their own actions; and which are not only made to act, like others; but which can act of themselves; for actions belong to singulars. Therefore also the individuals of the rational nature have a special name even among other substances; and this name is 'person'.

Thus the term "individual substance" is placed in the definition of person, as signifying the singular in the genus of substance; and the term "rational nature" is added, as signifying the singular in rational substances." (Summa theologiae Ia, q. 29, a. 1).

So a person is "an individual substance of a rational nature" according to Boethius' classical definition. And an individual substance is a hypostasis or supposit.  This is precisely what suppositum or hypostasis signifies: an individual substance. Therefore, a person is nothing but a rational hypostasis/suppositum.  So if we were to admit that there is a human, temporal, and created hypostasis or suppositum in Christ, then we would be saying that besides the Divine Person, there is in Him a human person as well.

Note especially that for St. Thomas, hypostasis and suppositum are synonyms.  The former term has a Greek etymology, and the latter is a native Latin term.  But they mean the same thing.  Moreover, hypostasis/suppositum is not synonymous with "person"; all persons are hypostases, but not vice-versa.  Whenever a hypostasis is rational, then we have a person.  Therefore, in St. Thomas' mind, to posit two hypostases in Christ, one Divine and another human (yet both rational), would be tantamount to positing two persons in Christ, which is clearly heretical.

He makes reference to this theory again, in q. 17, a. 1, and the way he does is very illuminating. This time he explains it in terms of the pronouns unus ([some]'one', in the masculine), unum ('one' [thing], in the neuter), and duo ('two' [things], in the neuter). According to the theory Aquinas is combatting, Christ, being one person and two supposits, is unus (“some-one,” signifying the complete ‘who’), but duo (“two things”). This is not the same as saying that he has two natures, humanity and Divinity, because natures thus construed are just abstractions, and not concrete, real things; saying he is duo, or “two things,” amounts to ascribing two supposits to Him, two concrete natures. But this is erroneous. St. Thomas corrects this theory, saying that in Christ there is not only one Person (unus, in the masculine), but also one supposit, ‘one thing’ (unum, in the neuter), despite this one thing having two natures. Therefore, Christ is not: (a) a divine thing plus (b) a distinct being that possesses human nature; but rather, He is one being that is both human and Divine in nature.  Christ is one thing and some-one (unus et unum, masculine and neuter), with one existence, though possessing two natures, humanity and Divinity. Thus, St. Thomas saves Christ’s unity of being (which is the subject of the following article).

But how is it possible for Catholic theologians, like Peter Lomabard, who were writing and teaching so many centuries after the Patristic era---where all these issues had already been settled---to fall into such a grave error?  Although this error of positing two hypostases/supposits/persons in Christ was already condemned in the early councils of the Church, such as those of Chalcedon and Ephesus, through the terminological imprecisions of the early Scholastics---an unwarranted distinction between person and hypostasis/supposit---, these errors crept back into the theological scene. St. Thomas’ contemporaries were not aware of these councils as much as he was.  If Lombard and his ilk understood what they are saying, they would be forced also to posit two persons in Christ when they place two supposita or two hypostases in Him.   Corey Barnes sheds light into the historical circumstances:

“Aquinas’ later presentations of Christology share knowledge of patristic and conciliar sources unparalleled in the thirteenth century. Among other things, these sources granted Thomas a privileged awareness of early Christological controversies and led him to suspect that some medieval approaches to Christology veered toward Nestorianism. Combating this unintended but nonetheless pernicious tendency required eliminating the imprecisions through which error could enter. The root Nestorian error, according to ST III, q. 2, a. 6, lies in positing an accidental union in Christ, against which Thomas affirms a substantial union, though Nestorius also erred in allowing two hypostases in Christ. Terminological imprecision had, in Aquinas’s own age, led some to allow a duality of hypostases or supposits in Christ. Thomas arrests this drift toward Nestorianism by specifying the relationship between supposits, hypostases, and persons and through the principle actiones sunt suppositorum.”¹

¹ Corey L. Barnes, “Aristotle in the Summa Theologiae’s Christology” in Gilles P. Emery, Matthew Levering, Aristotle in Aquinas’ Theology (Oxford; New York: Oxford University Press, 2015), 193.