Matter and form
Matter and form are closely related to potency and act. Matter is the principle of a physical thing that accounts for its ability to change: matter is the potency of a physical thing. If we asked the question what Fido has that accounts for his ability to change the answer would be, in a word, his matter. Every physical thing has matter and it is this matter that accounts in some way for all changes that take place in the things we sense around us.
Matter never exists alone but only in conjunction with the other intrinsic principle of a thing, called form. Substantial form is the principle that makes a thing to be the type of thing it is. Fido is a dog precisely because he has the substantial form of a dog. A substantial form determines what something essentially is, whether it be a dog, a tree, a human person, or a rock. Each thing is what it is because of its substantial form. So matter answers the question how a thing can change, either substantially change into something else or accidentally change in some way. Form answers the question how a thing continues to be the type of thing that it is. Heraclitus’s insight that things change is accounted for by matter while Parmenides’s insight that things endure is accounted for by form.
We are now able to define a physical thing. A physical thing is simply a thing of a particular type that is capable of changing into something else. In other words, a physical thing is a substance that has both matter and form as intrinsic principles. An immaterial substance is something that is a specific thing but is not able to receive a new substantial form since it lacks matter. An immaterial substance such as an angel has form but not matter. Angels do have potency, however. Only God is pure act without the slightest trace of potency or imperfection (more to follow when we discuss the distinction between esse and essence).
Given the intellectual climate in which we live, we need to be careful to properly understand matter and form. It is easy to fall into a common misunderstanding. Some think of matter as the stuff out of which a thing is made and form merely as the shape of the thing. Sometimes an example is given of matter and form that is partly helpful but also leads to this confusion. A block of marble is identified as matter that a sculptor fashions into the form of a horse. There is a sense in which this example points toward a correct meaning of matter and form but it is also misleading, especially today when it becomes confused with the way the word “matter” is used in modern atomic theory.
For Aristotle and realist metaphysics, matter is never without form. Matter is not simply a block of marble or a group of atoms or any other possible “construction material.” These things have form as well as matter. An atom is not simply speaking “matter.” An atom is a thing that has both matter and form. When a chemist thinks of a physical object as being composed of matter considered as certain atoms he is not thinking about the word “matter” in the same way as a realist. He is rather thinking about the component parts of the thing as they are mechanically arranged (a perfectly valid thing to do). For a realist, matter is the potency of a physical thing to undergo change of different types. Without the thing there is not matter. And the subsisting physical thing necessarily has two intrinsic principles: matter and form.
Without getting too technical, we need to make the distinction between matter and form in general on the one hand and prime matter and substantial form on the other. Every physical thing has matter, which is what accounts for its ability to change. A physical thing, like a piece of firewood, has the ability to change in many ways without ceasing to be a piece of firewood. It can be moved from one place to another, dried out through extended exposure to the air, or change color when bleached by the sun. All of these changes are called accidental changes. An accidental change is a change that involves modifying a thing in some way without changing what it essentially is. If the wood were to burn or rot into the soil it would be undergoing a different type of change called a substantial change. In both of these examples it would cease to be a piece of firewood and rather become something else: ash, carbon dioxide and water vapor in the first example and part of the soil in the second.
While any change is accounted for by a physical thing’s matter, a substantial change is accounted for specifically by what is called prime matter. Prime matter is matter that is receptive to receiving a substantial form. Prime matter is an abstraction and strictly speaking has no real existence. Prime matter is pure potency, pure “can become” and thus has no actuality, no actual existence. Even so, in the matter of a physical thing, there is the capacity to receive not only accidental changes but also certain substantial changes, changes that require receiving a new substantial form. Thus there is prime matter in a physical thing. But this does not change the fact that it is completely impossible for matter to exist without form.
Substance and accidents
In talking about matter and form we have already referred to substance and accidents. Even so, it is helpful to provide a bit more clarity here. A substance is the underlying reality that accounts for a thing existing by itself and not in another thing. Substance comes from the Latin word substare, meaning “to stand under.” A substance stands under and supports the accidents, which are multiple perfections that inhere in and determine a single subject. For example, a tree is a substance. It also has multiple accidents, such as its size, its color, the way its sap smells, its location, the way its leaves are oriented toward the sun and so on. Our five senses are only able to perceive the accidents. We are able to see the brown trunk and the green leaves, smell or taste the sap, feel the rough texture of the bark, and hear the sound of the leaves rustling in the wind. The substance is not directly perceived by our senses but rather grasped by the intellect through a process known as simple apprehension. We sense an object, form a mental image of the object and then abstract from this mental image to know the object as a type of thing. We know the object by a concept. Even so, in knowing a thing we know it as a whole composite of substance and accidents; it is not as if we first sense and then subsequently “figure out” what a thing is and then know it as a concept only.
The accidents can be further classified into different types. Quantity and quality are most directly rooted in the substance and are intrinsic to the thing. The other accidents are extrinsic and involve how the thing exists in the world: in relation to other things, acting upon other things, receiving action from another thing, a thing’s position, orientation, environment, and time. Realists refer to the substance and these nine accidents as the ten categories.
An example can help to clarify. A bear is a substance. He has intrinsic accidents like his black fur and his four legs and paws. These intrinsic accidents fall under the categories of quantity and quality. Intrinsic accidents can be further divided into mere and proper accidents (called properties). Fur color is a mere accident since he would be no less of a bear if some of his fur turned grey with old age. Having a particular paw is the type of accident known as a proper accident, since having this paw is proper to the nature of being a bear. The paw is not a substance in its own right: so long as it is attached to the bear the paw has its existence in another, specifically the bear. If the paw is cut off in a trap and the bear escapes without one of his paws he is still a bear (the paw is now its own thing, specifically a dead and decaying bear paw). But this three-pawed bear is lacking something that is proper to a normal bear and is in some way incomplete without it. Similarly we can say with confidence that skin color or hair color are mere accidents for humans, making no difference whatsoever as to someone being a complete human being. Having two arms is proper to being human but still accidental in that a person with only one arm is still completely a human person, just missing something that normally should be there. The location, position, state of resting or movement, and other such things are extrinsic accidents as applied to both the bear and the human. The human may be running from the bear along the Appalachian Trail in Maine during the evening of May 24. This scenario is full of extrinsic accidents for both bear and person.
The words substance and accidents are also appropriately applied to changes, as we have already started to do in the above section. An accidental change is one that modifies a subject without causing the subject to become something different, a new substance. The man running from the bear is undergoing accidental changes, including a change in location, becoming hot, and sweating. His goal in running from the bear is to avoid undergoing a substantial change, becoming dinner for the bear and ceasing to be a living human person. For living things, birth and dying, coming to be and passing away, are substantial changes. A piece of firewood can change substantially when burned. The ashes that remain are clearly no longer a piece of firewood. This same piece of firewood can undergo many accidental changes as well, changes such as a change in location, temperature or color.
 This position is referred to by the technical names of hylemorphism or hylomorphism.
 It is, however, possible for form to exist without matter. An angel is a form, a subsisting immaterial being.