§1. Textual References Chronologically Considered

Aquinas's observation that the term ‘being’ can be said in two ways (dupliciter) appears numerous times throughout his corpus. Sometimes he makes this observation with regard to the term ‘ens’ whereas other times he does so with regard to the term ‘esse’. As we have already seen, on occasion he observes that ‘being’ can be said in three ways. The occasion for Aquinas to draw these two- or threefold distinctions occurs most frequently when he wishes to explain how evil can be said ‘to be’ even though it is a privation. At other times, he does so to explain how we can know the ‘is’ in the assertion ‘Deus est’ even though we do not know the esse that is God's essence. And, still other times, he draws these distinctions in order to address whether there is only one esse in Christ. The question I am interested in here, however, is less about the context than the observations themselves and what Aquinas means by them.

In order to compare the various relevant texts, and to do so in a manageable way, I have produced the table in Fig. 1, which catalogs the various instances throughout Aquinas's corpus when he explicitly tells us that the terms ‘esse’ and ‘ens’ can be said, name, or signify, in two or three ways. In other words, it focuses on those texts in which Aquinas compares different senses of these terms.^{Footnote 5} Hence, this catalog does not pretend to be exhaustive, since there are a number of other, non-comparative statements by Aquinas regarding what the terms ‘esse’ and ‘ens’ signify. I will address some of these other texts in what follows. For now, it should be noted that the last three columns in the table follow the order in which he presents his considerations of the relevant term in a given text; my summaries in these columns attempt to be as literal as possible given the space provided.^{Footnote 6} The shaded cells are intended to highlight locations where Aquinas explicitly observes that ‘esse’ or ‘ens’ names or signifies essentia, or the related notions of quidditas and natura.

1 Texts on Ways that ‘Esse’/‘Ens’ is Said.

In reviewing this table, we find the following commonalities among the various texts. First, in all of these texts Aquinas's principal considerations are ones about ‘esse’ and ‘ens’ taken as terms, addressing how each either can be said (dicitur), name (dicitur quod), or signify (significat). Another common feature, present in all of these texts, with the exception of 13, is the explicit mention of a sense of the term ‘being’ that signifies the truth of a composition in a proposition. Considering this sense, he makes clear in some of these texts (4, 7, 8) that the verb ‘est’ functions in this way in its role as a copula, joining a predicate to a subject. For this reason, he notes at times that ‘esse’ taken in this way is present in the mind (intellect, reason), rather than in things (2, 4–8, 11, 14). In texts 5, 12, and 13 he also notes (or indicates) that this sense of ‘being’ answers the question ‘Is it?’ (an est).^{Footnote 7} Going forward, I will refer to this sense of ‘being’ as the copulative sense of ‘being’, or simply as copulative being.

Another common feature of these texts is Aquinas's identification of a sense of ‘being’ that is divided by the categories (1, 2, 5, 7–9, 12–14). Thus, nine of the fourteen texts indicate what is clearly a fundamental sense of the term for Aquinas, which I will refer to as the categorial sense of ‘being’, or simply as categorial being. In some of these texts he makes clear that—unlike the copulative sense—the categorial sense of ‘being’ signifies something that exists outside of the mind in reality (5: in natura existens; 6: extra animam existens; 8: in rerum natura). This categorial sense of the term is twice associated with a third sense of ‘being’ identified by Aquinas (7, 8), which I will refer to as the ‘actuality sense of ‘being’’ or ‘being as act’. We find this actuality sense mentioned in four other texts without reference to the categories (3, 4, 10, 11). As he makes clear, here ‘ens’ and ‘esse’ signify the act of existing (actus essendi) (10, 11) which is the act of an essence (3, 7) and, hence, the act of a being inasmuch as it is a being (actus entis in quantum est ens) (8). Still, although Aquinas mentions the division of the categories in texts 7 and 8 when presenting the actuality sense of ‘being’, we would be mistaken if we took that sense to be the same as the categorial sense of the term. If we look closely at text 7, he makes clear that, according to the actuality sense, the term ‘names the esse that belongs to the nature of a thing inasmuch as it [i.e., the nature] is divided according to the ten categories’.^{Footnote 8} Similarly, in text 8 he notes that the sort of esse that names the act of a being inasmuch as it is a being is ‘attributed only to the very things that are contained in the ten categories; hence, from such esse is named the [sort of] being (ens) that is divided by the ten categories’.^{Footnote 9}

In sum, the sense of ‘being’ that is divided by the categories is not the actuality sense of the term but, rather, the categorial sense, which receives the name of ‘being’ (ens) from the act of existing inasmuch as things receive, or have, such an act, which is the act of their essence (3, 4). As Aquinas makes clear in other texts (1, 2, 9, 12–14), it is essences (quiddities or natures), that are divided by the categories.^{Footnote 10} Hence, he identifies a sense of the term ‘being’ (whether ‘ens’ or ‘esse’), that signifies essence. Going forward, I will refer to this sense of the term as the quidditative sense of ‘being’, or quidditative being for short. And, from the foregoing analysis, we see that the quidditative sense of ‘being’ is identical to the categorial sense of the term for Aquinas, since it is quidditative being that is divided by the categories.

To sum up, in these texts Aquinas identifies the follows three senses of the term ‘being’, whether taken as ‘ens’ or ‘esse’:

• • Copulative Sense: Signifies, as the copula, a composition of the intellect and, hence, the truth of a proposition.

• • Categorial/Quidditative Sense: Signifies essence (quiddity or nature) and is divided by the categories.

• • Actuality Sense: Signifies the act of existing (actus essendi), which is the act of an essence and, hence, the act of a being inasmuch as it is a being (actus entis in quantum est ens).

If we review the texts in this table, we see that Aquinas acknowledges a quidditative sense of ‘being’ from the start of his career to the end. More commonly he presents ‘ens’ as a term that can signify essence (1, 2, 6, 9, 10, 13, 14). But, at times, he presents ‘esse’ as doing the same, notably in his early Commentary on the Sentences (4, 7), but even as late as the De potentia (10, 1265–66). This fact should help us recognize that Aquinas's acknowledgement of a quidditative sense of ‘esse’ is not a mere youthful reporting of an Augstinian/Boethian usage of the term by others. Indeed, the common authority whom he cites for support in his considerations of the different senses of the term ‘being’ is none other than Aristotle (1, 5, 7, 8, 9, 10, 12), including when Aquinas presents ‘esse’ as signifying essence (7, 10). His reference to the Philosopher on this point would suggest that Aquinas acknowledges a quidditative sense of the term ‘esse’ for substantive reasons in accord with his own philosophical thought. To get a sense of why that is, we should turn to the text of Aristotle that Aquinas references in these passages, namely, Metaphysics V, 7.

§2. Aquinas on the Quidditative Sense of ‘Ens’ and ‘Esse’ in Metaphysics V, 7

In Metaphysics V, 7 Aristotle considers the ways in which the terms ‘being’ (on) and ‘to be’ (einai) are said. He begins with an initial distinction between (1) accidental (kata sumbebēkos) and (2) per se (kath’ auto) senses of ‘being’ and treats the former first, by looking at different types of accidental predications. As regards the latter, Aristotle looks at the figures of predication, namely the categories. He then identifies a third sense of ‘to be’, noting that (3) the term ‘is’ can indicate that a statement is true and the phrasing ‘is not’ that it is false. Finally, he observes that (4) sometimes in statements the terms ‘to be’ and ‘is’ mean that something is potentially or that it is actually.^{Footnote 11}

A surface reading of this text might suggest that Aristotle is presenting only the categorial sense of ‘being’ as what he terms ‘per se being’.^{Footnote 12} But when considering this passage in lectio 9 of his Commentary on Metaphysics V, 7—in what I will call the CM text—Aquinas tells us that in fact each of the last three senses of ‘being’ pertain to per se modes of being. In summary, Aquinas sees the following senses of ‘being’ as presented in by Aristotle:

If we return to the texts treated in the table from Fig. 1, it is clear that when Aquinas speaks of ‘being’ as said in two or three ways (whether ‘ens’ or ‘esse’), he has in mind Aristotle's per se being and is leaving out of consideration per accidens being (presumably, precisely because it is per accidens). Thus, in text 1 from his early De ente et essentia, Aquinas explicitly tells us: ‘We should note that, as the Philosopher says in Metaphysics V, ‘per se being’ (ens per se) is said in two ways: [1] in one way, as it is divided by the ten categories; [2] in another way as it signifies the truth of propositions’.^{Footnote 13} We might wonder why Aquinas here leaves out being as divided by act and potency and how it relates to the actuality sense of ‘being’. An answer to this question is indicated in his prefatory remarks from the CM text, where Aquinas tells us that Aristotle does the following when treating of the modes of per se being:

1. (1) First, he divides [the mode of] ens that is outside of the mind (extra animam) by the ten categories, which is ens perfectum.

2. (2) Second, he sets out another mode of ens, which is only in the mind […]

3. (3) Third, he divides ens by potency and act. And ens divided in this way is more common than ens perfectum. For ens in potency is ens only in a qualified way and is imperfect […].^{Footnote 14}

As in the texts from Fig. 1, here too Aquinas presents categorial being as outside of the mind and copulative being as only in the mind. What is different here is his presentation of categorial being as ens perfectum. In describing categorial being as perfect, or complete, Aquinas goes beyond a literal commentary of the text. We see that the contrast he is drawing is not with copulative being but, as he notes, with being that is in potency, which is being only in a qualified and imperfect way. Does that mean that categorial being is simply to be identified with actual being and, hence, in that respect, actuality? The answer is clearly, no. As we proceed further into the CM text, it becomes clearer how he sees Aristotle's third mode of being as related to the first two:

Here, we find an implicit answer to our question of why, in the De ente, Aquinas presents only two of Aristotle's three senses of per se being. The reason is that being as divided by act and potency is not a third sort of being in addition to categorial and copulative being but, rather, is a division of each of those two since both can be divided by act and potency. Aquinas clearly sees, in a certain respect, a priority of the first two modes to the third—namely, the priority of the divisible to what divides it. Hence, in the De ente, as in other texts (e.g. 2, 5, 6, 9, 12–14), he focuses his attention only on categorial and copulative being.

We see, then, that categorial being is not limited to the actual. Indeed, later in his commentary on Book X of the Metaphysics (text 14), he tells us that ‘the being that is divided by the ten categories signifies the very natures of the ten categories that are [either] in act or in potency’.^{Footnote 16} Why, then, when commenting on V, 7 does Aquinas begin his consideration of categorial being by identifying it as ens perfectum? Presumably because, as he tells us elsewhere, the division by act and potency is a division of the analogical which is a division according to priority and posteriority.^{Footnote 17} Thus, the primary mode of categorial being pertains to actual beings, with potential beings said to be in the categories only with reference back to actual, or perfect, beings.

What is most of interest to us, however, is how Aquinas finds in Metaphysics V, 7 a quidditative sense of the verb ‘esse’ (‘to be’). Commenting on Aristotle's presentation there of categorial being, Aquinas goes beyond the text at hand by reminding us that being (ens) is not a genus and, hence, cannot be contracted to the diverse genera of the categories by means of the addition of differences. Instead, we are told, it is contracted to the categories according to diverse modes of predication (modi praedicandi) that follow upon the diversity of modes of existing (modi essendi). Aquinas explains this view both by quoting Aristotle and by explicating his words for the reader:

Thus far, we might wonder whether Aquinas is focusing on the actuality sense of the term ‘esse’, since he is identifying modi essendi: ways of existing. But the quidditative sense of this term is brought out more clearly in his semantic analysis that immediately follows:

What is noteworthy in this text is Aquinas's presentation of the term ‘esse’ (and its conjugated form ‘est’) as signifying—not actuality—but, rather, categorial natures, with the examples of substance, quantity, and quality. In other words, he presents ‘esse’ and ‘est’ here as signifying various types of quiddities. Surely, he is not excluding some sort of connection between predication and the act of existence. But to be clear: it is the act of existence as modified by some essence.^{Footnote 20} And, as he indicates here, this modification is seen in the very use of the verb ‘to be’ (esse). To illustrate this fact, let us consider the two example propositions he provides of distinct modi praedicandi offered to reveal distinct modi essendi:

1. (1) ‘A human is an animal’.

2. (2) ‘A human is white’.

As Aquinas presents the matter, the diverse modi essendi of the predicates in these propositions are revealed to us by the very way the predicates are affirmed of the subject using the verb ‘is’. In this way, diverse modi praedicandi—diverse ‘is-es’—somehow signify the fundamental kinds of essences that are the categories. It is here that we must turn to Aquinas's semantic theory to see how ‘esse’ can signify essence in this way.

§3. Modes of Signification and of Predication: How ‘Esse’ and ‘Est’ Signify Essence

Following Aristotle, Aquinas holds that words signify conceptions of the intellect, which conceptions in turn are the likenesses of things. Thus, in a mediated way our words signify things.^{Footnote 21} As Aquinas sums up this role, ‘The ratio that a name signifies is a conception of the intellect of the “thing” (res) signified by the name’.^{Footnote 22} Whereas the significatum of a name's ratio (account, analysis) is a conceptualization within the mind, then, this conceptualization is itself the likeness of some res; thus, the ‘thing’ signified (res significata) by a term is something beyond the concept.^{Footnote 23} Paradigmatically, this res significata is something outside of the mind (extra animam), in reality (in rerum natura).^{Footnote 24}

Here, we need to be careful not to confuse the medieval accounts of signification and supposition. The extramental res that a name signifies is typically not the same as what the name supposits for (i.e., references).^{Footnote 25} When we say that ‘A human is an animal’ (homo est animal) the res significata of ‘human’ is not an individual human being, such as Socrates. Rather, an individual human is what the term ‘human’ supposits for, or references, in the context of this proposition. By contrast, what the term ‘human’ signifies, Aquinas explains, is human nature.^{Footnote 26} In short, the ‘thing’ that is signified by a name is neither an individual, nor a collection of individuals, but some form (nature, property, perfection).^{Footnote 27} Thus, humanity (humanitas) is the res significata of the term ‘human’. With that said, humanity is also the res significata of the term ‘humanity’. In other words, both ‘human’ and ‘humanity’ signify the same res. Still, each does so in a different way—according to a different mode. Indeed, in accordance with Aquinas's terminist semantics, the res that is signified by a word is always signified according to a modus significandi.

Unlike the later speculative grammarians known as the Modistae, Aquinas himself does not provide us with any systematic treatment of the modes of signification of terms. With that said, he does clearly acknowledge throughout his corpus a distinction between a number of grammatical and logical modi significandi—for example, between male, female, and neuter nouns; between different cases of nouns; and between different tenses of verbs. And, what concerns our considerations, he identifies a distinction between concrete and abstract modi significandi.^{Footnote 28} Regarding these modi significandi, Aquinas is clear about this much: just as words signify an extramental res in a mediated way via concepts, so modes of signification follow upon extramental modes of existing (modi essendi) in a mediated way—namely, through modes of understanding (modi intelligendi).^{Footnote 29} In affirming this connection between this triad of modes—significandi, intelligendi, essendi—Aquinas shares something in common with the Modistae. But unlike these speculative grammarians, Aquinas does not see a simple isomorphism between word and reality. Modi significandi have a foundation in reality, but not a necessary one-to-one correspondence between that mode and a modus essendi.^{Footnote 30} Nevertheless, on this point he is clear: words that signify their res according to a concrete modus significandi do so because of some extramental composition.^{Footnote 31}

We find this view exemplified with the term ‘human’: it signifies the form humanity, but according to a concrete mode. This is because what ‘human’ fully signifies (its significatum) is a haver of humanity (habens humanitatem). Similarly, the concrete term ‘something white’ (album) signifies a haver of whiteness (habens albedinem).^{Footnote 32} In each case, the formality that is signified (humanity, whiteness), is signified as in a haver. In this way, although the concrete terms ‘human’ and ‘white [thing]’ do not principally signify composition, they nevertheless consignify it by implication, or in Aquinas's terms, ex consequenti. In this respect, the consignification of a term is a secondary, or additional, signification that is, as it were, an ‘accidental’ property of the term, which follows from a term's mode of signification. As with nouns and adjectives, so too with verbs: they have both abstract and concrete modes of signification, and following the latter, verbs consignify composition when taken according to a concrete mode of signification. And we find this to be the case for Aquinas no less with the verb ‘to be’ (‘esse’).

The relevance of these semantic distinctions for verbs is brought out nicely in Aquinas's Commentary on the De hebdomadibus where he famously draws a comparison between the verbs ‘currere’ and ‘esse’. Considering the former, he tells us that the term ‘running’ (currere), signifies according to an abstract mode, in a manner parallel to the term ‘whiteness’; by contrast, the term ‘someone who runs’ (currens) does according to a concrete mode, in a manner parallel to the term ‘white’. Similarly, he notes, ‘esse’ signifies abstractly, whereas ‘what is’ (quod est), or ‘a being’ (‘ens’) does so concretely. Thus, just as we say of a runner (currens) that ‘He runs’ (currat) inasmuch as he participates in running, so we say of a being (ens), that ‘It is’ (est) inasmuch as it participates in an act of existing (actus essendi).^{Footnote 33} The composition that is consignified by the concrete term ‘ens’ is brought out by Aquinas in other texts, where he tells us that this term signifies that which has esse (id quod habet esse), or a haver of esse (habens esse), and also the subject of esse (subiectum essendi).^{Footnote 34}

In light of these semantic considerations, we can begin to see how Aquinas can hold that ‘ens’ can signify essence. In saying this, he does not mean that essence is the res significata of this term. As we see in the various formulations above, the res that ‘ens’ signifies is esse taken as the act of existence. Nevertheless, it signifies this res according to a concrete mode of signification, and therefore consignifies composition ex consequenti; in this way, essence is indicated by the ‘quod’ of ‘quod est’ and the ‘habens’ of ‘habens esse’. Less evident is how Aquinas could see ‘esse’ as signifying essence. Unlike the participial noun ‘ens’, the infinitive verb form ‘esse’ signifies according to an abstract mode. As an abstract term, it signifies as something simple and as that by which something is (quo est).^{Footnote 35} Thus, the very modus significandi of the term ‘esse’ would seem to prevent it from signifying essence at all. Our question is thus heightened: Why would Aquinas say on occasion that ‘esse’ can signify essence?

We begin to get an answer to this question if we consider what Aquinas tells us in text 7, from Super Sententiis (1252–56). There, in the context of considering whether there is only a single esse in Christ, he notes that the term ‘esse’ is said in two ways: (1) as it signifies the truth of a proposition, inasmuch as it is a copula, and (2) as the act of a being (actus entis) resulting from the principles of that thing. Then, he adds a third way:

We find Aquinas in this text providing us with something of an etymological account for the quidditative use of the word ‘esse’. As I read him here, this third sense of the term ‘esse’ does indeed have a different res significata than the actuality sense of the term. If that is the case, then he is presenting two different terms that signify two different concepts. Let us refer to the actuality sense of the term as ‘esse _A’ and the quidditative sense signifying essence as ‘esse _E’. As Aquinas presents the matter, the rationale for ‘esse _E’ signifying essence is that the word in this case is imposed from (derived from) the act of existing, but by convention what it is imposed upon (applied to) is not that act itself but rather what is actualized by that act, namely essence. Complementing this etymological account of ‘esse _E’ is text 4 from Fig. 1, which appears earlier in the same work. There, Aquinas justifies the Boethian use of the term ‘esse’ to name the quiddity, or nature, of a thing with a distinctly Aristotelian account, namely, that ‘a definition is speech signifying what-it-is-to be (quid est esse), for a definition signifies the quiddity of a thing’.^{Footnote 37} In short, texts 4 and 7 indicate that the res significata of ‘esse _E’ is different from than that of ‘esse _A’ even though the former term is etymologically derived from the latter. If that is the case, then in these texts the two uses of the word ‘esse’ are indeed homonymous.

It is worth noting at this point, however, that in these texts Aquinas is not using the term ‘esse’ but identifying how it can be used. We have here a case of the use-mention distinction noted by analytic philosophers. Aquinas is indeed clarifying for us how the word ‘esse’ can be used, but when he tells us in these texts from Fig. 1 that ‘esse’ can name, signify, or be said in two or three ways, the term ‘esse’ is referencing the word. It is as if Aquinas were saying, ‘This word that is spelled e-s-s-e can be used in these different ways’. Or—to make the same point according to his own terminist semantics—when Aquinas tells us that ‘“Esse” is said in two/three ways’, the term ‘esse’ supposits with material supposition, referencing the word itself, even though the assertion as a whole concerns how the term signifies.

Highlighting Aquinas's application of the use-mention distinction is key for us to understand how he does and does not adopt as his own a quidditative sense of the term ‘esse’. On the one hand, if ‘esse’ is taken as suppositing only for the word spelled e-s-s-e, then Maurer is quite right to say that Aquinas reserved the term ‘esse’ to mean the act of existing (‘esse _A’). With that in mind, I believe that texts such as 4 and 7 can be fairly read as Aquinas reporting the conventional use of ‘esse _E’ by others (e.g., Augustine, Boethius, William of Auvergne). On the other hand, Maurer's assertion needs to be qualified in light of the later text 10 from the De potentia as well Aquinas's treatment of the meanings of ‘esse’ in the CM text. In texts such as these, I believe, Aquinas again makes mention of the term ‘esse’ with material supposition—not to supposit merely for that word as such—but, instead, as the name for the verb ‘to be’ along with its various grammatical modes, such as conjugated and participial forms. Conventionally in Latin, as in English, the infinitive mode of a verb acts as such a name. With that in mind, we must consider that in certain contexts when Aquinas speaks of about the term ‘esse’, he should be read as employing that verb according to its infinitive mode as a name inclusive of any of its various derivative grammatical forms, such as ‘ens’ and also ‘est’.^{Footnote 38} If we take the term ‘esse’ in that way, we find that at times Aquinas presents the verb as capable of signifying both actuality and essence simultaneously, albeit in different respects.

To get a sense of this usage, let us consider again Aquinas's analysis of categorial being in the CM text. As we have seen, Aquinas tells us there that ‘when we say, “A human is an animal”, ‘esse’ signifies substance. And when we say, “A human is white”, [‘esse’] signifies quality. And so forth for the others [i.e., other categorial modes of predications]’.^{Footnote 39} In looking at this text, we were left with the question of how ‘is’ (‘est’) could signify essence in these examples. The question is heightened by what Aquinas tells us in his commentary on Aristotle's Peri hermeneias. There, acknowledging the copulative sense of ‘is’ as signifying the composition of proposition, he clarifies that this role of the verb is in fact secondary to its principal signification:

If in propositions such as Aquinas's examples from the CM text the verb ‘is’ principally signifies actuality and consignifies the composition of a proposition (and hence its truth) ex consequenti, how does it quidditatively the various categories of being? The answer again lies in a consideration of mode of signification entailed in the term ‘est’. Like the participial noun ‘ens’, the verbal form ‘est’ signifies according to a concrete modus significandi.^{Footnote 41} And thus it consignifies implicitly and ex consequenti the composition of a subject with the res significata that it principally signifies. Granted, Aquinas tells us that ‘is’ said simply (i.e., on its own) signifies act taken absolutely. But simply to say ‘Is!’ is not to make an assertion at all. A complete assertion requires a subject as well as a predicate.^{Footnote 42} Thus, with the verb ‘est’ one must assert either that ‘x is’ or that ‘x is F’. As we have seen Aquinas indicate in the CM text, it is through the analysis of the latter sort of statement that we can discern the fundamentally diverse modi praedicandi that in turn point to the fundamental modi essendi, which are the ten categories of essences.^{Footnote 43} The reason, again, is that as a concrete term, ‘est’ not only signifies its res significata but does so according to a concrete mode. Aquinas indicates as much in his commentary on the Peri hermeneias where, after noting that ‘is’ said simply signifies to be in act, he adds,

Here, we have one of Aquinas's classic texts on the inherence theory of predication, which holds that the predication of a common term F of an individual x is true if and only if that form ultimately signified by F in x actually exists.^{Footnote 45} In presenting this account, Aquinas indicates a double role for ‘is’ in a statement of the sort, ‘x is F’. In one way, it signifies the actuality of the nominal predicate term, and in another it joins that term to the subject signifying ex consequenti that that actuality inheres in the subject. In this text, then, we find Aquinas acknowledging that both the actuality sense and the copulative sense of ‘esse’ are simultaneously at work, each in its own respect. To bring out the distinctions being made in this text, consider the following visual analysis of the proposition, ‘Socrates is white’, offered in Fig. 2:

Figure 2 The Copulative Role of ‘Is’

What we see is that Aquinas presents the term ‘is’ in such a proposition as looking, if you will, both forward and back. Its role as the copula, it looks back to the subject, joining the nominal predicate to the subject and, in doing so, signifies composition and, hence, the truth of a proposition. But he makes clear that this copulative sense of ‘is’ is secondary to its principal sense of signifying actuality. And in the context of such a proposition, ‘is’ no longer signifies actuality absolutely (as it does simply, on its own) but, rather, it signifies the actuality of the nominal predicate, namely by entering into composition with it to form the full predicate of the proposition. In our example proposition, then, the complete predicate is not simply the term ‘white’ but, rather ‘is-white’.^{Footnote 46} And that nominal predicate in turn, modifies the actuality signified by ‘is’ so that, in the context of the proposition, its concrete modus significandi takes on the modus praedicandi of consignifying the actuality of quality. And, as Aquinas tells us in the CM text, ‘it must be the case that for each mode of predication, ‘esse’ would signify the same’, namely, one of the ten fundamental quidditative categories.^{Footnote 47}

We find something similar, mutatis mutandis, with propositions of the form, ‘x is’. Aquinas tells us that ‘when we say, ‘Socrates is’ we intend by this nothing other than to signify that Socrates is in reality (in rerum natura)’.^{Footnote 48} In other words, we find ‘is’ again signifying actuality, but this time it is the actuality of the subject since, for Aquinas (pace Kant, Russell, and others) ‘is’ here functions as the entire predicate. And, as before, the quidditative sense of the word is simultaneously at work. We get a clear indication of this fact if we return to the CM text. There, after having considered Aristotle's treatment of categorial being as the first mode of per se being, Aquinas looks at Aristotle's presentation of the second, copulative, mode and draws a contrast between the two:

As Aquinas explains, the reason that ‘is’ taken as a copula is an accidental predicate of Socrates is that it is accidental to him whether or not we say anything of him at all. What is particularly intriguing about this text, however, is Aquinas's observation that the term ‘is’ in the proposition ‘Socrates is’ can sometimes be a substantial predicate. On the one hand, this would seem to follow since his actus essendi is the esse of a substance: esse substantiale. On the other hand, Socrates as a substance does not exist by his very essence. How then can this ‘is’ be a substantial predicate?

This question is heightened if we consider another text in which Aquinas addresses the same proposition. Composed shortly before his Commentary on the Metaphysics, Quodlibet 2 (Christmas 1269) addresses in question 2, art. 1 [3], whether an angel is substantially composed of essence and esse. In this text, Aquinas again provides a twofold distinction regarding being, but this time in terms of answers to two different questions: ‘Is it?’ (an est) and ‘What is it?’ (quid est).^{Footnote 50} Our concern is with Aquinas's consideration of the first of these two questions, regarding which, he explains,

As with the CM text, here Aquinas presents two ways in which we may take the ‘is’ of the proposition ‘Socrates is’. The second is clearly the copulative sense of the word. By contrast, according to the first way, ‘is’ signifies entitas rei. The terminology is less familiar to us, perhaps, but the context and question raised by the article make clear that this first way concerns the actuality sense of ‘is’. Suffice it to say, in this text Aquinas makes clear that in both ways, the ‘is’ in the assertion ‘Socrates is’ is an accidental predicate.^{Footnote 52} As before, we can see that the copulative sense is an accidental predicate because it is accidental to Socrates that we make any assertion about him at all; as regards the actuality sense, the remainder of the article makes clear why this too is accidental, namely, because the act of existence of a being such as Socrates is really distinct from his essence.

We might wonder whether Aquinas's observations in this quodlibet can be reconciled with those in the CM text where he identifies a sense of ‘is’ that is a substantial predicate. Indeed, we might wonder how any form of the verb ‘to be’ (‘esse’) can be predicated substantially of a creature, since Aquinas notably holds that only God is a being essentially since only God is by his very essence. Here, I would argue, we can draw upon the same semantic distinctions as we have before. In both the CM and quodlibetal texts, the non-copulative senses of ‘is’ in the proposition ‘Socrates is’ are the same term whose res significata is esse taken as the act of existing. But the two texts differ in their point of focus. In Quodlibet 2, Aquinas's point of focus is precisely upon the formality signified by that res, and so he concludes that what ‘is’ in this assertion is an accidental predicate since Socrates’ actus essendi does not belong to his essence. In the CM text, the point of focus is instead on the concrete modus significandi of the term ‘is’ together with the modus praedicandi it takes on in conjunction with the subject term of the proposition, which is a singular substance, namely, Socrates. From this perspective, even though what the ‘is’ in the statement ‘Socrates is’ principally signifies an act of existence that is accidental to his essence, it nevertheless consignifies it ex consequenti as a substantial predicate.

§4. Conclusion

What we have found from this review of texts throughout Aquinas's corpus is that these occasions in which he identifies the different ways in which ‘being’ is said reveal a consistent general position throughout his career: that in a way, the term ‘esse’ can signify essence. Nevertheless, in saying this Aquinas does not mean to indicate that in his view the term ‘esse’ itself should be directly employed as a synonym for the words ‘essence’, ‘quiddity’, or ‘nature’. Rather, what we have seen, is that when he speaks of ‘esse’ in this way, in his own voice, Aquinas tends to employ the term as the name of the verb, standing for its relevant conjugated and participial forms, such as ‘est’ and ‘ens’. Although these terms do not principally signify essence, following from their concrete modus significandi and relevant modus praedicandi they nevertheless consignify it. In this way, the quidditative sense of these terms does not as such exclude the other senses of ‘per se being’, whether copulative or actuality. More broadly, what we have found is that Aquinas looks to the modi significandi and modi praedicandi of terms to carefully draw conclusions about the modi essendi of real beings, illustrating for the reader of his metaphysical thought the importance of having a familiarity with his semantic theory. What these considerations have also revealed is that for Aquinas, although the grammarian and logician are in their own way concerned with semantic modes, it is not their job to employ them to discern the various senses of the term ‘being’ or the fundamental modes of being. In the end, this is a task for the metaphysician.^{Footnote 53}