[4.9.2] St Thomas Aquinas on Essence and Existence in Material Substances

St Thomas Aquinas (1225 – 1274 AD), in his work On Being and Essence analyzes substances using concepts like matter, form, hylomorphism, actuality, potentiality (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.
  • Substances are a composition of essence (which makes the substance what is) and existence (which is the act of being “esse“).
  • The essence of material substances contains both matter and form.
  • Actuality and potentiality (in conformity with Aristoele’s model) relate form and matter to the essence of the material substance.
  • God, an immaterial substance gives existence to all the other substances, be it material or immaterial.

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

Aquinas on essence and existence in material substances
ClassDescriptionRelations
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] 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.”subkind of Substance
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.”
Essence(ofMaterial Substance)Essence (ofMaterial Substance): “But the definition of things of nature contains not only form, but matter as well; otherwise natural definitions would not differ from mathematical definitions. And it cannot be said that matter is included in the definition of a natural substance as something added to its essence, or as something existing outside of its essence, for this sort of definition is proper to accidents, which do not have perfect essence, and therefore they have to receive in their definition their subject, which is outside their essence. It is clear, therefore, that essence comprises both matter and form. […]
The name “quiddity,” on the other hand, is derived from the fact that it is signified by the definition; but it is called “essence” [essentia], because it is that through which and in which a thing has its being [esse].”
subkind of Essence; contains Matter; exclusive part of MaterialSubstance
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“For by the form, which is the actuality of matter, matter is made into an actual being and this [particular] thing [hoc aliquid]. Therefore, whatever supervenes does not give actual being to matter absolutely speaking, but it makes it actual in some respect, just as accidents do, as whiteness makes something actually white. So, when a form of this sort is acquired, the thing is not said to be generated absolutely speaking, but only in some respect.”characterizes Form
MatterMatter, e.g. wood, bronze, or human flesh.
Existence“And reason agrees with this as well, for the act of being [esse, existence] of a composite substance is neither of the form alone, nor of matter alone, but of the composite itself; and the essence is that on account of which the thing is said to be.”characterizes and actuates Essence(ofMaterial Substance)
Actuality“For by the form, which is the actuality of matter, matter is made into an actual being and this [particular] thing [hoc aliquid].”relates Form with Essence(ofMaterial Substance)
PotentialityPotentiality is “possibility” that a thing can have.relates Matter with Essence(ofMaterial Substance)
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

Sources

  • 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: 17/09/2020
Updated: 19/09/2020 added Actuality, Potentiality
Updated: 23/09/2020 added God

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