Aquinas: The Process of Change

The Philosophical System of Thomas Aquinas, by Maurice de Wulf, edited and adapted for the web by by Jonathan Dolhenty

IX. The Process of Change

A. Actuality and Potentiality

Our supposition of a motionless and dead universe is after all only an artifice of our didactic method. For it is evident that the things which we have described are actors in a cosmic drams: they are borne on the stream of change, and nothing is motionless.

Molecules or atoms, monocellular beings or organisms, all are subject to the law of change. Substances, together with their accidents, are constantly becoming. The oak tree develops from an acorn, it becomes tall and massive, its vital activities are constantly subject to change, and the tree itself will eventually disappear. So also the lion is born, develops and grows, hunts its prey, propagates its kind, and finally dies. Again, human life, both in its embryonic and more developed forms, is a ceaseless process of adaptation. If we wish to understand the full meaning of reality, we must throw being into the melting pot of change. Thus the static point of view, or the world considered in the state of repose, must be supplemented by the dynamic point of view, or that of the world in the state of becoming. Here we come across a further scholastic notion, — namely, the celebrated theory of actuality and potentiality, which may well be said to form the keystone in the vaulting of metaphysics.

This theory results from an analysis of what change in general implies. What is change? It is a real passage from one state to another. Schoolmen reason thus:

If one being passes from state A to state B, it must possess already in state A the germ of its future determination in state B. It has the capacity or potentiality of becoming B, before it actually is B. To deny this quasi-preexistence, in fact, involves the denial of the reality of change, or evolution of things.For, what we call change would then simply be a series of instantaneous appearances and disappearances of realities, with no internal connection whatever between the members of the series, each possessing a duration infinitesimally small. The oak tree must be potentially in the acorn; if it were not there potentially, how could it ever issue from it? On the other hand, the oak is not potentially in a pebble rolled about by the sea, although the pebble might outwardly present a close resemblance to an acorn.

Act or actuality (actus) is any present degree of reality. Potency (potentia) is the aptitude or capacity of reaching that stage of reality. It is imperfection and non-being in a certain sense, but it is not mere nothing, for it is a non-being in a subject which already exists, and has within itself the germ of the future actualization [1].

The duality of act and potency affects reality in its inmost depths, and extends to the composition of substance and accident, matter and form.

B. The becoming of a substance

To say that a concrete substance — for instance, this oak tree, this man — is in a process of becoming means that it is realizing or actualizing its potentialities. A child is already potentially the powerful athlete he will some day become. If he is destined to become a mathematician, then already in the cradle he possesses this aptitude or predisposition, whereas another infant is deprived of it. All increase in quantity, all new qualities, activities exercised and undergone, all the new relations in which the subject in question will be engaged with surrounding beings, all its various positions in time and space, were capable of coming to existence, before being in fact. Substance is related to its accidents like potentiality to actuality.

Viewed in the light of this theory, the doctrine of substance and accident loses its naive appearance. A growing oak, a living man, a chemical unit, or any one of the millions of individual beings, is an individual substance which is in a process or state of becoming, inasmuch as its quantity, qualities, activities, and relations are actualizations of the potentialities of the substance. Leibnitz was in point of fact following this thomistic doctrine when he said: “the present is pregnant with the future.”

But while Leibnitz taught also the eternity and the immutability of substances, which called monads, Aquinas and the Schoolmen went further into the heart of things. It is not only the quantity or quality which changes when, for example, an oak tree grows, or its wood becomes tougher, it is not merely its place which changes when it is transplanted, or its activities which develop, — in all these cases it is the substance, the oak tree, which is so to speak the subject of these accidental changes. But the very substance of a body may be carried into the maelstrom, and nature makes us constant witnesses of the spectacle of substantial transformation. The oak tree dies, and from the gradual process of its decomposition there come into actual existence chemical bodies of various kinds. Or an electric current passes through water: and behold in the place of water we find hydrogen and oxygen.

C. Prime Matter and Substantial Form

When one substance changes into another, each has an entirely different specific nature. An oak never changes into another oak, nor one particle of water into another. But out of a dying oak tree, or a decomposed particle of water, are born new chemical bodies, with quite different activities, quantities, relations, and so on. Substances differ not only in degree, but in kind.

Let us look more closely into this phenomenon of basic change from one substance into another, or into several as in the case of water and the hydrogen and oxygen which succeed it. If Aquinas had been asked to interpret this phenomenon, he would have said that every substance that comes into being in this way consists ultimately of two constituent elements or substantial parts: on the one hand, there be something common to the old state of being and the new — to water and hydrogen for instance — and on the other hand there must be a specific principle proper to each. Without a common element, found equally in the water and in the hydrogen and oxygen, the one could not be said to ‘change’ into the other, for there would be no transposition of any part of the water into the resulting elements, but rather an annihilation of the water, followed by a sudden apparition of hydrogen and oxygen. As for the specific principle, this must exist in each stage of the process as a peculiar and proper factor whereby the water as such differs from the hydrogen or oxygen as such.

This brings us to the theory of “primary matter” and “substantial form” which is often misunderstood. It is in reality nothing more than an application of the theory of actuality and potency to the problem of the transformation of bodies: before the change, hydrogen and oxygen were in the water potentially. The primary matter is the common, indeterminate element or substratum, capable of receiving in succession different determinations. The substantial form determines and specifies this potential element, and constitutes the particular thing in its individuality and specific kind of existence. It enable it to be itself and not something else. Each man, lion, oak tree, or chemical unit possesses its form, that is, its principle of specific and proper reality. And this principle or form of any one thing is not reducible to that which is proper to another. The form of an oak tree is altogether distinct from that of man, hydrogen, and so on.

D. Role of matter and form – their relation

Each thing that concerns the state of indetermination of a being follows from its prime matter. This applies especially to quantitative extension; for, to possess quantitative parts, scattered in space, is to be undetermined.

On the other hand, each thing that contributes to the determination of a being — its unity, its existence, its activities — is in close dependence upon the formal principle. Thus form unifies the scattered parts, it provides the substance with actual existence and is the basic root of all specific activity.

It follows from the above that matter and form cannot be found independently of one another in beings which are purely corporeal. They compenetrate each other like roundness and a round thing. To speak of a prime matter existing without a form, says Thomas, is to contradict oneself, for such a statement joins existence — which is determination — with the notion of prime matter — which is that of indetermination [2].

We may now come back to the conception of individual substance from which we started (VIII, A). A corporeal being consists of two substantial parts — matter and form — neither of which is complete. Only the being resulting from the union of both is a complete or individual substance, to which belongs the proper perfection of self-sufficiency and of being incommunicable to any other.

E. Evolution or succession of forms

The material universe presents us with an harmonious evolution. Reality mounts step by step from one specific nature to another, following a certain definite order. Nature changes water into hydrogen and oxygen, but it does not change a pebble into a lion; nor ‘can one a saw out of wool.’ Things evolve according to certain affinities, and in a certain order, the investigation of which is the work of the particular sciences, and calls for patient observation. If there are any leaps in Nature, they are never capricious. Every material substance, at every stage and at every instant, contains already the germs of what it will be in the future. This is what is meant by the scholastic formula which states that “primary matter contains potentially, or in promise, the series of forms with which it will be invested in the course of its evolution.” Prime matter is related to each substantial form, like potentiality to actuality. Hence, to ask, as some do, where the forms are before their appearance, and after their disappearance, is to reveal a misunderstanding of the scholastic system.

To sum up. Two kinds of change suffice to explain the material world. We have firstly the development of substances already constituted; thus an oak tree is undergoing development or change in its activities, its quantity, qualities, and relations, but retains throughout the same substance: the change undergone is called accidental. In the second place, we have the change of one substance into another or into several, such as the change of an oak tree into a collection of chemical bodies: this change is called substantial.

Thus the evolution of the cosmos is explained as being a combinant of fixity and movement. Beings evolve, but everything is not new: something of the past remains in the present, and will in turn enter into the constitution of the future. The scholastic theory of the process of change is a modified one, a via media between the absolute evolution of Heraclitus and the theory of the fixity of essences which so much attracted Plato.

F. Principle of individuation

The theory of matter and form also explains another scholastic doctrine, that of the principle of individuation. The problem to be solved is this: How is it possible that there should be so many distinct individualities possessing the same substantial perfection, of ‘of the same kind,’ as we say? Why are there millions upon millions of oak trees, and not only one, corresponding to one forma querci, one ‘oak tree form’? Why should there be millions of human beings instead of one only? If everything was unique in this way, the universe would still manifest a scale of perfection, but there would be no two material things of one and the same kind. One things would differ from another specifically, as the number ‘three’ differs from the number ‘four.’

The ‘monads’ of Leibnitz present us with a conception of the world more or less on these lines. But the thomist solution is more profound. It is summed up in this thesis. Extension — which pertains to prime matter — is the principle of individuation.

My body has the limitation of extension, and in consequence there is room for your body, and for millions of others besides ours. An oak tree has a limited extension in space, and at the point where it ceases to occupy space there is room for others. In other words, without extension, or extended matter, there would be nothing which could render possible a multitude of individuals of the same kind. For, if we consider form alone, there is no reason why there should be a multiplication of a given form, or why one form should thus limit itself, instead of retaining and expressing within itself all the realization of which it is capable. Forma irrecepta est illimitata, — “A form which is not received in anything, i.e., an isolated form, is not limited or confined.” But the case is different if the principle of determination is one which must take on an extended existence.

There is an importance consequence which follows directly from this doctrine. If there exist some beings which are not corporeal, and whose principle of reality has nothing to do with extension and prime matter (pure forms; pure Intelligences, for instance), then no reduplication or multiplication is possible in that realm of being. Each individual will differ from one another as the oak-form differs from the beech-form or the hydrogen-form.

The last point explains why the problem of individuation is different from that of individuality. Each existing being in an individuality, and therefore a Pure Intelligence if such exists, also God, is an individuality. But individuation means a special restricted kind of individuality, i.e., a reduplication or multiplicity of identical forms in one group; hence the term specific groups, species.

G. Causality

The theory of cause is a complement of the theory of actuality and potentiality, for it explains how the actualizing of a potency takes place in any given being. Causality is fourfold, because there are four ways of regarding the factors which account for the evolution of individual substances.

(a). The first and apparent is efficient causality. It is the action by reason of which a being A which is capable of becoming A’ actually becomes A’. This action comes from without. No being which changes can give to itself, without some foreign influence, this complement of reality by virtue of which it passes from one state into another. Quidquid movetur ab alio movetur: whatever changes is changed by something other than itself. For if a thing could change its own state (whether substantial or accidental), unaided, it would possess before acquiring; it would already be what it is not yet, which is contradictory and impossible. Water is capable of changing into oxygen and hydrogen, but without the intervention of an electric current or something else it would never of itself take on these new determinations. A being which changes is of course a being which does not exist necessarily in this state of change. Hence the principle: whatever changes is changed by something other than itself, is an application of this more general principle: the existence of a non-necessary being demands an efficient cause (IV, B).

However, this acting cause is itself subject to the process of becoming. The electrical energy could not manifest itself unless it is affected in its turn by the action of other efficient causes. The whole process resembles that which happens when a stone is thrown into still water: the waves spread out from the center, each producing the next in succession. Moreover, there is an additional complication, for every action of a being A upon another B is followed by a reaction of B upon A. Nature is an inextricable tissue of efficient causes, developments, passages from potency to actuality. Newton’s Law of Gravitation, the Law of the Equilibrium of Forces, the Principle of the Conservation of Energy, are all so many formulas which set forth in precise terms the influence of one being upon another. Actions and reactions establish close connections between substances which are independent in their individuality.

(b) and (c). In addition to the efficient cause, scholasticism attributes a causal role to matter and to form, inasmuch as, in giving themselves to each other, these two constitute and explain the being which results from their combination. A particle of oxygen has for its constituent causes an undetermined element (primary matter), and a specifying element (substantial form), just as in turn the oak-substance or marble (secondary matter), together with the cylindrical shape or the human figure (accidental form), are constituent causes of a particular oak tree as a whole, or of a particular statue.

(d). Lastly, we have the final cause. The activities which flow from each individual being do not develop simply at random. Water is not indifferent to boiling at 90 degrees C. or 100 degrees D.: if it were so, we might expect to find all sorts of capricious jumps in nature. Since the same activities and transformations are continually recurring, we infer that there is in each being an inclination to follow a certain path, to obey certain laws. Deus imprimit toti naturae principia propriorum actuum. — God has impressed upon every nature the principles of its peculiar activities [3]. This inclination, which is rooted in the substantial form, and tends to produce the appropriate activities, constitutes the internal finality of each being. It is always present, even when an obstacle prevents its full exercise. Natura non deficit in necessariis. — Nature does not fail in necessary things.

In spite of disorders which appear at the surface of the physical world, and in spite of moral evil, both of which result from the contingent and imperfect character of the world, the internal finality proper to each being in the universe leads up to another finality, — which is external. The courses of the stars, the recurrence of seasons, the harmony of terrestrial phenomena, the march of civilization, are all indications of a cosmic order which is not the work of any single being — not even of man — but which proves to the mind of a Schoolman the existence of a Supreme Ruler of all, endowed with wisdom. Dante receives his inspiration from scholasticism, when he concludes the Divine Comedy by singing of the universal attraction of the world ever drawn towards its goal, which can only be God [4].

This twofold doctrine of internal and external finality furnishes us with a strong teleological interpretation of the universe.

The hierarchical order that exists between the four causes results from their nature. Finality attracts (consciously or not) and persuades a being to exercise its activities. Efficient causality tends towards the end in view, and the result of action is a new union of matter and form. When an artist undertakes to chisel a statue, it is his purpose which directs the designs, the choice of the material, the chiseling itself. The first intention of the artist is the last thing to be realized. It is not otherwise with the aim of nature: in the order of intention the final cause comes first; but in the order of execution it is the last to be realized.

H. Essence and existence

We have not yet exhausted the analysis of reality. Each individual has been distinguished into substance and accident, and in every material substance we have found matter and form. In all these stages we have been studying essence, ‘what a thing is.’ Essence, however, has existence, and existence presents us with a quite new aspect of reality. Existence is the supreme determination of any being (actus primus). Without existence, the several essential elements which we have been considering would be merely possible; they would resemble the legendary horse of Roland, which possessed all perfections, but did not exist.

Moreover, these manifold essential elements (matter, form, accidents) do not exist in separation. They exist, says Aquinas, by virtue of one existence alone. It is the concrete oak tree which exists, the concrete lion, the actual man, Pasteur or Edison.

They theory of essence and existence completes the analysis of reality. We shall return to it in another part (XI, B). We must first indicate the place of man in the world which we have been studying, and expound a body of doctrines sometimes known as the metaphysical side of scholastic psychology.

Scheme of metaphysical doctrines explained in parts VIII, IX, XI, B.:

Essence (essentia)

Substance (substantia)

Prime matter (materia prima)
Substantial form (forma substantialis)


Accidents (accidentia)

Quality (shape, power, habits)



  1. We deliberately abstain from translating potentia by “power,” as is sometimes done. “Power has practically always an active sense which is completely absent from potentia when contrasted with actus. An example will make our meaning clear. A sculptor is in potentia to the carving of a statue, but it is equally true that the block of marble is in potentia to becoming the statue. We should say that the sculptor had the “power” to make the statue, but we should hardly say that the block of marble had the “power” of becoming the statue. Hence the objection to the use of the word “power” here. A thing is in potency to that which will become, whether by its own activity, or the activity of something else.

  2. It is important to note that primary matter (material prima) is altogether distinct from matter as understood by modern science. Matter as now understood signifies a substance of a particular kind (comprising ‘matter’ and ‘substantial form’ of the Schoolmen together with extension in space, which is an ‘accident.’

  3. Summa Theol., Ia IIae, q. 93, art. 5.

  4. L’Amor che muove il sol e l’altre stelle.

The late Dr. Jonathan Dolhenty was the Founder and President of The Center for Applied Philosophy and the Radical Academy, and is Honorary Philosophy Editor at The Moral Liberal. The Moral Liberal has adopted these projects beginning with a republishing and preserving of all of Dr. Dolhenty’s work.