Appeared in Spring/Summer 1995, Vol. XXI, Nos. 1, 2 Download PDF here

In several of his works, Thomas Aquinas formulates some fascinating philosophical arguments for the existence of finite, purely spiritual beings, incisively exploring their essential characteristics as well. These creatures are named “separate substances,” because by definition they are devoid of material composition. More commonly they are called “angels,” a term of Greek etymology meaning “messengers” – by denomination from their relationship to mankind as narrated in the Bible. Since the actual reality of angels is an important article of orthodox Judeo-Christian faith (and a religious tenet of other belief systems, too), it would be desirable to provide a convincing defense of their existence and to work out a reasoned exposition of their nature.

In the first section of this paper we concentrate our attention on the pith of Aquinas’ metaphysical demonstrations for the existence of angels. Once their reality is granted (evenhypothetically), a cascade of propositions articulating their fundamental properties can be apodictically deduced. This program is readily accomplished in the second section. An ample treatment covering more than such a narrow span, one which encompasses not only their immanent activities but also a detailed elaboration of their relations among each other and their transitive actions on the physical universe (including the human race), would be prohibitively extensive in scope, as the Angelic Doctor’s vast writings on the topic attest – even prescinding from questions penetrable exclusively by a theological approach within the supernatural sphere.


Aquinas employs two principal lines of existential argumentation in his philosophical angelology. We explicate each in turn.

One of Thomas’ chief modes of demonstration invokes the continuity postulate of pseudo-Dionysus. This celebrated precept enunciates the doctrine that the highest creatures of a lower grade of being approach in dignity the lowest members of the next higher rank in the cosmic ontological hierarchy. Thus, in order to ensure the absence of metaphysical gaps in the universe, the rational soul, which (hylomorphically integrated as it is with human flesh) is elevated above the terrain of brute animality, must itself be surpassed by a nobler realm of finite beings wholly disconnected, in their inner constitution, from the physical plane. In other words, since it is man who uniquely holds the status of contact point between the spiritual and material domains, there must be, in accordance with the ordered structure of the cosmos, a class of completely separate intelligences superior to humanity on the ladder of ascending degrees of being. Along the scale of approximation of creatures to their Creator, an entitative vacuum between mankind and the Deity would be abhorrent: it is most fitting that there be intermediate gradations closer in nature to the Divine Essence, hence simply spiritual.1   

The second type of proof is grounded in an implicitseparability assumption: namely, that the more eminent feature of two essentially different facets of composite substances is found actually disjoined from the less excellent constituent in other substances. For instance, Aquinas observes that there are incarnate intellectual substances (i.e., human beings), yet inferior bodies can exist in themselves apart from conjoined minds (e.g.,brute animals, plants, and even minerals). Since the lower, corporeal aspects of the human composite are competent for independent existence (again, in the three mentioned cases of sentient, vegetative, or inanimate beings), a fortiori the higher, spiritual principle is capable of subsistence (viz., as exemplified by purely intellectual forms of life).2

Moreover, it is incidental to intellectuality as such that the powers of understanding and willing be united with bodily concomitants, because these operations are intrinsically spiritual, hence ontologically prior to (and thus thoroughly detachable from) matter. According to Aquinas, the mere fact of an essential distinction between intellectionand sensation guarantees the existence of creatures who have the former faculty alone, not requiring the aid of sense perception at all. Indeed, cognitive components that have been fused (i. e., in man) can also be isolated – on the one hand in beasts, and on the other hand in angels.3Aquinas advances here a thesis of symmetric separability between profoundly different noetic functions. (In fact, the continuity argument also relies on a principle of complete symmetry operative in the creaturely realm.)

Furthermore,as Aristotle explainsin his Metaphysics,“substance precedes the other categories”in every manner (whether in being, definition, or time). Therefore, although the accident of quantity cannot exist unless it has a natural inherence in some (corporeal) substance, substance can theoretically subsist free from any aptitude for dimensive extension. Similarly, although matter cannot exist except under some form, “form as such does not depend on matter.” So, from the axiomatic superiority of form to matter, the separability criterion has entailed a reduction to the theorem that form is existentially independent of matter. Consequently, it is entirely fitting that there should be forms whose nature it is to totally transcend embodiment. Such entities thereby epitomize form in the universe, being so perfect that they are unrestricted by the confines of physicality. Thus, the most sublime group of creatures (those closest to the Pure Actuality of God) are essentially subsistent forms.4As James Collins implies, pneumatology is equivalent to eidology.5

Aquinas’ philosophical arguments for the objective reality of angels evidently lack the taut probative force of his rigorous proofs for the existence of God. First of all, there is no a priori necessity for the existence of any creature: God was not obliged to create anything, whether material or spiritual. Secondly, this radical contingency also imposes a barrier on a posteriori inference, since any observable preternatural phenomena allegedly produced by invisible angelic agency could equally well derive as immediate effects of Divine efficient causality.

Nevertheless, Aquinas’ rational justifications display an attractive cogency. Indeed, their logical coercion is one of “convenience” or relative suitability, emanating from a trenchantly astute perspective on the proper arrangements that befit the cosmic order. The core insight guiding Thomas’ angelology is a teleology of harmonious symmetry: a complete universe adequately reflecting (to a great extent, at least) the Divine Wisdom cannot fail to comprise every conceivable entitative level, from the humblest inorganic things to the most exalted intelligences. Although the existence of separate substances is, prima facie, only metaphysically possible, nonetheless the fulfillment of the universe almost mandates that all generic natures below the Godhead be realized de facto.6

Aquinas’ appeal to the real organization of the cosmos, rather than to mere concepts, prevents him from falling victim to a version of so-called “ontological” argumentation; instead, his angelology is solidly rooted in “cosmological” argumentation. His theory has truly philosophical proportions, so that nowhere in his reasonings about the existence of angels does he attempt to import and incorporate strictly theological data (i.e., truths revealed solely in Sacred Scripture or Tradition). His conclusions proceed from the twin avenues of two undeniably philosophical principles (viz., the continuity and separability canons), which, from his global vantage point, concur and merge into a beautifully balanced celestial framework dictating the presence of angelic beings.

In light of Aquinas’ compelling dialectics, it would be rash to dismiss his thought as a “medieval relic” (in the pejorative sense), thereby relegating angels to the status of mythical symbols – rather than genuine persons possessing the spiritual powers of intellection and volition.


Having established the existence of angels, we now discuss the kind of beings that they are, in terms of what natural attributes they must exhibit.

Since an angel is a purely spiritual substance, bereft of material composition, its act of existence is bestowed directly on its form, which obviously cannot be separated from itself. Thus, it is intrinsically free from any susceptibility to corruption; in other words, it is naturally imperishable, once it begins to exist.7

Although angels are devoid of any physical admixture of matter with form, as creatures they nevertheless contain a metaphysical compound of potentiality and actuality: namely, an essence limiting a participated act of existence. Hence, unlike corporeal entities admitting a dual composition of act and potency (i.e., matter/form and essence/existence), angels are substantially simple – complex only in the ontological order. Therefore, the separate intelligences are “unlimited from below” (i.e., their natures are not contracted by matter), but “limited from above” (i.e., their being is received from God into a distinct essence).8

The preceding discussion leads us into the issue of the classification of angels according to species and genera. Aquinas furnishes several proofs for the thesis that each angel constitutes by itself a single species. First, since each separate substance is a subsistent form, it has a unique, incommunicable proper definition, and so must be a species in its own right: that is, species and individual wholly coincide. Secondly, since separate substances are utterly removed from matter, they lack the potentiality for the numerical diversification of their forms, because matter is the source of the individuating pluralization of a given species. As pure forms, angels admit only of specific differentiation.It follows that there cannot be two angels of identically the same species. Thirdly, the purpose for the multiplication of individuals within one species is to ensure the perpetuation of the specific nature, in any instance where such entities are corruptible. But separate substances are not subject to any decay whatsoever. Consequently, there is no need for a plurality of angelic individuals within the bounds of one species: each angel actually exhausts the full range of the possibilities inherent in its specific nature.9

Furthermore, in the case of simple substances, the notion of genus is entirely attendant on the meaning of the specific nature, which both determines and is determined by itself alone. That is, the absence of any determinable matter in the separate substance precludes the potentiality required for a genus. The specific essence per se locates each finite spirit within a definite “grade of being,” yielding a natural linear inequality of entitative degree among the angels. Nonetheless, although the proximate genus of an angel cannot be logically and properly distinguished from its specific difference, the human mind can legitimately broadly categorize all finite, purely spiritual beings as members of one so­called “genus” (taken in a loose sense).10

We next examine the natural powers of a separate substance, treating first the angelic intellect and then the angelic will. Indeed, lacking bodies and the concomitant psychosomaticpowers, angels have only the faculties of intellect and will. Certainly these powers, along with their actual exercise, are accidents inhering in the angelic substance, for a subsistent intellect or act of understanding would be unrestricted or infinite, hence unique and Divine.11

Nevertheless, even though the angelic mind is not identical with the angelic substance, the angelic intellect is always in a state of (second) act, with no interruption in the actual use of its faculty of understanding. Indeed, an angel is free from the hindrance of bodily connections which might impede the continuity of its mental operations. In fact, for any living being, some of its (immanent) natural activities are never totally suspended; however, intellectual acts comprise the whole extent of an angel’s natural (internal) operation. Consequently, an angel can never suffer a lapse of attention.12

Since an angel is a subsistent spiritual form, it itself is intelligible in act. Thus, it is an “intelligible species” ever present to itself and revealing itself to itself in the intentional order. Therefore, a separate substance directly, intuitively, and transparently understands itself through its own essence. The primary natural object of an angel’s knowledge is itself, and, as a result, the rest of its natural knowledge is instilled in it via an ordination to this fundamental cognitive act.13

Now the angelic essence is finite and confined to a definite entitative type; hence, unlike the infinite Godhead, it does not pre-contain by way of efficient causality the entire sweep of creation. Since any mind can know only through a form somehow present in it, in particular, an angel requires that its intellect be informed by distinct intelligible species in order to discern other things precisely.14These ideas are media by which an angel understands, inhering in its mind as noetic determinants. They are innately supplied to it from the first instant of its existence, since they are a connatural aspect of its perfection as an essence already fully realized. They adequately encompass the complete scope of an angel’s natural cognitive capacity. Thus, an angel’s immateriality liberates it from the limitation of being compelled to gather its natural knowledge from finite extrinsic sources; it depends only on the Creator’s original infusion of intelligible species into its intellect, and it knows by turning to look within itself.15As the human mind is initially a blank slate, so the angelic mind is an engraved tablet.16

Although the substance of each angel innately includes all the intelligible likenesses pertaining to the whole gamut of its natural knowledge, it does not make use of all these imprinted species at once in its actual understanding. Regarding its natural cognition, an angel can know simultaneously only the plurality of those things subsumable under a single intentional form –not the noetic contents comprised under distinct intelligible similitudes. In particular, it cannot contemplate the total spectrum of its native knowledge all at once. Thus, even though the angelic mind is never in a condition of absolute or essential passive potentiality with respect to its natural knowledge (as is the human possible intellect, which acquires knowledge from a privative state of primitive ignorance), yet, because it is not always exhaustively considering everything that it knows, it does undergo an accidental transition from relative potency to exercised act when it decides to advert to and reflect on some habitually possessed epistemic datum. As Aquinas declares, “[I]n the intellect of a separate substance there is a certain succession of understandings; there is not, however, movement properly speaking, since act does not succeed potentiality, but [rather] act succeeds act.” Still, this limiting feature does radically differentiate angels from the omniscient Deity, Who knows everything without successiveness in one eternal act of understanding identical with His Divine Essence.17

Besides its innate actuality in contrast with the basic potentiality marking human cognitive acquisition, angelic knowledge is distinguished from the human level by its thorough intuitiveness, as opposed to the discursiveness characterizing human reasoning from premises to conclusions. Without any mental movement, angels apprehend immediately in any naturally known principle all the consequences that necessarily flow therefrom. This perfection of an angel’s intellectual operation derives from its essence as a completely realized subsistent form requiring no intrinsic natural development, since things always act according to their mode of being. Because all angelic knowledge arises as a direct insight into quiddities, without combining terms to construct propositions and without laborious syllogistic mediations, an angel is immune from error about whatever falls within the purview of its natural cognition.18

Similarly, angelic volition differs from human volition in that angelic choice does not emerge from a process of inquiry and consultation, as is the case with human deliberative judgment, but rather issues immediately upon a glance at some truth.19Aquinas asserts that the free choice ofangelic will instantaneously tends to its object.20It follows that angelic election must be irrevocably permanent, since the angel will never discern any more facets of the object than are included in the angel’s original, naturally exhaustive penetration into the object’s essence (with all the relevant ramifications of that selection). The angelic mind and will are so powerful that, once the angel has attached itself to a choice, it will forever be tenaciously confirmed in that inflexible decision, with no possibility for adherence to the contrary -in stark contrast to the vacillation often marring unstable human fixation on an object of predilection.21

The fact, however, that angelic choice results from an all-consuming intuitive grasp of a situation, instead of from rational inference, does not make angelic appetition belong to an essentially nobler genus than the human will, because both simple intellection and discursive reason ultimately attain the same formal object (viz., being qua true). Indeed, both the angelic and human wills are oriented to the good in general (i.e., being qua good), and thus can find completely fulfilling satisfaction only in a union with the Infinite Good.22

We recall that the angels are intrinsically differentiated from each other as subsistent forms, each angel constituting a specific class unto itself. Now a major distinction among them, a criterion disclosing their natural inequality, lies in the degree of universality of their innate ideas. Indeed, since God understands all things through one Idea identical with His Infinite Essence and since intellectual substances rise in dignity the nearer they approach His supreme Unity, it follows that an angel occupying a stratum of greater eminence knows larger tracts of reality covered by fewer intelligible species than an angel of lower ontological status (thougheven the latter succeeds in comprehending the cosmos via a mental map of it). For a weaker intellect always needs a more multifarious variety of detailed concepts to grasp what a stronger mind can encompass with fewer, but more universally embracing, cognitive principles.23

Moreover, we note that, because angels do not abstract their ideas from sensible objects, their universal concepts are by nature prior (not posterior)to the material world.24Furthermore, the angelic universals are not merely general notions about broad classes of things, but rather unified media by which the angel also knows singulars in an intensely distinctive fashion: within one universal idea there is contained a representative similitude of the properly rich density of manifold species and specimens.25

We learned earlier that each angel understands itself through its own essence, which functions dually as a subsistent form and an intentional form intimately present to itself. However, it knows all other angels via additional intelligible species innately imprinted on its mind by God at its inception. God alone knows all things through His creative Essence. An angel knows only itself through its own essence; in order to know other angels it requires a plurality of instilled ideas.26Angels manifest their thoughts to one another via a spiritual locution dependent solely on a free act of volition, unimpeded by any interference of time, distance or finite external agents.27

Now we must patently affirm that the time which measures bodies cannot affect angels, who are non-corporeal. Nevertheless, there are diverse “moments” (as it were) in an angel’s existence, according to a succession of its acts. In fact, no creature can totally transcend duration, because every finite substance supports a succession in its operations. Yet, since the “instants” applicable to angels are differentiable only as distinct cognitional or volitional activities not subject to the continuous motion of the celestial orbs, the time measuring angelic vitality must be discrete, lacking intervals of fluid or extended duration.28

An angel can be said to be in place in an analogous sense. A body fills space according to the accommodation of its dimensive quantity with extrinsic surfaces. Although, unlike a body, an angel has no mass or volume by which it could be physically localized, it nonetheless has a virtual quantity, in that it can be equivalently regarded as occupying a place through the application of its power there. Evidently, it could never be locally contained; rather, the exercise of its will on a place causes that place to be contained by it.29

Unlike God Who is omnipresent through His universal efficient causality, an angel cannot be everywhere at once, but only at the finitely many places where it is currently exerting its transitive action, since its finite power dictates a limited range of operation. It further follows that no more than one angel can be in one place at the same time (according to the above interpretation of angelic “place”), because there is only one proximate cause of any application of power circumscribing a given physical place in a given respect.30

According to Aquinas, all angelic capabilities for exerting dominion over the physical universe reduce to their power of inducing local motion in material composites. Angels possess no direct control over either matter or form as such.31

An angel itself can be said to undergo local motion (again, by an analogical use of the term), in that it can successively apply its power to different places. Its so-called “movement,” though, does not betray any intrinsic potentiality on the part of the angel itself: the passive potency resides totally in the object receiving the action. Moreover, the angel’s “movement” is not necessarily continuous; indeed, its movement will be discontinuous if it instantaneously exchanges the focus of its causal efficacy from the whole of one arena to an utterly diverse locale.32In the latter case of discrete motion, it bypasses the entire intermediate magnitude linking initial and terminal positions.33Thus, an angel can violate the usual laws of space-traversal by effecting quantum leaps.

Concerning the relationship obtaining between the angels and the human race, Thomas thinks it appropriate that the angels enlighten and guide humanity (both in collective groups and as individuals), since the organization of the universe demands that higher beings should in some way govern lower creatures.34


Theologically, Christians believe from Divine Revelation (as proposed in Sacred Scripture) that the angels have had and will have a crucial role to play in the drama of human salvation history. Nevertheless, absolutely speaking, such intermediary spiritual creatures are not indispensable, for Divine intervention could have been planned without their auxiliary assistance in mankind’s affairs. Yet, according to St. Thomas’ philosophical perspective, God in fact rules the universe by endowing His creatures with the dignity of exercising the maximal causality of which they are naturally capable. Hence, Aquinas’ universe displays a dynamic network of interacting secondary causes, arrayed hierarchically. Consequently, finite separate substances constitute an integral part of the cosmos, through both their existence and their activity. Were there (per impossibile) no angels or angelic influence on inferior creatures, the world would suffer a grievous void in its being and in its operation.



1Summa Contra Gentiles (hereeafter SCG) II, trans. English Domincan Fathers (London: Burns, Oates, & Wash-bourne, 1923), c. 68[4] and c. 91[4]. Cf. On Spiritual Creature (henceforth DSC, trans. Mary FitzPatrick and John Well-muth (Milwaukee: Marquette University Press, 1949), q. 1, a. 5, c (secundo).

2SCG II, c. 91[7]. Cf. DSC, q. 1, a. 5, c (primo).

3SCG II, c. 91[3, 5, 8]. Cf. DSC, q. 1, a. 5, c (tertio). Also cf. Summa Theologica (henceforth ST) I, q. 50, a. 1, c; a. 2, c, ad 2; q. 51, a. 1, c. Note that two editions of the Summa Theologica have been consulted for these references. First, the translations of the Blackfriars, Kenelm Foster, William Wallace, and M. J. Charlesworth (New York: McGraw-Hill Book Company, resp. 1968, 1967, 1970). Secondly, the translation of the English Dominican Fathers (New York: Benziger Brothers, 1948), reprinted (Westminster, MD: Christian Classics, 1981).

4SGC III, trans. Vernon Bourke (Notre Dame, IN: University of Notre Dame Press, 1975), c. 97[6]. Cf. On Being and Essence (hereafter DEE), trans. Armand Maurer (Toronto: Pontifical Institute of Medieval Studies, 1986).

5James Collins, The Thomistic Philosophy of the Angels (Washington, DC: Catholic University of America Press, 1947), pp. 37, 39-40, 98. Cf. DEE, c. 4[4]: “…the essence of a simple substance is form alone.”

6SCG II, c. 91[6].  

7ST I, q. 50, a. 5, c, ad 3. Cf. again q. 50, a. 2, c. Cf. also Treatise or, Separate Substances (hereafter TSS), trans. Francis Lescoe (W. Hartford, CT: St. Joseph College,1959), c. 18[98-100],and SCG II, c. 55.

8SCG II, cc. 52, 53, 54[5-10]; ST I, q. 50, a. 2, ad 3, 4; DEE, c. 4[6, 8, 9, 10]; c. 5[4].

9SCG II, c. 93; ST I, q. 50, a. 4, c. Cf. DEE, c. 4[5] and c. 5[5].

10SCG II, c. 95[3, 5, 6]; STI, q. 50, a. 2, ad 1. CL DEE, c. 5[6-9].

11Cf. ST I, q. 54, aa. 1, 2, 3, 5; also q. 79, a. 1, ad 3.

12SCG II, c. 97[1, 3, 5, 6].

13ST I,q. 56, a. 1, c, ad 1, 2; SCG II, c. 96[6]; SCG III, c. 46[7].

14ST I, q. 55, a. 1, c, ad 3; SCG II, c. 98[7-9].

15ST I, q. 55, a. 2, c, ad 1. Cf. also SCG II, c. 96[7].

16Cf. ST I, q. 79, a. 2, c.

17ST I, q. 58, a. 1, c, ad 3; a. 2, c. Also SCG II, c. 98[11-12], and c. 101The textual quotation comes from the last citation.

18STI, q. 58, aa. 3, 4, 5.

19STI, q. 59, a. 3, ad 1.

20STI, q. 63, a. 5, c.

21ST I, q. 63, a. 6, ad 3; q. 64, a. 2, c.

22I, q. 59, a. 1, ad 1, and q. 60, a. 5.

23STI, q. 55, a. 3, c; SCG II, c. 98[9-11].

24STI, q. 55, a. 2; a. 3, ad1.

25St I, q. 57, a. 2; q.55, a. 3, ad. 3.

26St I, q. 56, a. 2; SGC II, c. 98[1-9, 13-19].

27ST I, q. 107, aa. 1, 4, 5.

28ST I, q. 62, a. 5, ad. 2; q. 63, a. 6, ad. 4; a. 8, ad. 1.

29ST I, q. 52, a. 1. Cf. TSS, c. 18[102].

30STI, q. 52, aa. 2, 3.

31Cf. STI, q. 65, a. 4; q. 110, a. 2, c, ad 2, 3; a. 3.

32STI, q. 53, a. 1. Cf. TSS, c. 18[103].

33ST I, q. 53, a. 2.

34STI, q. 111, a. 1; SCG III, c. 79. Cf. also ST I, q. 113, aa. 1-5.