[4.9.3] St Thomas Aquinas on Essence and Existence in Immaterial Substances

St Thomas Aquinas ( “Doctor Angelicus”, 1225 – 1274 AD), in his work On Being and Essence analyzes substances using concepts like matter, form, hylomorphism, potentiality, actuality (developed by Aristotle, see [1.3.5]), essence, and existence (developed by Avicenna, see [3.3.1]).

  • Substances are enduring primary existents, which can be material or immaterial, (see also [4.9.2]).
  • Substances are a composition of essence (which makes the substance what is) and existence (which is the act of being “esse“).
  • The essence of immaterial substances is their form.
  • For immaterial substances, the form is the potentiality, and existence is the actuality.
  • The essence (quiddity) of God, the first existent – is its existence. God gives existence to all the other substances, be it material or immaterial.

The following OntoUML diagram presents Aquinas’s model of immaterial substances:

Aquinas on essence and existence in immaterial substances

SubstanceAccording to Aquinas substances are what are primarily said to exist. They are enduring things like trees, horses, humans.
“Some substances are simple [immaterial], others are composite [material]”.

MaterialSubstanceMaterial substances [composite substances] are like: Socrates, a horse, a tree, a stone etc.
“Some substances are simple, others are composite, and both sorts have their essence, but the simple ones in a more genuine and excellent way, just as they have a more excellent way of being. For they are the cause of the composite ones; at least this is true of the first, simple substance, which is God.
However, since the essences of these substances are quite hidden from us, we should begin with the essences of composite substances, so that our discussion may more suitably proceed from the easier subjects.”
subkind of Substance
ImmaterialSubstanceImmaterial substances [simple substances]: are like: soul, intelligences [i.e., angels], and the first cause [i.e., God].
They have their essence “in a more genuine and excellent way, just as they have a more excellent way of being. For they are the cause of the composite ones; at least this is true of the first, simple substance, which is God. […]
Therefore, in the soul or in the intelligences there is no composition of matter and form in any way, so that they could be understood to have matter in the same sense in which corporeal substances do. But there is the composition of form and existence [esse] in them.”
subkind of Substance
ImmaterialSubstance (OtherThanGod)Immaterial substance other than God are like soul, intelligences [i.e., angels].
“It is clear that forms are not actually intelligible, unless they are separated from matter and its [individuating] conditions, and they are not rendered actually intelligible, except by the power of an intelligent substance, insofar as it receives them and works on them. Therefore, an intelligent substance has to be immune from matter in every way, so that it neither has matter as its part, nor does it exist as a form impressed in matter, as is the case with material forms.”
subkind of ImmaterialSubstance
Essence“For on the basis of what has been said so far it is clear that the essence of a thing is what its definition signifies.”exclusive part of ImmaterialSubstance
Essence (ofImmaterial Substance OtherThanGod)“The difference, therefore, between the essence of a simple substance and that of a composite substance is that the essence of a composite substance is not only the form, but it comprises both form and matter, whereas the essence of a simple substance [essence of Immaterial substance] is its form only. […]
since the essence of a simple thing is not received in matter, in their case there cannot be this kind of multiplication; therefore, in the case of these substances, there cannot be several individuals in the same species, but there are as many species as there are individuals, as Avicenna expressly claims. […]
the essence of a simple thing, which is its form, can only be signified as a whole, for there is nothing there apart from the form, so there is nothing that could receive the form. Therefore, no matter how we consider the essence of a simple substance, it is predicated of the simple substance. This is why Avicenna says that the quiddity of a simple substance is the simple substance itself, for there is nothing else [in the simple substance] to receive it.”
subkind of Essence; exclusive part of ImmaterialSubstance
Definition“the definition of things of nature contains not only form, but matter as well; otherwise natural definitions would not differ from mathematical definitions.”
The definitions are in the mind.
signifies Essence
Form“So, those forms that are the closest to the first principle are forms subsisting without matter, for the whole genus of forms does not require matter, as has been said, and forms of this sort are the intelligences; so, the essences or quiddities of these substances do not have to be other than their form itself. […]characterizes Essence (ofImmaterial Substance OtherThanGod)
Existence“Whatever is not included in the understanding of an essence or quiddity is coming to it from outside, entering into composition with the essence; for no essence can be understood without its parts. But every essence can be understood without even thinking about its existence, for I can understand what a man or a phoenix is, and not know whether it actually exists in the nature of things. Therefore, it is clear that existence is distinct from essence […]”characterizes Essence (ofImmaterial Substance OtherThanGod)
Potentialty“Therefore, the quiddity or form that is the intelligence has to be in potentiality with respect to the existence it receives from God […]”relates Form with Essence (ofImmaterial Substance OtherThanGod)
Actuality“Therefore, the quiddity or form that is the intelligence has to be in potentiality with respect to the existence it receives from God, while the existence received is its actuality.relates Existence with Essence (ofImmaterial Substance OtherThanGod)
GodGod, the first cause.
“Now, everything that a thing has is either caused in it by its own principles, as the ability to laugh in man, or it comes to the thing from an external source, as the light in the air is coming from the sun. But the existence of a thing cannot be caused by its form or quiddity itself (I mean, as by an efficient cause), for then a thing would be its own cause, and would bring itself into existence, which is impossible. Therefore, all such things, namely, those that have their existence as something distinct from their nature, have to have their existence from something else. However, since everything that is through something else [per aliud] is reduced to what is through itself [per se] as its first cause, there has to be something that is the cause of existence for everything, since it is existence only. For otherwise the series of causes would go to infinity, since every thing that is not existence only has a cause for its existence, as has been said. It is clear, therefore, that an intelligence is both form and existence, and that it has its existence from the first being that is existence only; and this is the first cause, which is God.”
subkind of ImmaterialSubstance; causes Existence
God’sExistenceGod’s existence is his essence (quiddity).subkind of Existence; characterizes God


  • All citations from:  Thomas Aquinas, “On Being and Essence”, in Medieval Philosophy: Essential Readings with Commentary, Edited by Gyula Klima with Fritz Allhoff and Anand Jayprakash Vaidya, Blackwell Publishing Ltd, 2007
  • Kerr, Gaven : “Aquinas: Metaphysics”, Internet Encyclopedia of Philosophy
  • McInerny, Ralph and John O’Callaghan, “Saint Thomas Aquinas”The Stanford Encyclopedia of Philosophy (Summer 2018 Edition), Edward N. Zalta (ed.)
  • The Cambridge Companion to Aquinas, CAMBRIDGE UNIVERSITY PRESS, Edited by  Norman Kretzmann and Eleonore Stump, 2010
  • Joshua P. Hochschild, “Form, Essence, Soul: Distinguishing Principles of Thomistic Metaphysics”

First published: 24/09/2020

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