The Role of Chalcedon

The objection to a proper being for Christ's human nature had its remote origin in Aquinas's reading of Chalcedon, the central claim of which, at least as Aquinas understood it, was that Christ was one person with a divine nature and a human nature, perfectly human and perfectly divine.Footnote ⁶ Now one of the quirks of this reading – and it was by no means unique to Aquinas – was that it committed its adherents to holding that Christ possessed all the essential attributes of a human person without also being a human person.

Hence if we note that it was the Word or the Logos which became incarnate, that the Word is a divine person, and that there was no change in God through the Incarnation – all claims which Aquinas would have accepted–these claims then, together with the Decree's claim that ‘…the union…comes together in a single person…’,Footnote ⁷ entail that the person in which the hypostatic union was made was a divine person. But if that is true and the union was made in one person, then it will also be true that Christ cannot be a human person. Since if he were, then there would be two persons in Christ and Nestorianism would be true. Further, given the Decree maintained that the union was ‘acknowledged in two natures which undergo no confusion, no change, no division, no separation’,Footnote ⁸ then Christ must possess a human nature. But if Christ possesses a human nature, then Christ will possess all the essential attributes of a human person without also being a human person.

One important corollary of this was that being a human person was in some sense accidental to human nature – at least insofar as being a human person was not an attribute entailed in virtue of possessing human nature. Eventually theologians inspired by the Decree sought to specify what this accident was. They were able to narrow their search by excluding from consideration accidents which the Decree implied Christ possessed in addition to his human nature. Thus since human beings come in different sizes, weights, and shapes etc. and the Decree held that Christ was ‘perfect in humanity…truly human…consubstantial with us as regards his humanity…’,Footnote ⁹ then it would be difficult to understand how the Decree's claim could be true here unless in addition to possessing all the essential attributes of a human person it also posited that Christ possessed many of the accidental attributes a human person enjoyed.Footnote ¹⁰ Or to specify it further, that Christ, in addition to possessing a human nature, also possessed a collection of accidental attributes which distinguished his humanity in the same way that a normal human being's humanity is distinguished from others of its kind.

In effect, the human nature which the Decree postulated in Christ was an aggregate of human nature together with certain accidents that rendered it distinct from other individuals but not a human person. To see how Aquinas dealt with this though, and to identify the proximate origin of the denial of a proper being for Christ's human nature, we need to turn to Aquinas's account of nature.

The Analysis of Nature

Aquinas was fortunate to be able to draw on the particularly rich analysis of nature and essence contained in his early opusculum, De Ente and Essentia.Footnote ¹¹ ‘Essence’, he reasoned, signified those material and/or formal characteristics in virtue of which a thing is able to be and to be categorised within its species and genera.Footnote ¹² Such an essence, which we can call a specific essence, can be considered with or without precision.Footnote ¹³ Taken with precision it signifies only that which is proper for an individual's inclusion within the categories to which the essence pertains. Taken without precision, however, it signifies that which is common to the individuals included within the categories to which the essence pertains. In each case what is signified is the same: the specific essence, that is, the essence considered without reference to determinate accidents. Yet the mode of signification differs insofar as the specific essence is determinable by accidents; if taken with precision such determinability is not considered, but when taken without precision it is.Footnote ¹⁴

In the case of material beings, specific essence can be distinguished from individual essence by designated matter, that is matter which is quantitatively terminated and qualitatively determined: the individual essence includes such matter determinately, the specific essence does not. Since designated matter individuates its possessor both numerically and qualitatively, the individual essence will be both denotable, something which can be pointed out, and incommunicable to others as inferiors.Footnote ¹⁵ On the other hand the specific essence taken without precision will be communicable to others as inferiors but not denotable, whilst the specific essence taken with precision will be neither communicable to others as inferiors nor denotable.

If we take a human being as an example, the specific essence taken with precision will signify rational soul and body as such, that is humanity. Whereas the specific essence taken without precision will signify rational soul and body as determinable, that is as human being. The individual essence is a substance and will consist of a particular rational soul united with a particular body. Moreover, if God elevates the human intellect so that it can cognise the designated matter of the individual essence, then the individual essence will characterise what is unique to the individual human being whose individual essence it is.

However, whether one considers the individual essence as such, or indeed the specific essence with or without precision, one is considering the same reality – the individual essence – albeit from different aspects in order to impart different information about it. The intellect can abstract from the individual essence's determinate accidents, but that does not mean those accidents do not inhere in the essence in rerum naturae and determine it.

The Role of Esse

Given the implications of the Chalcedonian Decree, together with the fact that the specific essence, whether considered with or without precision, merely abstracted from the individual essence, it is not surprising that Aquinas regarded the nature assumed, which we will call the assumptum, to be an individual human nature.

Aquinas characterised the assumptum as both ‘a certain individual in the genus of substance’Footnote ¹⁶ and a ‘particular substance’.Footnote ¹⁷ He also explicitly linked the assumptum with individuality when he remarked, ‘human nature in Christ…is able to be called a certain individual or singular’.Footnote ¹⁸ However, given designated matter individuated hylomorphic compounds, if the assumptum was to be individual, singular, and particular, then it could only be so in virtue of possessing designated matter. Only the individual nature possessed designated matter though; therefore, the assumptum had to be an individual nature.

Aquinas made the same point in a slightly different way when he contrasted the assumptum with nature taken as a Platonic-type form, separate from individuals. This construal, he argued, could not adequately characterise the assumptum because nature so taken could neither be individualised nor be perceptible by the senses.Footnote ¹⁹ By implication then the assumptum would be both individual and perceptible. Yet since it could only be such in virtue of possessing designated matter, then the assumptum would have to be an individual nature.

Now given the assumptum was an individual nature which contained designated matter, then ipso facto it would possess many of the accidental attributes of a human person. This gave rise to two closely related problems.

First, the assumptum, characterised as an individual nature, appeared to be very much like a human person. After all, it possessed all the essential characteristics of a human person and at the very least it had many of the accidental attributes as well. Yet if Aquinas's account was to be consistent with Chalcedon, the assumptum could not be a human person. To avoid this conclusion then Thomists had to identify an ontological ingredient which persons possessed but individual natures lacked, and which could therefore be used to distinguish person and individual nature.

Second, since it was possible that God could have created a world which consisted of one person only and nothing else, the ontological ingredient which distinguished person and individual nature had to be intrinsic to the person. However, all the accidental categories of being other than quantity and quality included in their ratios some reference to reality extrinsic to their subject and therefore were not suitable for specifying the ontological ingredient sought.Footnote ²⁰ Moreover, quantity and quality, which did not suffer from this lacuna, were nevertheless already included in the individual nature in virtue of it possessing designated matter, which after all is just matter quantitatively terminated and qualitatively determined. Consequently, none of the available categories of being appeared suitable for distinguishing person and individual nature, and almost inevitably Thomists' thoughts turned towards esse and bespoke solutions.

One such group of Thomists, led by Louis Billot SJ, interpreted Capreolus's analysis to mean esse was the characteristic persons possessed but individual natures lacked.Footnote ²¹ Another group inspired by Cajetan argued that individual natures were only remotely ordered to esse. To be made ready to receive esse an individual nature had first to be terminated by a substantial mode which rendered it a person (or suppositum) and thus proximately ordered to esse.Footnote ²²

Although the two theories disagreed over which ontological ingredient distinguished person from individual nature, nevertheless, both theories accepted that once esse was on the scene so also a person was. Applied to the case of the Tertia pars text then, the one-esse account holds that Christ's individual human nature could not possess a proper esse, since if it did it would be a person, regardless of whether one also thought esse constituted it such or not. Garrigou-Lagrange put the objection forward clearly when he remarked, ‘if there were two substantial existences in Christ, there would be two beings’.Footnote ²³ ‘Existence’ here is used to translate ‘esse’, and since the beings postulated would each be of a rational nature, then two esses in Christ would entail two persons, which clearly would be inconsistent with the Chalcedonian Decree and not at all what St Thomas had in mind.

Being and Unity

In the sed contra of the Tertia pars text, Aquinas linked the question of the assumptum’s being to the convertibility of being and unity. In the text he remarked, ‘each thing, insofar as it is called a being (ens), is called one; because oneness (unity) and being are convertible. If therefore there were two esses (beings) in Christ, and not one only, Christ would be two, and not one’.Footnote ²⁴ From this one might infer, not unreasonably, that two esses entails two beings (ens) and that therefore the assumptum could not possess a proper being because if it did Christ would be two, not one, with all the deleterious consequences that entailed. Clearly such a conclusion would provide strong support for the one-esse view.

Yet the reasoning moves quickly at this point. Before we accept it we ought to wonder how Aquinas intended being to be understood in the text. One possibility would be according to an absolute consideration of being.Footnote ²⁵ That is independently of the existential status of the being thus considered. There are at least two good reasons for interpreting Aquinas's remarks in this way. First, because it is natural to read the first half of the sed contra, ‘each thing…convertible’, as a general claim which extends to all of ens commune: being as it is divided through the categories. Second, because when Aquinas says, ‘each thing, insofar as it is called a being’, it is natural to read it as a logical claim.Footnote ²⁶

However, despite the seeming reasonableness of the interpretation, problems emerge if we persist with it. Hence, because being is analogous and being and unity are convertible, things in different categories have different kinds of being and therefore different kinds of unity. Indeed, Aquinas had made the same point implicitly in the article immediately preceding the sed contra when he contrasted the diversity that follows upon accidents with that which follows upon substance.Footnote ²⁷ Yet this means none of the accidental categories of being would possess the requisite unity to entail the desired conclusion and thus the argument of the sed contra would be invalid. In other words, it would not entail what the one-esse view would like it to entail.

Therefore, a different interpretation of the text is required, and despite the seeming generality of the first half of the sed contra it is more likely that what Aquinas has in mind was an existential consideration of being. So taken, only substances are properly called beings whilst accidents are said to be of a being.Footnote ²⁸ The reasoning of the argument will extend only to substance which uniquely among the categories has the requisite unity to entail Aquinas's conclusion.

Yet this all comes at a price. Substances possess the requisite unity the argument requires only in virtue of their subsistence. In which case Aquinas's point in the sed contra is that the assumptum cannot subsist; whatever esse the assumptum may possess it cannot be such as to make it subsist. On this interpretation all the two-esse view need do is defend some distinction between what being a subsistent possesses and what being the assumptum possesses. It does not even need to make that distinction in terms of being, as long as it can distinguish the subsistent and the assumptum. Therefore the sed contra and its interpretation does not entail a denial of proper being to the assumptum and as such it is neutral in the debate between the one and two-esse views.

No new esse

A second concern follows upon Aquinas's claim in the Tertia pars corpus that the Word received no new esse from its human nature. As Aquinas put it, ‘according to the human nature no new personal esse comes to him’.Footnote ²⁹ The one-esse account can accommodate this claim by arguing that if Christ's individual human nature did not possess a proper esse, then it could not contribute any esse to the divine person, the Word, whose human nature it was. In effect the Word's lack of new personal esse follows from the assumptum’s lack of a proper esse; the assumptum cannot give what it does not have.

This claim, however, is significantly different from the further claim that if Christ's human nature possessed any being of its own, then it would contribute esse to the divine person whose nature it was: essentially that a nature in possession of esse could not but contribute esse to its suppositum or person. Whilst the former claim follows from the assumptum’s lack of a proper esse, it is not at all clear that the latter does as well. At the very least the one-esse account needs to defend that position.

One has a sense here that the one-esse view was influenced by the decision to group the Tertia pars text with three other texts where Aquinas discussed Christ's being, namely Sentence-Commentary III d. 6, q. 2, a. 2, Quodlibet IX q.2 art. 2, and Comp. Theol. I, c. 212, and argue these four texts shared a common doctrine – that of a one-esse account – which was distinct from a fifth text, De unione Verbi Incarnati art 4, where alone Thomas had experimented with a two-esse account.Footnote ³⁰

Since the De unione famously allowed for ‘another esse of this…[i.e. the divine]…suppositum’Footnote ³¹ whilst the other four texts, including the Tertia pars text, did not, then one might surmise the latter claim, that if Christ's human nature possessed any being of its own, then it would contribute esse to the divine person whose nature it was, depended upon treating these other four texts in contradistinction to the De unione.

However, if one rejects that procedure and seeks instead to evaluate the Tertia pars text on its own merit, then there is no reason why a two-esse account needs to be linked to the De unione. In fact one could defend a two-esse account which was inconsistent with the De unione.

The logical priority of nature over person

Third, one might consider whether person or nature has logical priority in a human being and apply the results of those considerations to the question of the assumptum’s being.

Now I would argue that nature should have logical priority over person. Hence, given Aquinas, as is well known, followed Boethius and defined person, taken generally, as ‘an individual substance of a rational nature’,Footnote ³² then one can infer that nature was logically prior to person in two ways. First, insofar as the thing which is a person must be in the genus of substance. Second, insofar as the thing which is a person must possess a rational nature. Whatever else a thing might be, if it is neither in the genus of substance nor in possession of a rational nature, then it will not be a person. To that extent then, being in the genus of substance and being in possession of a rational nature are logically prior to being a person. Yet since a thing is in the genus of substance and in possession of a rational nature in virtue of its essence, whilst essences, as we have seen, are synonymous with natures, then nature will be prior to person in the two ways thus identified.

At this point we need to consider the corpus of the Tertia pars text. There Aquinas argued that those things which pertained to Christ's natures were two, whilst those which pertained to Christ's hypostasis (person) were one.Footnote ³³ Esse, however, pertained both to person and to nature,Footnote ³⁴ ‘to the hypostasis as to that which has esse; to nature however, as to that by which something has esse’.Footnote ³⁵

However, given Aquinas attributes esse in some sense to the nature, no matter how attenuated an entity one regards that nature to be, the only way a one-esse account can explain that esse is by maintaining it is derived from the divine esse of the Word. After all, if Christ's individual human nature did not possess a proper esse, then it must have been actualised by the divine esse.

But if Christ's human nature depends upon the esse of the divine person, how will that affect the logical priority of nature to person? Perhaps, though, the one-esse account would respond by saying Christ's human nature is a special case, a unique case in fact. As such it can depend upon the Word for its esse because the Word already constituted a person prior to assuming its human nature.

Two points could be made in reply, though. First, it is not easy or obvious to construe Aquinas's claim that esse pertains both to person and to nature as just a claim about the unique situation of Christ. Anyone who wants to defend Aquinas as a moderate realist, for instance, has reason to say something along these lines. It is much more plausible to read it as the application of a general point to a particular issue.

Second, Aquinas insists that the assumptum, ‘does not lack proper personality on account of a defect of something which pertains to the perfection of human nature’.Footnote ³⁶ Yet if that were the case, even if one thought Aquinas's claim about esse only applied to the unique situation of Christ, then one would still have to choose between the lack of defect claim and the logical priority of nature to person claim. If there is good reason to think nature is logically prior to person, but esse only pertains to Christ's human nature in virtue of the divine person, then Christ's human nature will not be doing something – specifying esse – which it ought to be doing. As a result then Christ's human nature will lack proper personality on account of a defect.

However, can a two-esse account deal with the problem any better? Certainly, in virtue of attributing a proper esse to Christ's human nature it will be able to maintain the twofold logical priority of nature to person in all persons. Yet that leads to the question of how it maintains the person's unity of esse. After all, in all the texts in which he addressed the question, Aquinas insisted on the unity of a person's esse.

It seems that the two-esse account will want to say that the divine person becomes the person of a thing which is a man. It does so in virtue of assuming an individual human nature into the unity of its person. The divine esse becomes the esse of a man in virtue of being the act of being of a thing which is a man. Even though that divine act of being does not activate that by virtue of which that thing is a man, i.e., its human nature.

The esse of relation

The fourth problem I want to address concerns the esse Christ's human nature possessed in virtue of the hypostatic union. Aquinas described the hypostatic union as, ‘a certain relation which is considered between the divine and human nature which come together in the one person of the Son of God’.Footnote ³⁷ Because the hypostatic union began in time this relation was created or, to put it more precisely, concreated.Footnote ³⁸ It was also real because first it inhered in a real subject, Christ's human nature, which itself was created – a certain creature, we are told. Second, it had a real foundation in its subject, and third it had a real term, the Person of the Word. However, like any relation between a creature and God, it was mixed, that is, real only in the creature and of reason in God.

Given the relation's character – concreated and real – it is hardly surprising Aquinas linked it to a proper created esse, a relational esse, which its subject – Christ's human nature – possessed in virtue of this relation inhering in it. Hence Aquinas remarked that the esse of relation, ‘depends upon its subject. And because such a union does not have real esse accept in a created nature … it follows that it has created esse’.Footnote ³⁹

At this point though we need to ask how a one-esse account explains the created esse of this relation? That relation depends upon its subject, Christ's human nature, which, as we said, is created. Yet according to the one-esse account Christ's human nature will not possess any proper esse. Its only esse will be the divine esse of the Word, which gives rise to two difficulties: how can the divine esse render Christ's human nature a created nature? Second, even if it can, how can that divine esse account for the relation's created esse?

Let us set aside the first problem for the moment and focus instead on the second. The subject of an accident, Aquinas suggests, ‘is compared to it as potency to act, because an accident is a certain form, making [the subject] to be in act according to an accidental being’.Footnote ⁴⁰ That accidental being, Wippel remarks, was consistently understood by Aquinas to be really distinct from the substantial esse of its subject.Footnote ⁴¹ However, if the subject of the relation – Christ's human nature – is in act in virtue of the divine esse, which esse as we know is both infinite and uncreated, then how can the relation's limited and created esse be added over and above the divine substantial esse of its subject? Surely the divine esse will exhaust all the potency in Christ's human nature, both substantial and accidental?

On the other hand, if one defends a mere distinction of reason between accidental and substantial esse, then how can the relation's limited and created esse really be limited and created? It cannot because it will just be the divine esse considered under a different aspect. Moreover, the same problem applies to the being of any accidents Christ's human nature might possess.

A two-esse account faces no such problem, however. Because it admits a proper esse for Christ's human nature that esse will be limited and thus allow for further realisation by accidental esse.

Divine esse as actualiser

The fifth problem was touched on in the previous one; how is Christ's human nature rendered actual by esse? A two-esse account will attribute a proper esse to that nature in virtue of which it is rendered actual. A one-esse account will argue, to the contrary, that it is rendered actual by the divine esse. But can that really happen? The divine esse is the divine essence and as such is a quiddative principle. How could one quiddative principle, the divine essence, actualise another, Christ's human nature?

Joseph Owens in an analysis of being as a real nature, admittedly within the context of the real distinction, has said being, ‘is a real nature which includes every perfection, every act, in its own indivisible unity. If it entered into composition with anything else as a nature, it would absorb that thing into itself’.Footnote ⁴² But the being which is a real nature is the divine nature, and if the being which is a real nature cannot enter into composition without such consequences, then nor can the divine nature. Yet is this not precisely what the one-esse account of the Incarnation requires? At best this appears to lead to Monophysitism, at worst, incoherence.

Esse and the individual nature

We have seen earlier that both Capreolus-type and Cajetan-type accounts of personality agreed that once an individual nature possessed esse then it would be a person regardless of whether esse constituted it such or not. Given the Tertia pars text is discussing an individual nature then some account needs to be given of why that nature can possess esse yet not also be a human person. Such an account can be derived, I think, from an analysis of the principles Aquinas employed to determine the criteria for assumption into the unity of a divine person, or to put it in a slightly less cumbersome way, for Chalcedon-consistent assumption.

It is helpful to begin with the criterion which determines what cannot be Chalcedon-consistently assumed. Thus Aquinas insisted in Book IV chapter 43 of the Summa contra Gentiles that no nature which had existed prior to a putative assumption could be Chalcedon-consistently assumed.Footnote ⁴³ Such a nature, he reasoned, would have to exist in a person.Footnote ⁴⁴ Yet what would happen to that person on the supposition of a putative Chalcedon-consistent assumption? If it remained, then there would be two persons in Christ, which is Nestorianism rather than Chalcedon-consistent assumption. If it did not remain after the putative assumption, then it must have been corrupted because, as Aquinas pointed out, ‘no singular ceases to be what it is except by corruption’.Footnote ⁴⁵ However, if the person was corrupted then the nature would be also and likewise the human being who had existed prior to the putative assumption. Hence no Chalcedon-consistent assumption would have occurred. Moreover, since there are no further options – either the person remains, or it does not – then one must conclude Aquinas denied that any nature which had existed prior to a putative assumption could be Chalcedon-consistently assumed.

The next criterion to consider specified how Chalcedon-consistent assumption could occur. It was articulated in response to an objection that distinguished implicitly between two kinds of created supposita: angelic persons, which were ‘complete in their personality from the moment of their creation’,Footnote ⁴⁶ and what we might call non-angelic supposita, which were not ‘complete in their personality from the moment of their creation’. The foundation of the distinction was that angelic persons were not ‘subject to generation or corruption’, whereas by implication non-angelic supposita were and the consequence of this distinction, so the objection went, was that angelic persons were not Chalcedon-consistent assumable, whereas it was implied, non-angelic supposita were.

Aquinas never explicitly identified the non-angelic supposita in question. But given the discussion occurred in an article addressing whether human nature was more assumable than any other nature, we can infer he was distinguishing angelic persons from human persons. The criterion itself, ‘God is able, by producing a new angelic nature, to join it to himself in the unity of person’,Footnote ⁴⁷ supported such an interpretation because it turned out to be applicable to both angelic and to human natures.

What Aquinas appeared to have in mind was that God by creating a new nature – angelic or human – could assume that nature into the unity of a divine person and thereby impede the principle(s) of that nature causing its proper personality.Footnote ⁴⁸ On this view natures could be Chalcedon-consistently assumed only at the moment of creation and that moment would be logically prior to the moment when the principles of those natures caused their proper personalities. Moreover, since there was nothing which pre-existed such an assumption, nothing would be corrupted, and, therefore, this criterion would be consistent with the previous one.

Aquinas's distinction between the capacity of angelic and human natures for Chalcedon-consistent assumption led to a refinement of the theory. That distinction depended upon the latter being ‘subject to generation or corruption’ and hence not ‘complete in their personality from the moment of their creation’. We might surmise that Aquinas drew the distinction because in the natural generation of human beings – at least as he understood it – ‘the body was successively formed and disposed for the soul’.Footnote ⁴⁹ That is, the complexity of the body increased successively as it was informed by a series of transient substantial forms the culmination of which was the body's disposition to receive a rational soul, its permanent and abiding substantial form. At that point the body would be a human being complete in its personality and therefore no longer apt for Chalcedon-consistent assumption. Prior to that however, the body would not be informed by a rational soul. It would not be complete in its personality. It would, therefore, be apt for Chalcedon-consistent assumption.

What made the pre-rational human body – the embryo – Chalcedon-consistent assumable though? Well it was certainly not esse. The embryo possesses matter and form, and clearly it exists. It would be arbitrary in the extreme to deny it possessed esse. So we cannot maintain the simple absence and presence of esse explains why the embryo can and a human person cannot be Chalcedon-consistently assumed.

Perhaps, though, one might wish to defend esse on the grounds the embryo is an exceptional case because it is imperfect. Certainly there is textual evidence in support of this. In the De Potentia Dei for instance, Aquinas said, ‘before the embryo receives the rational soul it is not a perfect being but is on the way to perfection: and therefore it is not in a genus or species except by reduction, just as the incomplete is reduced to the genus or species of the complete’.Footnote ⁵⁰ So given the embryo is imperfect, and its perfection consists in receiving a rational soul so that it can become a human person, one might suggest that the embryo's imperfection vis-à-vis a human person could account for its Chalcedon-consistent assumability. Whereas if the embryo were not imperfect then its esse would prevent it being Chalcedon-consistent assumable.

However, when discussing what would have happened if Christ's flesh – his body – had not been formed in an instant as actually happened in the Incarnation, Aquinas insisted ‘one of two things would follow, either that which was assumed was not yet flesh or the flesh was conceived before it was assumed’.Footnote ⁵¹ The second disjunct does not concern us here; the first, however, suggests the assumption of an embryo without the infusion of a rational soul. And if that interpretation is correct, then the embryo is Chalcedon-consistently assumable in and of itself. But if the embryo is Chalcedon-consistently assumable in and of itself, then it is the embryo's intrinsic character prior to the infusion of a rational soul that makes it Chalcedon-consistently assumable not its imperfection vis-à-vis a human being and thus no escape route has been found for esse.

But what might do the trick? Well, Aquinas observes that rational substances have ‘dominion over their actions’Footnote ⁵² and as a result are given the special name ‘person’. We could therefore suggest ‘dominion over action’ as a criterion which distinguishes the non-Chalcedon-consistent assumable from the Chalcedon-consistent assumable. In effect, if a thing has dominion over action then it will be a person and not Chalcedon-consistent assumable. But if a thing does not enjoy dominion over action then it will not be a person and therefore Chalcedon-consistent assumable. The idea would be that having such dominion is a perfection whose destruction is not in keeping with the goodness of the assumer,Footnote ⁵³ yet that is precisely what would happen if its possessor was Chalcedon-consistently assumed.

This seems promising and would certainly explain why the pre-rational embryo was Chalcedon-consistent assumable. However, it is too broad a criterion, since it would make any non-rational nature Chalcedon-consistent assumable, whilst Aquinas only spoke of rational natures being such. Accordingly, the dominion criterion would need to be supplemented and the best way to do so is by introducing the embryo's imperfection vis-à-vis the human person. We might say any nature is Chalcedon-consistent assumable which (i) does not enjoy dominion over action, and (ii) is ordered to such dominion.

The embryo fulfils both criteria readily. The first because it does not have dominion over action. The second because it is ordered to such dominion, as the imperfect to the perfect. Rational nature, whether angelic or human, also fulfils both criteria readily. The first because it does not have dominion over action – it is not a person. The second because it is ordered to such dominion, as a source to its final perfection. Non-rational natures, however, fulfil the first criterion because they do not have dominion over action. They fail in the second criterion, however, because they are not and indeed cannot be ordered to such dominion; they lack the requisite nature.

Still, regardless of whether the qualified dominion criterion turns out to be a criterion for Chalcedon-consistent assumability, esse certainly is not. Yet if neither esse alone renders the embryo incapable of Chalcedon-consistent assumption, nor esse qualified by the embryo's imperfection vis-à-vis a human being, then it is hard to see how esse can do the job at all. In which case there is no reason why the individual human nature Christ possesses cannot also possess esse without being rendered incapable of Chalcedon-consistent assumption and therefore a person. Admittedly some other account will be needed to explain how persons and individual natures differ but that need not concern us at this time.

Nature in the Tertia pars text

At this point it is helpful to discuss the contrast Aquinas draws between nature and hypostasis or suppositum in the Tertia pars text. Aquinas says,

The contrast Aquinas draws is between suppositum as that which has esse and nature as that by which something has esse. From here however, it is tempting to argue that natures cannot be the subject of esse, only supposita can, and that therefore Christ's human nature cannot possess a proper esse of its own.

However, I think this is mistaken because one can argue that a terminological shift occurred in Aquinas's account of natures in the period between the De Ente et Essentia and the Summa Theologiae. As we have seen earlier, in the De Ente et Essentia Aquinas distinguished between the essence of a species considered as a whole and the essence of a species considered as a part. In each case one was dealing with the same essence though, the individual essence. It was just that when taken as a whole the essence was considered as determinable by accidents but when taken as a part it was not.

Lawrence Dewan has noted however, that as Aquinas's career developed he tended to restrict his use of essence to the essence of a species considered as a part in distinction from the suppositum.Footnote ⁵⁵ One unfortunate consequence of this is that when combined with Aquinas's teaching on the real distinction between nature and suppositum the terms of that distinction tend to be construed as between the essence considered as one part of the suppositum and the suppositum as the whole. In effect the suppositum became what the essence considered as a whole had been previously.

In the case of the Tertia pars text, then, if we read essence considered as a whole in place of suppositum or hypostasis, there will no longer be any need to regard the essence as incapable of possessing a proper esse of its own. Moreover, since essence as a whole and essence as a part are just ways of considering the same essence, then if essence as a part has esse so does essence as a whole: same essence, same esse. Further, since I have also argued that whether one considers the individual essence as such, or the specific essence with or without precision, one is considering the same reality, then similarly if the essence considered as a part has esse so does essence considered as a whole and so does the individual essence.

A further concern might arise from comparing natures and accidents as that by which something is such and such. The essence, so taken, can end up being treated as an accident, having no esse of its own, but merely acting as that in virtue of which a suppositum has such and such esse. Yet this does not work either. If supposita are just essences considered as a whole then it will not make any sense to say the essence considered as a part has no esse of its own, but the essence considered as a whole does. It is the same essence in each case. Further, the essence is not an accident it is a substance. We can treat it as like an accident in some respect but that is just a way of considering it, not a way in which it exists.

The individual nature does not contribute esse

Still if an individual nature can possess esse without being a human person and such a nature is the reality referred to in the Tertia pars text, then one still needs to explain why the divine person whose nature it is does not receive any further esse from that individual nature.

In the Tertia pars text Aquinas offers the following account of two ways in which a person can receive esse. He writes,

Esse, therefore, can pertain to the person as such or in a qualified manner. In the former case it pertains to the person simpliciter and cannot be multiplied; it can only be one. Socrates is given as an example of the thing to which such esse pertains. However, since Socrates is a person and a person is a special type of substance, we can say at the level of created beings personal esse and substantial esse refer to the same esse: that in virtue of which Socrates exists simpliciter. In the latter case esse pertains to the person secundum quid – in a qualified way – and can be multiplied. A predicamental accident, whiteness, is given as an example of the thing in virtue of which this esse pertains to a person. Further, just as whiteness differs from musicality, so also does the esse which pertains to the person in virtue of possessing these accidents.

Yet how is the distinction between esse simpliciter and esse secundum quid drawn? It is worth noting that the distinction is intended to illustrate the relationship between Christ's human nature and any putative esse the Word might receive in virtue of that human nature. Since Christ's human nature is not an accident, however, the distinction cannot be glossed in terms of substance and accident. Something else will be required and the most obvious explanation is in terms of essences.

That is, take Socrates and ask what is he? The answer is a human being. Since we want to establish a universal claim, we will have to focus on what is common to human beings specifically. Thus we can say the personal esse of Socrates will be that esse which is common to all human beings and only to human beings. Since whiteness does not satisfy this criterion – there are white cats for example – then white esse will be esse secundum quid.

How might we apply this to a divine person? Well again focus on what is common to divine persons, in which case the personal esse of the Word will be that esse which is common to all divine persons and only to divine persons. What is not common to divine persons and only divine persons will be esse secundum quid.

Does that mean any putative esse the Word might receive in virtue of Christ's human nature would have to be esse secundum quid? I do not think so. The divine esse is identical to the divine essence, the divine essence is infinite, and, therefore, the divine esse is infinite. No created substance could add esse to the divine esse because (i) that esse is infinite and more cannot be added to it. (ii) the divine essence is participated by all creatures in some degree of likeness,Footnote ⁵⁷ and therefore any esse a subsistent might possess is already contained within the divine esse. Therefore as long as the esse of Christ's individual human nature is the same as that a human person would have then there is no reason to think the Word who possesses that individual human nature will thereby acquire new esse.

Two further points need to be mentioned. First, accidents do not subsist and, therefore, will neither imitate the divine esse nor participate it. As a result when Christ possessed accidents in virtue of assuming human nature Christ also acquired the esse secundum quid corresponding to those accidents.

Second, nor need this account mean Christ's individual human nature becomes part of Christ's divine nature and Aquinas's position collapses into monophysitism. In the Tertia pars text Aquinas draws an analogy between the way in which parts such as the head, hands, and eyes relate to the person of Socrates and the way Christ's individual human nature relates to the divine Word. In his Commentary on Boethius's De Trinitate however, Aquinas argues that fingers, feet, hands, and other such parts – parts which are essentially the same as the parts Aquinas is discussing in the Tertia pars text – are not parts of the definition or essence of humans but are material parts which depend upon the definition of humans.Footnote ⁵⁸ Yet if material parts are not parts of an essence in the natural order, then there is even less reason to think they are parts of an essence in the supernatural order. Therefore allowing that Christ's individual human nature is a material part of a divine person, it will not be part of the divine essence.

In sum the divine essence as imitable explains why the divine person receives no new esse from Christ's individual human nature, whilst the characterisation of Christ's individual human nature as a material part explains why that nature does not become part of the divine essence, thereby rendering Aquinas a Monophysite.