In Thomas Aquinas' philosophy, angels are conceived as pure forms without any matter, like God, but contrary to God they still possess potentiality.

Although there is no composition of matter and form in an angel, yet there is act and potentiality.

(Summa Theologiae - First Part: Question 50, article 2, reply to objection 3)

I already asked (in the context of human souls) how forms can exist or subsist by themselves and how forms can be understood not to be forms of matter. In the case of angels this may be even more mysterious, because existing as a pure form is the “natural” state of an angel. But the situation is still probably sufficiently similar enough to not warrant repeating those questions.

If we put this aside, it still leaves us with the question how we can understand the potentiality of forms. What does this mean? I read Aquinas' own words but didn't understand them.

For potentiality to exist, doesn't there have to be a possibility of change – how can immaterial forms change? Of humans we could say that the immaterial part of the form does not change, it's only the material part that changes – the brain changes every second, for example when acquiring new memories. [NB: this is probably an incorrect interpretation, so please correct me]. But in the case of angels this option does not exist.

Or is the assumption “potentiality = possibility of change” wrong? Then I misunderstood a lot. What then would the potentiality of a form imply instead?

  • You can see on all Aquinas' metaphisyscs : Anthony Kenny, Aquinas on Being, Oxford UP (2005); specifically on angels, see page 166-on. – Mauro ALLEGRANZA Mar 7 '17 at 14:21
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    @MauroALLEGRANZA the problem is that Kenny is simply too critical of Aquinas. Before I conclude that the theory of pure forms doesn't make any sense, I would like to read an explanation from somebody more sympathetic to Aquinas. Also Kenny claims that an angel's form is like a Platonic form. I suspect that this is not a correct interpretation. – wolf-revo-cats Mar 7 '17 at 14:42
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    For Aquinas, potentialilty of angels relates to the potentiality of their communication with us. The change in us is not a given, thus it remains a potential. A good summary of possible ways to treat this idea can be found here with this article, if you can access it: dx.doi.org/10.1080/14791420.2011.567806 – ClearMountainWay Mar 7 '17 at 18:07
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    @Mr.Kennedy What is incoherent about Aquinas' discussion of angels? I'm no way Christian or Catholic, but I don't see this being in any way incoherent. He lays out a fairly well thought out ontology and logic in the Summa Theologiæ. He is 100% philosophical in his discussion of them, not poetic. – ClearMountainWay Mar 8 '17 at 0:40
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    If one removes the theological shell of the discussion, you have an ontology of incorporeals and incorporeal action/creation that is very distinct from that inaugurated from the Stoics (e.g. Cicero's De Fato), that touches on the nature of intellect, imagination, and the matter and form of communicative acts. Or if one merely has ones prejudgments set, and eyes closed, there's absolutely nothing at all. What one learns from an author depends on us as readers. – ClearMountainWay Mar 8 '17 at 0:49

On the 7th of the 24 Thomistic Theses,

Creatura spiritualis est in sua essentia omnino simplex. Sed remanet in ea compositio duplex: essentiae cum esse et substantiae cum accidentibus.

The spiritual creature is as to its essence altogether simple. Yet there remains a twofold composition in it: that, namely, of essence with existence and that of substance with accidents.

The real distinction between essence (essentia) and existence (esse) (as well as between potentiality and actuality) is fundamental to Thomistic thought. It is the subject of St. Thomas's first key philosophical work, De Ente et Essentia.

Benedict Ashley explains, in The Way toward Wisdom: An Interdisciplinary and Intercultural Introduction to Metaphysics, "Chapter IV: The Culminating Foundational Theorem of Natural Science," § "2). Proof that a First Immaterial Cause Exists":

For Aristotle natural science demonstrates that motion (or change in general) is the natural and proper act of existing changeable, material substances and hence manifests the nature or essence of these existents, that is, that they are essentially changeable. The reason for this is that they are composed of matter (potency) and form (act). Basing himself on this conclusion of Aristotle, Aquinas then shows that existent substances composed of matter and form must also be composed of essence and existence. Hence no body can give itself esse, the act of "to be," any more than it can give form to its matter, that is, move itself. Aquinas' reasoning for this further conclusion is that matter receives its existence from its correlative, but cannot do so unless the form is not merely possible but actual. Forms of material things, however, do not have existence of themselves, since they exist only as an actualization of a matter.

Aquinas considers the possibility that a form might be able to exist without matter, as he really believes to be the case with the angels. Even such a pure form, however, unless its very essence is to exist, would have to receive its existence from the absolutely Unmoved Mover. A fortiori this is the case for the human soul that naturally requires a body and for all material things composed of matter and form. That ordinary substances composed of matter and form do not exist by their very nature is evident in our experience, since we observe that all such substances come into existence and pass away. Even if we consider the hypothesis that there are some material substances (Aristotle's celestial spheres) composed of an extraordinary kind of matter that exists eternally in motion, yet if they have a matter-form composition their existence cannot be self-explanatory but requires another agent. If such an agent is material, it cannot produce eternal motion, and, if immaterial, cannot be a finite intelligence, but must be the absolutely Unmoved Mover. No step in this argument requires a metaphysical notion of Being as ens commune but only the analysis of ens mobile proper to natural science. If the argument were proper to metaphysics, it would be circular since, as we have seen, metaphysics presupposes its conclusion, namely, that immaterial substances exist.

That work also discusses angels extensively because of the importance of the existence of immaterial beings for metaphysics to be a real science (as sciences must have real objects of study).

Also, see Réginald Garrigou-Lagrange's The Essence & Topicality of Thomism, which discusses all this concisely.

Although all analogies break down at some point, it could be thought of this way:
matter : form :: essence : existence.

Lumbreras gives this explanation of the 7th Thesis:

Commentary: The essence of angels is only Act, for the actuality of the form is not received into the potentiality of matter. Angels, indeed, are but intellectual substances, since to understand is a wholly immaterial operation. …
[Summa Theologiae, Iª q. 50 a. 1 ff.; De spiritualibus creaturis, a. 1]

The semiotician/logician João Poinsot, O.P. (1589-1644), a.k.a. John of St. Thomas, wrote more about angels than St. Thomas. Here's his definition of angels:

Spirituality properly speaking [that is, in the substantial order of first act, whence esse comes, and not merely in the operational order of second act, whence esse is sustained] is rightly demonstrated on the basis of intellectuality. But that angelic beings are pure spirits in no way informing or forms of bodies is proved by this: the fact that angels are perfect intellectual substances, and not imperfect as we are. Whence, since intellectuality of itself abstracts from body, and does not seek but is rather impeded by bodiliness, if there are bodily intellectual creatures bespeaking imperfection in the intellectual order, there must needs be yet other creatures perfect in that order of understanding, which means creatures lacking bodies and every intrinsic connection with bodies.

Poinsot 1643: d. 39, a. 1, 456 ¶36: "In Angelis vero magis est nobis notum quod intelligant, eo quod effectus eorum apud nos ex locutione et aliis intelligentiae actibus magis innotescunt, et ex intellectualitate recte probatur spiritualitas. Quod vero ita sint puri spiritus quod nullum corpus informent, ex eo probatur: quia sunt substantiae intellectuales perfectae, et non sicut nos. Unde cum intellectualitas de se abstrahit a corpore, nec petat illud, sed potius impediatur per corpus, necesse est quod si dantur creaturae intellectuales cum unione ad corpus, quod imperfectionem in eo genere dicit, dabiles sint aliquae creaturae in illo genere intelligendi perfectae, atque adeo omni corpore et corporeo affectu carentes."

Source: "The Semiosis of Angels" by the prominent semiotician John Deely (author of the bestselling The Basics of Semiotics)


The key to understanding what is meant by Aquinas' use of the term potential with the matter and form of Angels is to go back to Artistotle's use of the terms: energeia or entelecheia, and dynamis.

But let me back up. In the Articles and Questions preceding the Response to Objection 3 he's already established the grounds for which he'll state that the nature of Angels is as incorporeal/immaterial and existing under the genus of Substance rather than matter. He's also established via a critique of the Metaphysics that he believes that Aristotle makes an error in assuming that all could be explained via bodies (physical reality) through sense and imagination and responding instead that incorporeals can only be apprehended through intellect.

When he says "there is no composition of matter and form in an angel, yet there is act and potentiality", he's invoking the Aristotelian idea that in all beings (Bodies) there is both potentiality and actuality. Potentiality exists in a subject which, though undetermined, is capable of determination. Aquinas uses this framework and applies it to these immaterial entities as well.

He thus created a concept of existence distinct from Aristotle to be something along the lines of "form subsisting in itself". That which is is that which subsists in itself as a kind of form. This is the type of existence held by Angels for him, so, no composition of matter and form is necessary, only act and potentiality. The act and potentiality of these entities are explained elsewhere but relate to their incorporeal effects within human subjects. Hylomorphism is thus limited to the physical world for his perspective. Being qua Being and Substance (God) likewise become inclusive of incorporeality/intellect as well as corporeality/sensual world/imagination. Angels being somewhere between God and bodies are acts of intellect (from the perspective of God) but are finitely so, so they possess the same quality of actuality/potentiality as a corporeal entity would from the perspective of the infinite, but from our finite perspectives, we mistakenly perceive them as infinite (as is explained in the Reply to Objection 4).

So, regarding the question "is the assumption “potentiality = possibility of change” wrong?" I think that's correct, that it would be wrong insofar as Aquinas accepts Aristotle's distinction between potentiality and possibility, with the latter being solely an affair of logic and potentiality being an affair of the world of what "exists" actually. What has the potential to change CAN have the possibility to change as well, but what has the possibility to change logically may or may not have potentiality nor actuality as they relate to different things.


This is a good observation, the answer is that the notion of matter shifted away from the Aristotelian one around the time of Aquinas, and he was instrumental in effecting the change. According to the Aristotelian doctrine, matter was indeed the expression of any potentiality, any capacity for change. This led to a hierarchy of "spiritual matters", and form-matter composites turning into matter for higher order composites (in Aristotle's Physics, book II wood is a composite of matter with "woodenness", but at the same time it serves as mere matter for a bed).

To summarize the shift: yes, all change requires potentiality, but not all potentiality is material potentiality, that is the provenance of corporeal things subject to locality and motion. One can hear a distant echo of this shift in the idea of extension as the chief attribute of matter in Descartes and Spinoza. For spiritual "pure forms", like angels, Aquinas replaces matter as the principle of individuation and change with esse, the (act of) being, the third ontological ingredient he adds to the Aristotelian matter and form (a good short exposition is Maurer's article in A History Of Philosophical Systems).

Pasnau gives some history in Form and Matter:

"This conception of matter lends itself naturally to universal hylomorphism: the doctrine that every (created) substance is a composite of form and matter... Many early scholastic authors, especially Franciscans, embraced this sort of view. From the time of Albert the Great and Thomas Aquinas, however, the view fell entirely out of favor, replaced by the idea that matter occurs only in the corporeal realm. Hence there arose the linkage we take for granted today, between corporeality and materiality, so that to be a body (corpus) is just to have matter."

Aquinas wholeheartedly embraces interpreting matter as potentiality, but only as long as one is willing to make distinctions between different kinds of potentiality. And his preference is to avoid the "equivocation" altogether by restricting "material" to the tangible, corporeal. Kenny discusses the issue in detail in Aquinas on Mind, Ch.11. Here is Aquinas himself in Quaestiones Disputate de Anima:

"...to receive, to be a subject, and other things of this sort, are not found in the soul and in prime matter in the same specific way. For prime matter is actuated by means of change and motion, and since every change and motion may be reduced to local motion, as the primary and most universal type of motion, as is proved in the Physics [VIII, 7, 260b 6], it follows that matter is present only in those things in which there is potency to place (ab ubi). Moreover, things of this kind, which are circumscribed by place alone, are corporeal. Hence, in accordance with the way in which the philosophers have spoken about matter, matter is present only in corporeal things; unless, of course, someone wishes to employ matter in an equivocal sense."

We can only speculate why the shift occurred. Perhaps, it made for a cleaner separation between the lowly and the spiritual than malleable Aristotelian "matters", and/or "souls as pure forms" were more credibly incorruptible than the ones made of "spiritual matter". Aquinas does draw a distinction between the latter and angels ("and the very fact that the soul in a certain way requires the body for its operation, proves that the soul is endowed with a grade of intellectuality inferior to that of an angel, who is not united to a body"), but it is moot for the present purposes since for him even the human soul can subsist without a body. Aquinas personally also had additional reasons related to perception of universals in his theory of cognition. Here is from Summa Theologica:

"The intellectual soul, therefore, is pure form, not something composed of matter and form. For if the intellectual soul were composed of matter and form, the forms of things would be received in it as individual. In that case it would not have knowledge of anything except singulars. [...] Now the receptive potentiality in the intellectual soul is other than the receptive potentiality of first matter, as appears from the diversity of the things received by each. For primary matter receives individual forms; whereas the intelligence receives absolute forms. Hence the existence of such a potentiality in the intellectual soul does not prove that the soul is composed of matter and form. [...] In intellectual substances there is a compounding of actuality and potentiality, but not of matter and form, but of form and the being in which it shares."

Replacing "spiritual matter" with esse may have worked for souls and angels, but interpreting the non-composite ("prime") matter as "the" material of the corporeal realm, while retaining its "pure potentiality", led to quaint conclusions about the said material. It appeared to be devoid of any content. Duns Scotus, and more forcefully Ockham, took this position to task, and completed the actualization of (now corporeal) matter. From Ockham's Summula Philosophiae Naturalis:

"That matter is a certain actual entity is clear, because that which does not exist can be a part or principle of no being. But matter actually is a part and principle of a composite being. Therefore it is actually an entity in act. Further, every substance is in act in the natural world. But matter is a substance, since it is a part and principle of substance, and that which is a substance is composed only of substances, according to the Philosopher. Therefore matter is truly in actuality."

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