This paper presents a reconstruction of the “existential argument” for the existence of God that seems implicit, if somewhat elusive, in the writings of Thomas Aquinas. The reconstructed argument corresponds to no single passage of Thomas’s but gathers and synthesizes arguments used by him throughout his writings. The paper then attempts to evaluate the argument’s soundness against the background of Thomas’s metaphysical principles. There is ample motivation for desiring such an evaluation. John Haldane has recently described the existential argument as perhaps Thomas’s “most original contribution to the search for theistic proofs.” Yet, ironically, several prominent Thomistic philosophers have denied the soundness, and indeed the very possibility, of any such existential argument, leveling criticisms based on textual issues in Thomas’s works, alleged similarities to Anselm’s ontological argument, and perceptions of circular reasoning. The paper will attempt to show that these criticisms are unfounded.
In a few places in his later works, Thomas Aquinas presents formally-organized arguments for the existence of God.1 By his own account, these arguments were not original with him but were collected by him from “both philosophers and Catholic teachers.”2 In many other passages found throughout the whole of his works, Thomas employs arguments that seemingly demonstrate the existence of God as an intermediate step in the course of discussing other points. The latter passages, while not formally organized or presented as arguments for the existence of God, often reflect more distinctly Thomas’s own metaphysical thought. In particular, these passages often make use of Thomas’s unique view of esse as the actus essendi and the consequent real distinction of essence and esse in all finite beings. A well-known example of this more metaphysical approach occurs in De Ente et Essentia,3 but there are a great many other instances of this approach in Thomas’s writings.
This paper presents a reconstruction of the “existential argument” for the existence of God that seems implicit, if somewhat elusive, in the writings of Thomas Aquinas and attempts to evaluate the argument’s soundness against the background of his metaphysical principles. There is ample motivation for desiring such an evaluation. John Haldane has recently described the existential argument as perhaps Thomas’s “most original contribution to the search for theistic proofs.”4 A variant of the argument situated within the framework of analytic philosophy has been developed at length by Barry Miller.5 Yet, ironically, several prominent Thomistic philosophers have denied the soundness, and indeed the very possibility, of any such existential argument. The paper will attempt to show that the charges of unsoundness, as formulated by these critics, are unfounded.
The reconstructed argument to be presented corresponds to no single passage of Thomas’s but gathers and synthesizes arguments used by him throughout his writings. The argument will first be sketched in a schematic form, stating its individual steps. The elements of Thomas’s theory of demonstration that bear on the evaluation of the existential argument will then be highlighted. Each step of the argument will then be briefly discussed to show its accordance with metaphysical principles regularly employed by Thomas.6 Finally, the soundness of the argument will be evaluated in light of the criticisms advanced by some contemporary Thomists.
The reconstructed existential argument can be stated in schematic fashion as follows:
Thomas conducts his discussion of arguments for the existence of God within the context of a particular theory of demonstration or scientific argumentation that he draws largely from Aristotle’s Posterior Analytics.7 Certain details of Thomas’s theory of demonstration are crucial for his treatment of per se nota propositions and will therefore be important for evaluating critiques of the existential argument. Within his theory of demonstration, propositions are formed from terms, which are linguistic expressions of concepts. For example, the proposition “man is mortal” is composed from the term “man,” serving as its subject, and the term “mortal,” serving as its predicate. Every term has a definition that expresses its meaning. For some terms, a “real” definition that expresses what the thing is (quid est) is known. Thus, “rational animal” is a real definition for “man,” in that it characterizes the real essence of man. For other terms, only a “nominal” definition, one that characterizes linguistic usage of the term (quid significat nomen), may be known.8 These two types of definition give rise in turn to two types of demonstration. If a real definition of a term is known, then the term may be used in a demonstration “because of which” (propter quid), in which a proposition is verified by identifying the proper cause of the predicate’s obtaining in regard to the subject. If only a nominal definition of a term is known, the term may only be used in a demonstration “that” (quia), in which a proposition is verified, but not explained by a proper cause.
However, not all propositions need to be verified by demonstration. Depending on their conditions for verification, propositions can be classified into three types: the evident, whose truth is verified immediately through the senses; the per se nota, whose truth follows from the meaning of the terms employed; and the demonstrable, whose truth must be verified by demonstration. Evident propositions cannot be demonstrated, nor need they be in order to be verified. Conversely, demonstrable propositions always need to be demonstrated in order to be verified. The demonstrability (or lack thereof) of per se nota propositions is a more subtle and complicated affair.
Thomas describes a per se notum proposition as one in which the predicate is contained within the definition (ratio) of the subject. Thus, the proposition that “a square is four-sided” is per se notum because four-sidedness (the predicate) is included within the definition of a square (the subject). Thomas then distinguishes between a proposition that is per se notum “to us” (quoad nos) in which real definitions of the predicate and subject are universally known, and a proposition that is per se notum only “in itself” (secundum se), in which real definitions are not universally known.9 Propositions that are per se nota “to us” are also called first principles, and they strictly exclude all demonstration, whether propter quid or quia. However, propositions that are per se nota only “in themselves” require demonstration quia to be verified. Because real definitions for the subject and predicate are not known, the inclusion of the predicate within the subject will not be obvious and must be discursively inferred. Hence, in Thomas’s theory of demonstration, a proposition can be per se notum with regard to its intrinsic structure and yet require a demonstration quia to be verified. Of course, the most prominent example of the latter is the proposition “God exists,” but Thomas expressly indicates that it is not the only such example.10
The starting point of the existential argument, as presented above, is the observation that many things exist. This proposition is evident and therefore needs no demonstration. Nevertheless, two aspects of this proposition are important. First, the subject of the proposition is many things. The note of multiplicity, in contrast to uniqueness, plays a central role in the next step of the argument. Second, the proposition affirms the actual existence of these things. Human knowledge of such actual existence begins with sensory contact with real sensible beings, proceeds to the act of apprehension whereby the intellect grasps their essences, and culminates in the act of judgment whereby the intellect affirms their actual existence.11 Hence, the reconstructed version of the existential argument “starts from some thing or situation empirically given in sense knowledge,” as must any “truly Thomistic proof of the existence of God,” and not from “the abstract consideration of the act of being.”12 In contrast, other versions of the existential argument, particularly those that are more closely drawn from De Ente et Essentia, tend to have a starting point that is “more rarified because more removed from sensation.”13
The next step of the argument affirms that every existing thing, with at most one possible exception, is composed of principles of essence and esse that are really distinct. This proposition is surely not evident, as the intellect cannot apprehend esse in a concept as though it were an essence and therefore cannot compare esse with essence as it might compare and distinguish two essences. For the same reason, the proposition cannot be per se notum “to us.” As noted, esse cannot be apprehended in a concept as though it were an essence and is grasped only with difficulty. Hence, esse is not known to all as distinct from essence. If the proposition is per se notum, it is so only “in itself” and, like the existence of God, would require a demonstration quia “by things that are more known to us.”14 The real distinction of essence and esse is intended to apply not only to the sensible beings whose existence can be immediately affirmed through sense knowledge, but to all beings whatsoever, with at most one exception. This hypothetical exception would, of course, be a being in which essence and esse are not really distinct. The actual existence of such a being is in no way presupposed by this step of the argument; on the contrary, the demonstration of this step will entail that at most one such being can exist, without regard to whether even one does. In other words, the argument excludes the possibility of multiplicity for such a being independently of whether such a being actually exists.
The real distinction of essence and esse in all beings, with at most one exception, can be demonstrated in three interrelated ways from metaphysical principles regularly employed by Thomas. The first directly addresses the central issue of the multiplicity of beings. The second and third are variations of the first that argue from the participation and limitation of beings, respectively. The argument from the multiplicity of beings proceeds as follows. Suppose there existed some being in which essence and esse were not really distinct. In this being, essence and esse would have to be really identical, so that the quiddity of this being would be esse itself. Such a being could in no way be multiplied. A specific form is multiplied in individuals by its reception in matter, but esse itself contains no matter. A genus is multiplied by the addition of specific difference, but esse itself contains no specific difference. In general, esse can only be multiplied by its reception in a subject really distinct from itself, but the hypothesized being is precisely esse not so received.15 Just as a specific form separate from matter could only be one, as a genus separate from specific difference could only be one, so esse separate from any receiving subject could only be one.16 Thus, whether or not there actually exists a being in which essence and esse are really identical, it is impossible for there to be more than one such being.17 For esse to be not received in a really distinct subject and yet be multiplied entails a metaphysical contradiction. Yet, many things exist. Therefore, in each of these existing things, with at most one exception, essence and esse are really distinct.
The second argument proceeds from the perspective of the metaphysics of participation. Every thing that subsists is either identical to its esse or participates in its esse. If a subsisting thing were identical to its esse, it could only be one, as shown above. Therefore, every other subsisting thing must participate in esse.18 Every participant is composed of that which participates and that in which it participates, as two principles that are really distinct.19 In a subsisting thing, that which participates in esse is the essence of the thing. Therefore, in every subsisting thing, with at most one exception, essence and esse are really distinct.
The third argument turns on the limitation of finite beings. In every being, esse is either received in a subject or not so received. A being whose esse is not received in a subject would not be a limited being, but an infinite one, for it is only the receiving subject that limits esse.20 Esse itself is in no way self-limiting because it is the act of acts and perfection of perfections.21 But there cannot be more than one infinite being, since without limitation there is no source of diversification.22 Hence, there is at most one being whose esse is not received in a subject.23 Every other being has an esse that is received in something else and thereby limited. It follows that every being, with at most one exception, is composed of two really distinct principles: esse and that which receives esse, or essence.
These three arguments for the real distinction of essence and esse are interconvertible, as they are all applications of the act-potency relation to esse, understood as the actus essendi.24 If act is limited only by potency,25 and esse is the actus essendi, then in every limited thing there must be a distinct potency that receives and limits the actus essendi. The arguments from multiplicity, participation, and limitation express various aspects of this fundamental relationship.
The final step of the argument affirms that every being that is composed of essence and esse depends for its esse on a being outside of itself and, in the final analysis, upon a Being in whom essence and esse are identical, and whom we call God. The argument for this step is as follows. Every perfection that a thing possesses is either caused by its essence or comes to it from some cause external to the thing. But esse cannot be efficiently caused by a thing’s essence, as the thing would then be its own cause, which is impossible.26 Therefore, every thing composed of essence and esse derives its esse from another being that serves as its efficient cause.27 The esse of the compound being is an effect, and “from every effect the existence of its proper cause can be demonstrated,”28 where the proper cause is an actually existing being that is present to the effect. The proper cause itself is either identical to its own esse or receives its esse from another. If it receives its esse from another, then it is simply an intermediary cause, and by repeating the analysis a series of intermediary causes is constructed. A series of intermediary causes is united in the order of causality as a single intermediary, itself in need of a cause whose esse does not depend on another. Hence, such a series cannot regress infinitely, but must terminate in a being in which essence and esse are identical. It has already been demonstrated that there can be at most one such being. It now follows that there actually exists exactly one such Being, whom we call God.
The soundness of such an existential argument has been criticized by some contemporary Thomists, both in regard to its particular formulations in the works of Thomas and on broader philosophic grounds.29 These criticisms do not involve a rejection of the principles used in the argument (the real distinction of essence and esse and the reduction of compound being to simple Being) but rather deny that these principles can be properly combined to form an argument for the existence of God in the manner attempted by the existential argument. The most common criticisms can be conveniently gathered into four groups: criticisms specific to the text of De Ente et Essentia, more general textual criticisms, alleged similarities to the ontological argument, and the charge of circularity.
The existential argument is often discussed in connection with De Ente et Essentia, wherein Thomas gives one of his more extended presentations of some of its component parts.30 In arguing for the real distinction of essence and esse, Thomas begins with the intellectus essentiae argument, according to which we can know what a thing is without knowing whether it exists. The interpretation of this argument has been controversial, with Thomists divided as to both what the argument actually demonstrates and what Thomas intended to demonstrate by it. Two main interpretations have been advanced. The first takes the intellectus essentiae argument to apply to all of finite being, nonsensible as well as sensible. This interpretation holds that the argument concludes only to a logical distinction of essence and esse, not a real one, and that such was all that Thomas intended by it.31 Of course, a merely logical distinction of essence and esse is insufficient to support the existential argument. The second interpretation takes the intellectus essentiae argument to apply only to sensible beings but maintains that, within that restricted range, the argument concludes to a real distinction of essence and esse. Additional arguments, such as the argument from multiplicity, would then be required to extend the real distinction to all finite being.32 In any case, the soundness of the reconstructed existential argument is independent of the precise range and strength of the intellectus essentiae argument, as there are other arguments, described above, that establish a real distinction of essence and esse.
It is also sometimes noted that the arguments in De Ente et Essentia are not formally organized as a proof for the existence of God, as are, for example, the Five Ways in the Summa Theologiae.33 In fact, the same observation holds true when examining the entire corpus of Thomas’s writings. Whenever presenting the principles on which the existential argument rests, Thomas never formally organizes them into an argument for the existence of God, but rather employs them to demonstrate some attribute of God or creatures. Conversely, whenever formally presenting arguments for the existence of God, Thomas never includes the existential argument.34
From a strictly philosophic perspective, the relevance of these observations may surely be questioned. Philosophy, even Thomistic philosophy, is not a matter of biography. The soundness of metaphysical arguments is decided not by what Thomas did or did not explicitly write, but by what follows from credible metaphysical principles.35 If Thomas consistently employed certain metaphysical principles, then the use of those principles in an argument for the existence of God cannot be forbidden to Thomistic philosophers merely because Thomas never expressly so used them.36 This conclusion is supported by the fact that the very arguments that Thomas formally presents as arguments for the existence of God in one work, he sometimes uses in another work for a different purpose. For example, the Third Way of the Summa Theologiae is omitted from the series of arguments for the existence of God given in the Summa Contra Gentiles,37 but the same argument from generation and corruption is employed later in the Summa Contra Gentiles to demonstrate that God is eternal.38 Obviously, the use of the argument from generation and corruption in the latter context does not preclude its use elsewhere as an argument for the existence of God. As for the reason that Thomas never includes the existential argument in his formal presentations of arguments for the existence of God, one should note that Thomas has chosen to present arguments previously set forth by “both philosophers and Catholic teachers.”39 It is probable that he was guided by pedagogic concerns to present arguments with which his readers were already familiar, rather than those he considered the most original or profound.
It is sometimes suggested that the existential argument seems to be reminiscent of Anselm’s ontological argument for the existence of God. This suggestion usually arises in connection with Thomas’s use of the intellectus essentiae argument and its apparent movement from a logical distinction to a conclusion about reality.40 It has already been noted that the intellectus essentiae argument is not necessary for the existential argument since the real distinction of essence and esse can be demonstrated otherwise, as shown above. Moreover, the existential argument, in the reconstructed version presented above, begins with the actual existence of many things known in judgment and not with a concept of God, as does the ontological argument.41
Nevertheless, at least one writer, Joseph Owens, has pressed this criticism further, suggesting that even the argument from multiplicity raises “shades of the ontological argument.”42 Owens maintains that, even after the argument has demonstrated that there can be at most one being in which essence and esse are identical,
... there is as yet no ground for projecting [the distinction of essence and esse] as real. It would be reasoning from the presence of a distinction in the mind to a corresponding distinction in reality. In that perspective the reasoning would seem to have an ontological cast.43
It is true that, on Thomistic principles, one cannot reason from the concept of a being to the actual existence of that being, even if the concept should include “necessary existence” as one of its notes. Likewise, one cannot reason from a merely logical distinction to a real one. Owens errs, however, in supposing that the argument from multiplicity employs either form of reasoning. Rather, the argument from multiplicity reasons from the contradictory nature of a thing to its lack of actual existence. This form of reasoning is entirely sound; it is employed when one concludes that a concept with contradictory notes, such as “the square circle,” cannot, and so does not, actually exist. The argument from multiplicity observes that it is contradictory for a being’s essence to be really identical to its esse and for it also to possess any of the notes that are necessary for multiplicity, such as specific difference or reception in a distinct subject.44 It is therefore impossible for there actually to exist more than one such being.45 (Yet many things exist, so in them essence and esse are not really identical.) The key step of the argument, that which concludes that a being in which essence and esse are identical cannot be more than one, rests on the principle of contradiction, understood in its metaphysical and not merely logical sense. It is problematic to attempt to bring actual existence under the scope of some concept by a priori reasoning, but the argument from multiplicity does not attempt such. Rather, it excludes existence from a concept based on the principle of contradiction. In his Treatise on Separate Substances, Thomas employs the argument from multiplicity in just this way to conclude to a real distinction.
For there can be only one being that is esse itself; just as some form, if it should be considered by itself, can only be one. . . . Hence, it is impossible that other than it, there should be something that is esse alone. Now everything that is, has esse. Therefore, in every being other than the first, there is present both esse as the act, and the substance having esse as the potency receptive of the act that is esse.46
Thus, the reconstructed existential argument remains a posteriori, beginning with the actual existence of things as known in judgment.47
The final criticism of the existential argument is the charge of circularity, i.e., that the argument presupposes something that it claims to demonstrate. As with the comparisons with the ontological argument, the charge of circularity centers on the demonstration of the real distinction of essence and esse. Owens, for example, holds that “a real distinction between a thing and its being cannot be shown until after completion of the demonstration that God exists.”48 He argues for this position on the grounds that Thomas often reasons from the identity of essence and esse in God to their real distinction in creatures.
There is no doubt that the argument “from God to creatures” is indeed one of the major arguments that Thomas uses for the real distinction of essence and esse.49 However, even in this argument, Thomas appeals not so much to the actual existence of God as to the conclusion that God, as esse subsistens, could not be more than one.50 The latter conclusion follows from the contradiction entailed by a multiplicity of beings whose essence is identical to their esse and not from a prior demonstration of God’s actual existence. An analogy with God’s necessity may be helpful here: Thomas held that one can demonstrate that God, if he exists, does so necessarily, without thereby demonstrating God’s actual existence. Indeed, Anselm’s ontological argument, on a Thomistic interpretation, unwittingly provides just such a demonstration.51 In like manner, one can demonstrate that God, if he exists, is one, without first demonstrating God’s actual existence. The logic of the existential argument requires no more: it suffices that there can exist at most one being whose essence is identical to its esse.
Etienne Gilson has set forth the charge of circularity in a more complex manner. Like Owens, Gilson holds that the real distinction of essence and esse can only be known subsequently to the proof of the existence of God.52 However, he makes two additional claims in this connection. First, he claims that the real distinction of essence and esse is a first principle and therefore not subject to demonstration.
Now, to the best of our knowledge, Thomas Aquinas has never attempted such a demonstration. Nor does one see how the thing could be done. . . . What here is at stake is the metaphysical intuition of the first principle, which is the notion of being. At this highest metaphysical level, it is not a question of proof, but of sight.53
Before evaluating the claim that the real distinction is a first principle, it is instructive to examine what its implications for the existential argument would be. Recall Thomas’s distinction between propositions that are per se nota “to us,” which he identifies with first principles, and those that are per se nota “in themselves” but not “to us.” If the real distinction were a first principle in the sense of being per se notum “to us,” this would in no way weaken the reconstructed existential argument as a whole. Rather, it would simply mean that the second step of the argument is immediate, requiring no further justification. Thus, the arguments from multiplicity, participation, and limitation, as presented above, would be superfluous, but the conclusion they are intended to establish would nevertheless be verified. Indeed, as a first principle, the real distinction would be unerringly known as true,54 and nothing would preclude its subsequent use in the existential argument.
However, it is implausible that the real distinction is a first principle in the sense of being per se notum “to us.” Gilson himself stresses the difficulty rather than the ease of obtaining knowledge of the real distinction, noting the “large number of Christian philosophers and theologians, even among the so-called Thomists, who have rejected the distinction of essence and existence understood in its Thomistic meaning.”55 Thus, it is more likely that the real distinction is per se notum “in itself” but not “to us,” i.e., that it is per se notum only “to the wise,” in Boethius’s terminology. As discussed above, only propositions that are per se nota “to us” strictly exclude all demonstration, whether propter quid or quia. Propositions that are per se nota only “in themselves” require a demonstration quia to be verified. The arguments for the real distinction of essence and esse proceed in just this manner, as well as employing the judgment that many things exist.
Second, Gilson states that “all the arguments one can use to establish the distinction between being and essence in Thomas Aquinas’s doctrine presuppose the prior recognition of the notion of the ‘act of being’ (esse).”56 At first sight, this might seem to be a puzzling criticism, as the arguments for any proposition necessarily “presuppose” the notions used in the proposition. Gilson’s point, however, is that to understand esse as the actus essendi is tantamount to understanding it already as really distinct from essence. As earlier noted, given the notion of esse as the actus essendi, one need only apply the principle that act is limited only by potency in order to deduce that in every limited being there must be a distinct potency that receives and limits the actus essendi. Yet, this conclusion is still deduced, even if in a compressed manner. There is no circularity involved. Even less need there be a circularity in the arguments from multiplicity, participation, and limitation, as these arguments need make no explicit appeal to the act-potency relationship. The presence of a deeper metaphysical significance, such as that illuminated by the act-potency relationship, does not vitiate an argument in which that significance is not explicated.
This paper has presented an argument for the existence of God based on the metaphysical principles of Thomas Aquinas. The argument begins with the actual existence of a multiplicity of things known in judgment and thus has an a posteriori character, as do all Thomistic arguments for the existence of God.57 The argument then proceeds to a metaphysical analysis of the many existing things with respect to the relationship of essence and esse in them. A central conclusion is the incompatibility of a real identity of essence and esse with the notes required for multiplicity. This, in turn, leads to the conclusion of a real composition of essence and esse in all existing things, with at most one exception. Finally, the argument uses a causal analysis to reduce the many compound things, which do not exist through themselves, to a single simple Being, who does.
The paper then examined a number of criticisms advanced by contemporary Thomists against the soundness of such an existential argument. Some of these criticisms were directed at the intellectus essentiae argument, which was not employed in this paper. Others pertained to textual observations that can be explained by the primarily pedagogic purpose of Thomas’s works. Some of the criticisms, mistaking use of the principle of contradiction for a priori reasoning, alleged a similarity between the existential argument and Anselm’s ontological argument. Finally, the argument was charged with circularity, for which the textual and philosophic evidence was found wanting. The existential argument for the existence of God is therefore found to be both sound and in conformity with the philosophic principles of Thomas Aquinas.
1 Thomas Aquinas, Summa Theologiae I, 2, 3; Thomas Aquinas, Summa Contra Gentiles I, ch. 13.
2 Thomas Aquinas, SCG I, ch. 13, #1.
3 Thomas Aquinas, De Ente et Essentia, ch. 4.
4 John Haldane, “Common Sense, Metaphysics, and the Existence of God,” American Catholic Philosophical Quarterly 77, no. 3 (2003): 387-8. Also notable is John Paul II’s urgent recommendation in Fides et Ratio, par. 97 of a “philosophy of being” that “is based upon the very act of being itself” and that “views reality in its ontological, causal and communicative structures” so as to surpass “every limit in order to reach the One.” See John Paul II, Fides et Ratio (Boston: Pauline Books & Media, 1998), 119-120.
5 Barry Miller, From Existence to God: a Contemporary Philosophical Argument (London; New York: Routledge, 1992).
6 Due to limitations of space, the individual principles invoked will not themselves be discussed in detail. Such discussion can readily be found in the references to be cited.
7 Thomas Aquinas, ST I, 2, 1-2.
8 Jacques Maritain, Formal Logic, trans. Imelda Choquette (New York: Sheed & Ward, 1946), 76-80.
9 In ST I, 2, 1 and I-II, 94, 2, Thomas refers to Boethius’s equivalent distinction between propositions that are per se nota “to all” and those that are so “only to the wise.” This analysis derives from Aristotle’s distinction in the Posterior Analytics between axioms and postulates. Both axioms and postulates are “immediate” propositions, but the latter are formed from terms that “are not known to all” (Thomas Aquinas, Exposition of the Posterior Analytics, I, lect. 5). For further discussion, see Leo J. Elders, S.V.D., The Metaphysics of St. Thomas Aquinas in Historical Perspective (Leiden, New York, Koln: E. J. Brill, 1993), 156.
10 Ibid., where Thomas endorses Boethius’s example, “incorporeal beings do not exist in a place,” as a proposition per se notum “in itself” but not “to us.”
11 Joseph Owens, C.Ss.R., St. Thomas Aquinas on the Existence of God, ed. John R. Catan (Albany: State University of New York Press, 1980), 80.
12 Etienne Gilson, Elements of Christian Philosophy (New York: Mentor-Omega, New American Library, 1960), 61.
13 Steven A. Long, “On the Natural Knowledge of the Distinction of Essence and Existence,” Nova et Vetera (English Edition) 1, no. 1 (2003): 96.
14 Thomas Aquinas, ST I, 2, 1.
15 Thomas Aquinas, De Ente, ch. 4.
16 Thomas Aquinas, De Substantiis Separatis, ch. 8, #42.
17 John F. Wippel, Metaphysical Themes in Thomas Aquinas, ed. Jude P. Dougherty, Studies in Philosophy and the History of Philosophy, vol. 10 (Washington, D.C.: Catholic University of America Press, 1984), 120; idem, The Metaphysical Thought of Thomas Aquinas: From Finite Being to Uncreated Being, Monographs of the Society for Medieval and Renaissance Philosophy, vol. 1 (Washington, D.C.: The Catholic University of America Press, 2000), 155.
18 Thomas Aquinas, Quodlibet 3, 8, 1; Wippel, Themes, 156; idem, Thought, 168.
19 Wippel, Themes, 147; idem, Thought, 169.
20 Wippel, Themes, 157; idem, Thought, 170.
21 Thomas Aquinas, SCG I, ch. 43, #5.
22 Ibid., II, ch. 52, #4.
23 Ibid., II, ch. 52, #3.
24 Herman Reith, C.S.C., The Metaphysics of St. Thomas Aquinas (Milwaukee: Bruce Publishing Company, 1958), 191.
25 W. Norris Clarke, S. J., Explorations in Metaphysics: Being, God, Person (Notre Dame: University of Notre Dame Press, 1994), 81.
26 Thomas Aquinas, De Ente, ch. 4, #6.
27 Thomas Aquinas, ST I, 3, 4.
28 Ibid., I, 2, 2.
29 Etienne Gilson, The Christian Philosophy of St. Thomas Aquinas, trans. L. K. Shook, C. S. B. (Notre Dame: University of Notre Dame Press, 1956), 81-82; Gilson, Elements, 142-143; Joseph Owens, C.Ss.R., “Stages and Distinctions in De Ente: A Rejoinder,” The Thomist 45 (1981): 99-123.
30 Thomas Aquinas, De Ente, ch. 4, #6-7.
31 Gilson, Elements, 138-9; Armand Maurer, C.S.B., introduction to Thomas Aquinas, On Being and Essence, trans. Armand Maurer, C.S.B., 2d rev. ed. (The Pontifical Institute of Medieval Studies, 1968), 22-3; Wippel, Themes, 113, 121; idem, Thought, 143.
32 Long, 93-94.
33 Owens, Existence, 181.
34 Mauer, 25-26; Gilson, Thomas, 82.
35 John F. X. Knasas, Being and Some Twentieth-Century Thomists (New York: Fordham University Press, 2003), 231-2.
36 Owens, Existence, 182.
37 Thomas Aquinas, SCG I, ch. 13.
38 Ibid., I, ch. 15, #5; Owens, Existence, 133; Gilson, Elements, 60.
39 Thomas Aquinas, SCG I, ch. 13, #1.
40 Patrick Quinn, Aquinas, Platonism and the Knowledge of God (Brookfield: Avebury, 1996), 18; Reith, 188.
41 Owens, Existence, 204.
42 Owens, “Stages,” 99-101.
43 Owens, “Stages,” 120; Knasas, 195-6, n. 30.
44 Wippel, Themes, 125-6; idem, Thought, 152-3.
45 Steven Long argues that any conclusion about essence and esse must begin with the case of physical things because human knowledge is founded on “quiddity in corporeal matter.” See Long, 82-83. The observation that the argument from multiplicity is fundamentally an argument from contradiction concluding to the non-existence of a putative kind of being shows that an important conclusion about essence and esse in all finite being can be obtained directly, without first considering physical things as a separate case. It is true, however, that the principle of contradiction itself rests on knowledge derived from sensible beings.
46 “Nihil enim per se subsistens, quod sit ipsum esse, potest inveniri nisi unum solum; sicut nec aliqua forma si separa consideretur potest esse nisi una. . . . Impossibile est igitur quod praeter ipsum sit aliquid subsistens quod sit esse tantum. Omne autem quod est esse habet. Est igitur in quocumque praeter primum et ipsum esse tanquam actus et substantia rei habens esse tamquam potentia receptiva hujus actus qui est esse.” De Substantiis Separatis, ch. 8, #42.
47 Thomas expounds the a posteriori character of the first principles, such as the principle of contradiction, in his Exposition of the Posterior Analytics II, lect. 20. For a good discussion of this point in contrast to recent a priori approaches, see Knasas, 286-7.
48 Owens, “Stages,” 110.
49 Thomas Aquinas, SCG II, ch. 52, #2-3; Wippel, Themes, 139-149; idem, Thought, 585-590.
50 Wippel, Themes, 145.
51 Thomas Aquinas, ST I, 2, 1.
52 Gilson, Thomas, 82; idem, Elements, 143.
53 Gilson, Thomas, 82.
54 Thomas Aquinas, ST I, 17, 3.
55 Gilson, Thomas, 82.
56 Gilson, Elements, 142.
57 Thomas Aquinas, ST I, 2, 2.