A substance (sometimes called a "suppositum" or "hypostasis") is that which exists in itself as a natural object in the world. Examples of substances are: this apple, those birds, that man, etc. A substance is a natural object and should therefore be distinguished from an artifact or a mere aggregate of substances. A house or a car is not a natural object, but an artifact made up of several substances.


People sometimes speak of apples, birds and men as "substances." But unless they are actually existing, they are merely universal concepts of essences in the mind. A true substance is not a universal concept, but an existent thing outside the mind and is, therefore, singular rather than universal. There is no such substance as "apple in general." The true substance is this apple, or that apple.

  • The word "substance" is sometimes used to represent a universal quiddity in the mind, as when one says "Man is a substance." (Note: A quiddity is anything that answers the question "What?") To avoid confusion Aristotle calls this quiddity "secondary substance" to distinguish it from the "primary substance" or suppositum that actually exists in the world. 

  • Everything that exists must exist as some kind of being, because nothing exists indeterminately. This means that all substances must have an essence, or something by which they are constituted as the kind of beings that they are. The substance "apple" has the essence of an apple. The substance "man" has the essence of man. When you form a universal concept of a substance in your mind, the concept is the likeness of that substance's essence in your mind.

  • The substance is the subject of accidents, or of its non-essential features and characteristics.


  1. The color of an apple is not an essential feature of an apple, because the color can change and the apple will still be an apple. The size of an apple is also not an essential feature of an apple because the apple can grow in size and still be an apple. Things like color, size, weight, its place on the table, etc. are "accidental features" or "accidents" of an apple because they are not essential to the apple as such.

  2. The substance is the subject in which the accidents exist. The redness of this red apple exists in this red apple, not in this apple's essence. Essences do not have accidents.

  3. Although the substance is what supports the accidents, inasmuch as it is the subject in which the accidents exist, the supporting of accidents is not what formally constitutes a substance. A thing is not a substance because it supports accidents, but because it exists in itself and not in another subject as an object in the world. This means that a substance can exist without any accidents and still be a true substance. 

  • The substance is also the subject of existence and action.


  1. Every man has a human nature, but when a man sings, it is not his human nature that sings, but the man. The substance man is the one who sings and is the true subject of the act of singing.

  2. Existence is also an act, and is sometimes called the "act of existing." Existence cannot be received without exercising it, because existence is an exercised act. When a man exists, it is the man who exercises the act of existing. Therefore, the substance man, not his essence, is the subject exercising the act of existing.


The essence of a substance is that by which the substance is constituted to be the kind of being that it is. The essence of an apple is that by which an apple is an apple. The essence of a man is that by which a man is a man.

  • Objectively the essence of a substance does not exist in the world as an entity. It only exists in the substance of which it is the essence.  I exist, and I have a human essence. But my essence, or my humanity, does not exist in the world as an entity apart from me. The same is true of the essences of other substances. They exist in substances and are real, but they are not physical objects that you can point to. That is why they are called metaphysical, rather than physical, beings. You can touch and see an apple, but you cannot touch or see its essence. The essence is not perceived by the senses, but is understood or grasped by the mind. After showing me a few examples of what you call a horse, then I understand what you mean by a horse, and I form the likeness of the horse's essence in my mind. In my mind the likeness of the essence is called a concept.

  • Since the essence is that by which a substance is specified to be the kind of being that it is, the essence of a substance consists of everything that enters into its definition. The essence is that by which a substance is defined as a certain kind of being. Man is defined as a rational animal because being a rational animal is the essence of man.

  • Man can be defined in one short sentence: Man is a rational animal. This is called an essential definition because it gives the essence of the thing defined. It is the best kind of definition. But many substances other than man cannot be defined in the same way. This is especially true of material substances because the essences of these things are not evident to us in the same way that our own essence is to us. Their essences are concealed by their matter. Often, material substances are not given an essential definition, but are defined simply in terms of their proper accidents, distinctive features or defining characteristics. For example, the following are distinctive features of Oxygen:

  • a colorless gas

  • odorless

  • reacts easily with other substances

  • has an atomic number 8

  • has an atomic weight 16.

These features or characteristics are used to form a descriptive definition of Oxygen:  "Oxygen is a colorless, odorless, reactive gas that has an atomic number 8 and an atomic weight of 16."  Not knowing the true essence of Oxygen, this definition is as close as we could get toward forming a substitute for an essential definition. The example defines the standard Oxygen as it appears in the Periodic Table. By adding more distinctive properties, you narrow down the extension of the concept to fewer individuals; by reducing the list of distinctive properties, you widen the extension of the concept. Thus, when you remove the atomic weight from the definition, then you include the isotopes. When you remove “colorless gas” from the list, you include liquid Oxygen, which is bluish in color.


The nature of a substance is its essence considered as a defining principle of its operations. My essence, "rational animal," defines me as a certain kind of being. It also defines the kind of actions or operations I can do. Insofar as this essence is the principle by which I perform specifically human acts, such as laughing, thinking, loving, working, etc., this essence is my nature. 

  • The concept, insofar as it is a likeness of a substance in the mind, is universal. Peter, James and John are merely the individuals represented by the universal concept 'man.' But the actual essence that exists in an individual substance, by which it is specified to be and to act as a certain kind of being, is always an individual essence, and is equivalent to its nature. Therefore, every nature is an individual nature, and every substance that exists has an individual nature.

  • When the Second Person of the Blessed Trinity, the Son of God, assumed human nature, He did not take human nature in general, but an individual human nature.



Subsistence is another principle in a substance by which the substance is constituted as a subject of accidents, existence and operations. For a substance to exist, it is not enough that it has an individual nature. It requires a co-principle added to individual nature by which the substance is constituted as an agent or doer of its acts, particularly the act of existing, and by which the acts performed by the substance belong to it alone and to no other.

  • Without subsistence, by which the substance becomes a subject or doer of actions, the substance with its particular nature cannot receive existence, because existence can only be received by exercising it. Subsistence may therefore be regarded as something that perfects nature and by which a substance is constituted as a subject of existence and operations.

  • Therefore, individual nature + subsistence + existence = substance. Note that in reality the constituents of substance are not given successively but simultaneously.  Nature does not exist first, to which subsistence and existence are afterwards added. No, because it is the substance that exists. Nature and subsistence are merely principles by which the substance exists; so, they both exist when the substance exists. But, although the existence of nature and subsistence are simultaneous with substance in the order of time, it is also true that nature and subsistence have priority over substance in the order of nature.

  • In the mystery of the Incarnation there was no human subsistence. For, subsistence was only necessary to produce a subject or doer of actions. But in the case of the Incarnation there was already a subject, the Second Person himself. The Son of God merely took an individual human nature, and did not need another subject to be the doer of human acts. That is why there was only one substance, suppositum or Person in Christ, the Son of God.

  • Christ is only one substance (suppositum) or Person with two natures - human and divine. By his divine nature Christ is a doer of divine acts.  By his human nature, he was also a doer of human acts.  His substance (or Person) is both human and divine. It is human by the human nature that He has, and divine by the divine nature that He is. Christ is true God and true man.



Existence is that by which a thing is placed outside of nothingness. It is that by which a thing is rendered actual, rather than just a possible being. The Latin word for existence is esse ("to be"). Existence is a thing's act of existing. Existence or esse is the "to be" of things.

  • Every substance that exists must have both essence and existence. For, essence is that by which a substance is specified as a certain kind of being. But without existence, this essence will just be a possible, but not an actual being. Existence is the act by which a being is rendered an actual being (or is placed outside of nothingness). Existence is the act by which a substance exists, but essence is the principle by which it is constituted to exist in a certain way, as a bird, as a man, or something else. Therefore, essence is what limits the existence of a substance to a certain form of being.

  • In God there is no distinction between essence and existence because God's essence is to be. God's essence is existence itself. This fact is proved in natural theology (a branch of philosophy), but is confirmed by Holy Scripture when God said, "I AM WHO AM" (Ex 3:14). In all creatures there is a distinction between their essence and their existence. And the essence limits created substances to exist in a certain way, that is, as a man, as a bird, etc. In God, where essence is identical to existence, there is nothing that limits the way God exists. He simply is, boundlessly and without limitation. It is because He is pure existence (or the pure act of existing) that His Being is without limit and, therefore, infinite in every respect - in power, in duration (eternal), in knowledge, in wisdom, etc. All of God's attributes (power, wisdom, knowledge, etc.) are identical to His substance, essence, or existence and, therefore, do not differ from each other in reality, but only in our way of thinking. 


An accident of a substance is some feature or characteristic that ordinarily and naturally exists in a substance, but which does not form part of the substance's essence or nature. 

  • Created substances usually have more than their nature. They also have "accidents," which further modify the way they exist in the world. Two birds, say two pigeons, may have the same essence, but they differ from each other because one is darker, bigger, heavier, or flies faster than the other. The individual qualities - color, size, weight, etc., - are called "accidents" because they are "accidental to," and are not part of, the pigeon's essence. A pigeon may grow in size, or have a different color, and still be a pigeon. The accidents are not what make them pigeons, but they make them this pigeon or that pigeon.

  • In man, who is a composite of body and soul, the accidents include, not just his color, size or weight, but also his relations (father, son, spouse) and spiritual qualities such as his knowledge, his wisdom, his virtues, etc. In God, whose existence is already perfect, there can be no accidents that could further improve or determine the way He exists. Therefore, in God there are no accidents. It is permissible to say that God is wise, but wisdom does not exist in Him as an accident, but is identified with Himself or His Substance. Thus God is Subsistent Wisdom itself.

  • In the real world accidents don't naturally exist except in a substance. The color red may exist in a red apple, but redness does not naturally exist in itself apart from a substance. Unlike substances, which exist in themselves, accidents do not normally exist in themselves, but need a subject to inhere or exist in. The substance is the subject in which the accident naturally exists.

  • By God's power, however, accidents can be made to exist supernaturally and extraordinarily without an underlying substance or subject, which is what happens in the Eucharist. When the priest at Mass utters the words of consecration, the substance of the bread is changed into the Body of Christ, and the substance of the wine is changed into the Blood of Christ, without the accidents of bread and wine becoming the accidents of the Body and Blood of Christ. The substances of the bread and wine cease to exist, but their accidents remain. The accidents of the bread and the wine, which naturally cannot exist without a substance, are supernaturally conserved in being by the divine power of Christ in the Eucharist. This is why Christ is really present wherever the consecrated species of bread or wine exist, for it is He who sustains them in being without them becoming accidents of His substance.

  • Accidents are similar to essence in that they both exist in a substance. But, unlike essences, accidents do not constitute a substance's essence or nature. For this reason accidents are not included in the essential definition of a substance. However, proper accidents (or accidents that arise from a substance's essence) are distinctive features that can be used to formulate a descriptive definition of a material substance. 


A person is an intelligent substance which, because it is intelligent, is responsible for its act. Not all substances are persons, but only substances with an intelligent nature. A pigeon has a nature, and acts by that nature, but it is not a person. In contrast, man does not merely have a nature, but an intelligent nature by which he understands and be responsible for his act. Thus, man is a person.

  • It is necessary or essential for a substance to be of an intelligent nature to be a person, but it is not necessary that the substance be actually able to exercise intellectual functions to be a person. Thus, a man who is impeded by sickness or injury to exercise his mental powers is still a person. Likewise, a human fetus, which is not yet able to perform intellectual operations, is still a person. In the same manner, it may be of the essence of a dog to be a four-legged animal. But it does not cease to be a dog if it loses a leg or two.


A quiddity is anything that answers the question, "What is it?" It may also be called the "whatness" of a thing. It is an old concept but a useful one. The essence of a substance is a quiddity because it answers the question, "What is it?"  However, things that have no essences may still be definable and are called quiddities. For example:

  • Mathematical entities, such as lines, circles, etc., do not have essences because they are not substances. However, they have a quiddity or "whatness" which can be defined. For example, a circle is defined as "a plane figure all of whose points are equidistant from a common point, called the center." What was thus defined is the quiddity of a circle, not the essence of the circle. People who do not have "quiddity" in their vocabulary often use the word "essence" to refer to the circle's quiddity. But that is using the word "essence" loosely. Everything has a quiddity, and every essence is a quiddity; but not every quiddity is an essence.

  • Every substance has an essence because every substance has to exist as a certain kind of being. However, substance in general, that is, as "secondary substance" in the mind, is not an essence; it is a quiddity. As a quiddity substance is defined as that to which it appertains to exist in itself, and not in another as in a subject of inherence.

  • Likewise, an accident is neither a substance nor an essence. It is a quiddity, and is defined as that to which it appertains to exist in another as in its subject of inherence.

  • Imaginary entities and beings of reason, such as a mermaid, a 4D world, the square-root of -1, etc. also have quiddities that can be described or defined. They are what these entities represent in our minds.

"The Thinker"

By Auguste Rodin (1840-1917)

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