An Introduction to Scholastic Philosophy

The philosophy of St. Thomas Aquinas was the marvel of the Middle Ages and the crown jewel of Catholic scholarship. Yet, Aquinas’ philosophy was in many ways the culmination of 1,600 years of Greek philosophy. Much to the boon of Christian philosophers at the time, the philosophy of Aristotle was rediscovered during the Middle Ages after having been lost during the downfall of the Roman Empire 1,200 years earlier. The philosophy of Aristotle gave St. Thomas Aquinas and his contemporaries the tools they needed to prove the reasonableness and rationality of the Catholic faith. This unique blend of pagan wisdom (the philosophy of Aristotle) and revealed wisdom (the Scriptures and teachings of the Church) was called Scholastic philosophy. The most famous Scholastic philosophical work was St. Thomas Aquinas’ five-volume treatise Summa Theologica. In the Summa Theologica, Aquinas used the doctrines of the Church as a springboard for exploring the deepest questions of philosophy.

Aristotle’s Definition of the Nature of Being and the Material

Since scholastic philosophy relies on many of the principles of Aristotle’s philosophy, it is necessary to explore Aristotelian principles before delving into scholastic works such as Summa Theologica. Perhaps the most important Aristotelian principle is the idea of being and an understanding of what all things are composed of. In modern thinking the answer to such questions as, "what is the nature of things?" seems simple. We are taught at an early age that all things are composed of atoms, and that atoms are the building blocks of all matter. Modern scientific teaching holds that the nature of a human is strictly material; a man is nothing more than matter (composed of atoms) and energy. Since science can show us that atoms truly exist (we can see them in electron microscopes), and that we are composed of carbon based molecules, water and electrolytes it seems reasonable to conclude that we are strictly a collection of atoms and energy. Aristotle, however, did not buy into this line of reasoning. Although some of his contemporaries believed that the ultimate basis of being is atomistic, Aristotle believed that it was false to say that being is defined only by the sum of its parts. The reason the atomistic theory is fallacious is that the substantial form of a material or being can not be located in any part of the substance (such as atoms, sub-atomic particles, quarks, etc.) rather, parts such as atoms are predicated to an already existing substance. In other words, something must be the basis of material things, and simply subdividing the material into smaller and smaller particles does not allow us define the nature of the material thing.

Therefore if we want to understand what a thing is in the philosophic sense, we must define a thing as a whole before predicating parts and quantity to it. Aristotle believed that when we ask the question "what is a thing composed of?" or "what is the nature of a being or thing?" the answer lies in the substance of that thing. Substance is a philosophic term that is defined as the primary mode of being. All things are composed of a substance. Therefore, the basis of reality lies in substances. Atoms and other particles are real, yet they are only parts of the whole. This is why substance is called the primary mode of being. Now, it is important to realize that substance is not an imaginative concept, it is a rational concept. This means that we can’t picture what a substance is in our imagination; rather we must use rationality and logic to understand it.

In addition to the primary mode of being, substance, all things have secondary modes of beings called accidents. Accidents are those things that allow us to imagine a being or thing. Accidents inhere in a substance and give it physicality. Aristotle defined ten categories of being which allow us to answer the question, "what is a being or thing composed of?"

The Ten Categories of Being

  1. Substance—Substance is the primary mode of being and defines what a thing is. Substance is the foundation of reality and cannot be pictured in the mind without also picturing the accidents that inhere in the substance.
  2. The remaining categories of being are accidents: secondary modes of being.

  3. Quantity—quantity allows us to define the parts in a substance. For example, a tabby cat has two ears, two eyes and a multitude of atoms and genetic material.
  4. Quality—quality is a descriptive term such as, the softness and brown color of a tabby cat.
  5. Relation—relation identifies the relative state between two objects. For example, that tabby cat has the same color fur as the Angolan cat.
  6. Action—the action of the subject is also an accident. Action does not necessarily imply motion or change. For example, "the tabby cat is sitting still" is a valid action accident of the cat.
  7. Passion—In the philosophic sense, passion is defined as change. For example, we can say that the aging process of the tabby cat is a passion.
  8. Location—Location is also an accident. For example, we could say that an accident of the tabby cat is that it is sitting on top of the sofa.
  9. Posture—Posture identifies the spatial orientation of the subject. For example, we can say the tabby cat is sitting with all four feet on the floor and it’s tail is in motion.
  10. Temporality—Temporality is the affectation of time on the subject. For example, the tabby cat is seven years old.
  11. State—State seems to imply change in the subject which allows us to identify it from other subjects. For example, the tabby cat currently has both eyes closed and is falling asleep.

We can clearly see that nine of the ten states of being are accidental. Yet, only the primary mode of being, the substance, defines a being or object. For example, we could say that the length of President Bush’s hair is a quantitative accident. If he were to cut his hair there would be a change to his accidental qualities (the length of his hair), but he would still be President Bush because his substance has not changed. We should also note that accidents do not exist separately from a substance. Rather, they inhere in a substance. The length of someone’s hair does not exist separately from a person. Therefore, no substance can be imagined without its inherent accidents and no accident can exist separately from a substance.

Potentiality and Actuality

The concept of potentiality and actuality is critical to scholastic philosophy. Actuality and potentiality are opposites that describe the state of a substance. Actuality describes the exact form, state, position and the rest of the ten categories of being as they apply to some object. If we use the tabby cat example, we can say that the categories of being such as the cat’s posture, location, qualitative properties, quantitative properties, etc. actualize the cat. In other words, the cat is actual simply because it exists. However, the cat not only has actuality, it also has potentiality. The cat is capable of changing any of its accidents by simply moving, shedding fur, aging or even changing its substance (such as when the cat changes from a living cat into a corpse after death). In this sense, we say that the cat has a potential to change. All material things have both actuality and potentiality simply because of their virtue to change.

The reason the concept of potentiality and actuality is important to scholastic philosophy is because it is the central pillar in Saint Thomas Aquinas’ cosmological proof for the existence of God. Consequently, an understanding of the Aristotelian concepts of potentiality and actuality is necessary for scholastic and Christian philosophers.

Prime Matter and Substantial Form

Now that we understand the difference between a substance and an accident as well as the ten categories of being, we can attempt to further understand what exactly a substance really is. Aristotle believed that a substance is composed of prime matter and form. Prime matter is not the same thing as physical matter (the kind we usually think of as composed of atoms). Instead, prime matter should be thought of in terms of potency and actuality. Prime matter is nothing more than pure potency. It is the substantial underlying reality of all things, and as such, has the potential to become anything. Prime matter (since it is substantial) has no physical appearance, quality or quantity. Prime matter cannot even be thought of separate from form. Form is what allows prime matter to become substance. For instance, all of the elements on the Periodic Table (such as iron, gold, silver, mercury, etc.) can be thought of as comprised of prime matter. The thing that differentiates the elements from each other is their form. Silver and gold are composed of the same prime matter but have different form. Form and prime matter constitute the substance of any object, and the substance of that object is inhered with accidents that give the substance a physical character.

Substantial Changes vs. Accidental Changes

We have already learned that the accidents of any object can change. An accidental change of this nature does not affect the substance of the object. If a cat changes its posture, its accident of location changes, yet it is still a cat. A change of accident will not cause the cat to become anything else.

However, there can be changes to the substance of an object. If an object’s form were to change, then the object’s substance would likewise change. This type of change is called a substantial change. For example, if a cat were to walk off a cliff and fall 60 feet to its death, it would undergo both an accidental change (due to the change of the cat’s posture and location) and a substantial change (caused by the death of the cat). We can no longer call the cat a living animal; instead it has substantially changed into a corpse. A similar thing occurs in chemical reactions. If a substance such as iron were to combine with oxygen, the iron would become iron oxide: rust. The change from iron to rust involves a substantial change in the form of the iron. The oxidation process renders a change in the substance of the iron into something new.


One criticism to Aristotle’s unique view of the world is perhaps the fact that modern science seems to view the world in a substantially different manner. It can be argued that in the example of the cat, the feline DNA comprises the real nature of the cat; not some strange idea such as substance. In other words, the parts determine the whole. However, this view, mechanistic materialism, is fundamentally flawed. If we were to appeal to DNA as the fundamental reality of the cat (as if DNA was a building block for the cat) we would still need to explain the parts of the DNA. Otherwise, how do we explain what these parts are if DNA is the prime building block of the cat? A mechanist materialist would probably then appeal to the atom as the prime building block of DNA and hence the cat. Of course, atoms are composed of sub-atomic particles, which demand explanation. These particles are also made of sub-particles, and so on… Eventually, the materialist must arrive at some particle which is the fundamental reality of the cat; the one thing that makes the cat real. However, even if he were to stop his sub-atomic chain at such a particle (such as pure energy or super-strings) he would still have to explain where the particle derives its "reality". In this event, the materialist would just admit that the energy particle (for arguments sake lets call it a super-string) just is real; it has no explanation for itself and must be a completely simple reality. If he did this he would find himself agreeing with Aristotle. Why? The reason is that the materialist will have agreed that the super-string is a fundamental reality. The super string has mass, quality (such as brightness or texture), quantity and possibly other modes of existence called accidents by Aristotelian thinkers. The materialist must agree that the fundamental particle (the super-string) has accidents, otherwise where does the quantity, quality, position, posture, etc. of the cat come from? If the materialist agrees to the fact that the super-string is a fundamental reality with accidental qualities, then he has just defined an Aristotelian object. His object has substance (a primary mode of being which fundamentally just is) inhered with accidents (secondary modes of being which give the substance discernable characteristics). In the end, the materialist will be forced to admit the reality of substance and accidents over the philosophy of an endless string of mechanistic materialism. We can also simply say that the cat just is a simple substance inhered with accidents that are divisible into parts (DNA, proteins, atoms, quarks, super-strings, etc.) The cat is defined as a whole rather than a sum of parts. The cat is fundamentally composed of primary matter with the form of a cat. The parts (such as cat DNA and its constituent atoms) are also feline since the form of the cat is inherent in every part of the cat. The substance of the cat determines the parts to be what they are. An individual atom separate from the form of the cat is substantially different because the substance of the atom is atomic, but an atom in a cat is only part of the whole: therefore its substance is feline. The bottom line is that a whole must be defined first before parts can be predicated to it.

I will leave the reader with one last example to drive home the difference between the mechanistic materialist view of matter and the Scholastic view of matter. Certainly, most of us have learned from science that an atom is almost entirely composed of empty space; there is very little material in the atom (protons, neutrons and electrons). Rather, the mass of the atom is constituted in the nucleus and the majority of the atom is an empty void with electrical charges (electrons) orbiting the nucleus. A materialist will often point out that a solid object such as a table is nearly entirely composed of empty space due to the spacing of the mass particles in the atoms. The question which remains unresolved is then why does the table seem qualitatively solid (it has color, weight, firmness) if it is composed only of empty space with comparatively tiny subatomic particles? The quality of the table cannot come simply from the quantity of the parts. This suggests that the table is essentially prime matter and form which has the secondary properties of quality and quantity.


Aristotle’s philosophy is latent in the works of the Scholastic philosophers. The idea of substance and accidents form the basis for the Christian metaphysical understanding of the world. St. Thomas Aquinas and his peers use the metaphysics of Aristotle to explore everything from the nature of man to the nature of God. Additionally, Aristotle’s ideas of actuality and potentiality are manifest within the Scholastic understanding of causation and the creation of the Universe. For the Scholastics, it seems that a great debt is owed to Aristotle. It was in Aristotle, that the philosophers first truly learned to seek for wisdom in the intellect rather than the imagination and sought for knowledge in rationality without limiting themselves to the world of sensation.

--S.M. Miranda

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