[4.9.1] St Thomas Aquinas on Substance and Change

St Thomas Aquinas (1225 – 1274 AD) in his works On Being and Essence and The Principles of Nature writes about substance as a primary metaphysical category.

  • His model is an evolved version of Aristotelian hylomorphism (see [1.3.5], and for humans [1.3.7]), where substances are enduring primary existents composed of prime matter and substantial form.
  • The soul is the form of the living human body.
  • Incidental (or accidental) changes modify the substance’s incidental forms (or properties) like quality, quantity, place, while substantial change modifies its existence.

The following OntoUML diagram presents Aquinas’s model of substance and change:

Aquinas on substance and change
SubstanceAccording to Aquinas substances are what are primarily said to exist.
“[…] enduring things like men and trees and horses and the like have also come into being and are destined some day to cease to be. Such things are called substances.
contains PrimeMatter
PrimeMatter“The subject of a surface or incidental change is a substance [its incidental form]. The subject of a substantial change cannot be a substance; if it were, the result would be a modification of that substance, that is, an incidental change. But we are trying to understand how a substance itself comes into being as the result of a change. There must be a matter or subject but it cannot be matter in the sense of a substance. In order to signal this, we can call the matter prime matter, first matter. But it is important to recognize that this prime matter is not a substance, and does not exist apart from any particular substance. It is always the matter of some substance that exists.”
SubstantialForm“The form in a substantial change must be that which makes the substance to be what it is. Call it substantial form.”characterizes Substance; inherits from Form
IncidentalFormIncidental form (or accidental form) can be: “like size or location or temperature” characterizes Substance; inherits from Form
FormThe “elements of the change get the names that stick from another example, whittling wood. The term for wood in Greek is hyle and the term for shape, the external contours of a thing, is morphe. In English, form, a synonym of shape, is used to express the characteristic that the subject acquires as the result of the change, e.g. musical.”
LivingHumanBodyA living human bodysubkind of Substance
Soul“When the discussion moves on from what may be said of all physical objects as such to an inquiry into living physical things, the analyses build upon those already completed. Thus, “soul” will be defined as the form of living [human] bodies. […]
Thomas merely concludes from this fact that the soul is a ‘particular thing’ and thus a subsistent after the death of the body. He argues that what belongs to the notion of ‘this particular thing’ is only that it be a subsistent, and not that it be a substance complete in a nature. A subsistent is something with an operation of its own, existing either on its own or in another as an integral part, but not in the way either accidental or material forms exist in another. Existing on its own is not distinctive of substances alone. A chair is a particular thing, and thus a subsistent. But on Aquinas’ account it is not a substance; it is rather an accidental unity of other subsistents which may or may not be substances. A hand has an operation distinctive of it as an integral part of a living body, an operation different from the operation of the stomach; it is a particular thing and also a subsistent. (Summa Theologiae Ia.75.2 ad1; also Quaestiones Disputate de Anima 2.) And yet being an integral and functional part of a substance, it does not have the complete nature of a substance.”
inherits from SubstantialForm; characterizes LivingHumanBody
ChangeChange is can be a change of one substance into another substance, or a modification of an already existing substance.
“Aristotle had to begin with a particular example of change, one so obvious that we would not be distracted by any difficulties in accepting it as such. ‘A man becomes musical.’ Someone acquires a skill he did not previously have. Thomas pores over the analysis Aristotle provides of this instance of change and its product.
The change may be expressed in three ways:
[1] A man becomes musical.
[2] What is not-musical becomes musical.
[3] A not-musical man becomes musical.
These are three different expressions of the same change and they all exhibit the form A becomes B. But change can also be expressed as From A, B comes to be. Could 1, 2 and 3 be restated in that second form? To say ‘From the not-musical the musical comes to be’ and ‘From a not-musical man the musical comes to be’ seem acceptable alternatives, but ‘From a man musical comes to be’ would give us pause. Why? Unlike ‘A becomes B’ the form ‘From A, B comes to be’ suggests that in order for B to emerge, A must cease to be. This grounds the distinction between the grammatical subject of the sentence expressing a change and the subject of the change. The definition of the subject of the change is ‘that to which the change is attributed and which survives the change.’ The grammatical subjects of 2 and 3 do not express the subject of the change. Only in 1 is the grammatical subject expressive of the subject of the change.
This makes clear that the different expressions of the change involve two things other than the subject of the change, namely, the characteristics of the subject before (not-musical) and after (musical) the change. These elements of the change get the names that stick from another example, whittling wood. The term for wood in Greek is hyle and the term for shape, the external contours of a thing, is morphe. In English, form, a synonym of shape, is used to express the characteristic that the subject acquires as the result of the change, e.g. musical. The characterization of the subject prior to the change as not having the form is called privation. Using this language as canonical, Aristotle speaks of the subject of the change as its hyle or matter, the character it gains as its morphe or form, and its prior lack of the form as its privation. Any change will involve these three elements: matter, form and privation. The product of a change involves two things: matter and form.”
IncidentalChange“The analysis of change and the product of change begins with surface [incidental] changes [or accidental changes]. Some enduring thing changes place or quality or quantity. […]
As the analysis of incidental change makes clear, the substance previously existed without the form it acquires in the change and it could lose it and still be itself.”
inherits from Change; changes IncidentalForm
SubstantialChange“In a substantial change, the substance itself simply comes to be, or ceases to be.” inherits from Change; changes existence of SubstantialForm


  • All citations from: McInerny, Ralph and John O’Callaghan, “Saint Thomas Aquinas”The Stanford Encyclopedia of Philosophy (Summer 2018 Edition), Edward N. Zalta (ed.)
  • Kerr, Gaven : “Aquinas: Metaphysics”, Internet Encyclopedia of Philosophy
  • The Cambridge Companion to Aquinas, CAMBRIDGE UNIVERSITY PRESS, Edited by  Norman Kretzmann and Eleonore Stump, 2010

First published: 10/09/2020

[1.3.13] Aristotle on Motion

Aristotle (384-322 BC) writes about motion in the books 5-8 of its Physics. In his analysis he uses the concepts of Potentiality, Actuality, and Causes (see in [1.3.4]), and introduces the idea of Nature (of a thing, object), based on which he sets apart natural from forced motion.
“A major presupposition on Aristotle’s part is that this division is exhaustive: there are no changes to which the nature of the entity would be indifferent or neutral. The major consideration behind such a presupposition is that natures regulate the behaviour of the entities to which they belong in a comprehensive manner, and not merely partially. Any influence the entity is exposed to interacts with its nature in a substantive manner. The entity does not possess potentialities for change which would not be directly related to the tendencies emerging from its nature.”

The following OntoUML diagram shows the main classes in this model:

Aristotle on motion
ObjectObjects are essential particulars (see [3.1]), like statues and houses, horses, and humans. can move Object
Mover (ActiveObject)Mover (Active Object) includes (Active) Potentiality which initiates motion, change in a Passive Object through its Passive Potentiality.is role of Object
UnmovedMover The unmoved movers are methaphysiscal, non-material entities, which are not part of the physical world.
“Aristotle postulates that the processes of the universe depend on an eternal motion (or on several eternal motions), the eternal revolution of the heavenly spheres, which in turn is dependent on one or several unmoved movers”
is Mover (ActiveObject); can move the Object
Moved (Passive Object) is set in motion, changed by the Active Object.is role of Object
NatureNature [of an Object], according to Aristotle, is an inner principle of change and being at rest… This means that when an entity moves or is at rest according to its nature reference to its nature may serve as an explanation of the event…
Natures, then, in a way do double duty: once a nature is operative, neither a further active, nor a further passive capacity needs to be invoked.”
is mandatory part of the Object
Cause“Natures,.. can feature in any… four causal functions. However, when the matter of an entity functions as its nature—i.e., when its natural motion and rest are explained in terms of the matter it is made of—this matter must possess some causally relevant features, bestowed upon it by its own formal aspect.” relates to Nature XOR ActiveObject
MatterMatter provides the Potentiality for the Object.is contained by the Object
Potentiality“change always requires the existence of a potentiality which can be actualised… Aristotle’s formulation strongly suggests that the potentiality actualised in the process of change is not a separate and independent potentiality for motion… Accordingly, potentialities of change are admitted into the ontology. They, nevertheless, do not need to feature as potentialities in their own right, but as the incomplete variants of the fundamental potentiality for an end result…
The definition of motion as the actuality of a potentiality of the entity undergoing motion in so far as it is potential requires that in each case the passive potentiality for the change is present in the changing object.”
relates between Matter and Object
ActivePotentiality active powers or potentialities (dunameis),,, are external principles of change and being at rest (Metaphysics 9.8, 1049b5–10), operative on the corresponding internal passive capacities or potentialities (dunameis again, Metaphysics 9.1, 1046a11–13)… is Potentiality; initiates motion in PassivePotentiality; is mandatory part of the Mover (ActiveObject)
PassivePotentiality“the passive potentiality, is in the object undergoing change”is Potentiality; is mandatory part of the Moved (PassiveObject)
Motion(Change) Motion is “the actuality of a potentiality of the entity undergoing motion.”
“Aristotle speaks about four kinds of motion and change only—those in substance, in quality, in quantity and in place…”
relates and characterizes Object
NaturalMotion“when the matter of an entity functions as its nature—i.e., when its natural motion and rest are explained in terms of the matter it is made of—this matter must possess some causally relevant features, bestowed upon it by its own formal aspect…
The presence of the potentiality can, nevertheless, be in accordance with the nature of the object—in which case the change is natural (phusei)”
is Motion; relates to Nature
ForcedMotion“When a change, or a state of rest, is not natural [is Forced Motion], both the active and the passive potentiality need to be specified…
The presence of the potentiality… can happen in the face of a contrary disposition on the part of the nature of the entity—in which case the change is forced (biâi) or contrary to nature (para phusin).”
is Motion; relates to ActivePotentiality and PassivePotentiality
InSubstance is a characterization of the Motion (Change)relates to Motion
InQuantity is a characterization of the Motion (Change) relates to Motion
InQuality is a characterization of the Motion (Change) relates to Motion
InPlaceis a characterization of the Motion (Change) relates to Motion


First published: 26/09/2019