Aristotle: Substance, Essence, and the Actuality of Being

Aristotle (384-322 B.C.E.) did not simply inherit philosophy from his teacher Plato-he reshaped it. Where Plato looked upward to a world of abstract, eternal Forms, Aristotle turned his attention to the world we actually live in: the concrete, changing, physical world of everyday experience. His metaphysical project sought to answer a deceptively simple question-what does it mean for something to exist? In pursuing that question, he developed a set of concepts-substance, essence, form and matter, potentiality and actuality-that became the scaffolding of Western metaphysics for over two thousand years.

Metaphysics as “first philosophy” 🔗

Aristotle described his own metaphysical inquiry in several ways: as “first philosophy,” as “the study of being qua being,” and as “theology.” According to the Stanford Encyclopedia of Philosophy, he did not use the word “metaphysics” himself. The title was assigned by a first-century editor who compiled his writings and placed them after the works on natural science (ta phusika)-hence “meta-physics,” literally “after the Physics.”

The phrase “first philosophy” does not mean the subject you should study first. It refers to the most fundamental level of inquiry-the study of the deepest principles and causes of everything that exists. Other sciences, such as physics or mathematics, study specific aspects of reality. Physics examines things insofar as they move and change; mathematics studies things insofar as they can be counted and measured. Metaphysics, by contrast, studies beings simply as beings-not as moving things or countable things, but as things that exist at all.

This is what Aristotle means by “being qua being.” The phrase does not point to some mysterious entity called “being qua being.” It describes a method: studying everything that exists, but focusing exclusively on what makes it a being in the first place. What are the fundamental causes, principles, and structures that underlie all of reality? That is the central question of Aristotle’s metaphysics.

The concept of substance (ousia) 🔗

Substance is the cornerstone of Aristotle’s metaphysical system. The Greek word is ousia, and it occupies a privileged position among all the categories of being. Aristotle’s other categories-quantity, quality, relation, place, time, and so on-all depend on substance. You cannot have “redness” floating around on its own; redness must belong to some red thing. That red thing-the individual object-is a substance.

In his early work, the Categories, Aristotle identifies primary substances as particular individual things: this particular horse, this particular human being. Species and genera (like “horse” or “animal”) are classified as secondary substances because they tell us what kind of thing a primary substance is, but they do not exist independently of individual things.

Substance in the Metaphysics: form takes priority 🔗

In the Metaphysics, Aristotle deepens his analysis. He asks: what is the substance of a substance? What makes an individual thing the kind of thing it is? His answer shifts the emphasis from the concrete individual to form (eidos). As discussed in the Stanford Encyclopedia of Philosophy’s entry on substance, Aristotle analyses substance in terms of matter and form, where matter is the material a thing is made of, and form is the organizing principle that makes it what it is.

Consider a marble statue. The marble is the matter-the raw material. The shape and design of the statue represent its form. Neither the marble alone nor the shape alone constitutes the statue; it is the combination of both. But Aristotle insists that form has priority over matter, because it is the form that determines what the thing actually is.

Essence: the “what it was to be” 🔗

Closely related to form is the concept of essence (in Greek, to ti ēn einai-literally “the what it was to be”). Essence is the defining nature of a thing, the set of properties that make it what it is and without which it would not be that kind of thing at all. If you remove a human being’s rationality, for example, you no longer have a human being; rationality is part of the essence of being human.

Substance and essence are related but distinct. Substance addresses the ontological question-what exists and endures? Essence addresses the definitional question-what is this thing? What accounts for its nature? Aristotle ultimately brings the two together: the form of a thing serves both as its substance (in the deeper metaphysical sense) and as its essence (when expressed as a definitional account).

This marks a key departure from Plato. For Plato, the essence of a thing (its Form) exists in a separate, transcendent realm. For Aristotle, essence is not off in another world. It is right here, embedded in the concrete things around us, inseparable from the matter it organizes.

Form and matter: the hylomorphic framework 🔗

Aristotle’s theory of hylomorphism (from the Greek hylē, matter, and morphē, form) holds that every physical substance is a composite of matter and form. This framework allows him to explain both what things are and how they change.

Matter, for Aristotle, is not a specific kind of stuff. It is whatever serves as the underlying material for a given kind of object. The bronze of a statue, the wood of a table, the flesh and bones of a human body-all of these are matter relative to the forms they take on. Form, in turn, is the organizing principle: the arrangement, structure, and defining characteristics that make the thing what it is.

Importantly, form is not just physical shape. For living things, Aristotle identifies the soul (psychē) as the form of the body. The soul is not a ghostly substance trapped inside flesh. It is the functional organization of the living being-the set of capacities (nutrition, perception, thought) that make the organism alive and functioning. As the Internet Encyclopedia of Philosophy explains, for Aristotle, to be a thing in the world is to be at work, to belong to a particular species, to act for an end, and to form material into enduring organized wholes.

Potentiality and actuality: explaining change 🔗

One of Aristotle’s most powerful contributions to metaphysics is his distinction between potentiality (dunamis) and actuality (energeia or entelecheia). This pair of concepts provides his central explanation for how and why things change.

Potentiality refers to the capacity or possibility that a thing has to become something or to do something. An acorn has the potentiality to become an oak tree. A block of marble has the potentiality to become a statue. Actuality is the fulfilment or realization of that potential-the oak tree fully grown, the statue completed.

How potentiality and actuality explain change 🔗

Before Aristotle, Greek philosophers struggled with the problem of change. Parmenides had argued that change is impossible because something cannot come from nothing. Heraclitus had argued that everything is in constant flux. Aristotle’s solution was elegant: change is the process by which something moves from potentiality to actuality. The acorn is not “nothing”-it is a real thing with a real potential. When it grows into an oak, it actualizes what was already latent within it.

Aristotle also distinguishes between two types of potentiality. Passive potentiality is the capacity to be changed by something else (clay can be moulded into a pot). Active potentiality is the capacity to bring about change in something else (a sculptor has the ability to shape the clay). This distinction helps explain the dynamic interactions between things in the natural world.

The priority of actuality 🔗

A crucial point in Aristotle’s system is that actuality is prior to potentiality. He argues for this priority in three ways. First, actuality is prior in definition: we define potentiality by reference to actuality (we call something “buildable” because it can be built). Second, actuality is prior in time in the sense that an actual thing must already exist to produce a potential one (an actual human being produces a child who is a potential adult). Third, actuality is prior in substance because the actual, fully realized state of a thing is more real and complete than its merely potential state.

This priority of actuality over potentiality is connected to Aristotle’s broader commitment to teleology-the idea that natural processes are directed toward ends or goals. The acorn exists for the sake of the oak tree; the potential exists for the sake of its actualization.

The Unmoved Mover: pure actuality 🔗

Aristotle’s metaphysics leads him to a striking theological conclusion. If everything that changes must be changed by something else, and if this chain of causes cannot extend infinitely, then there must be a first cause that initiates all motion and change without itself being moved or changed. Aristotle calls this the Unmoved Mover-an eternal, unchanging substance that stands at the apex of his causal hierarchy.

The Unmoved Mover is pure actuality (actus purus). It has no potentiality whatsoever-no capacity for change, no unrealized possibilities. This is what allows it to be eternal and unchanging while still serving as the ultimate source of all movement in the cosmos. It does not push or shove things into motion like a physical force. Instead, it causes motion as a final cause-as an object of desire and aspiration. All things in the universe are drawn toward the perfection and completeness that the Unmoved Mover represents.

The Unmoved Mover as God 🔗

Aristotle is willing to identify the Unmoved Mover with God, though his conception of the divine is strictly philosophical rather than religious. This God does not intervene in human affairs, answer prayers, or create the world out of nothing. Instead, this God is engaged in eternal self-contemplation-what Aristotle calls “thought thinking itself” (noēsis noēseōs). Because the value of a thought depends on the value of its object, and the Unmoved Mover is the highest being, it must think about the highest thing-which is itself.

This idea had enormous influence on later philosophy and theology. Thomas Aquinas drew heavily on Aristotle’s concept in formulating his famous Five Ways-five arguments for the existence of God. The first of these, the argument from motion, is a direct elaboration of Aristotle’s reasoning about the necessity of an unmoved first mover.

Why Aristotle’s metaphysics still matters 🔗

Aristotle’s metaphysical framework provided the dominant vocabulary for discussing reality, existence, and change throughout the medieval period and well beyond. His concepts of substance and essence shaped centuries of debate in both philosophy and theology. The potentiality-actuality distinction remains a valuable tool for thinking about change, development, and purpose in nature.

Even where modern science and philosophy have moved away from Aristotle-abandoning his cosmology, for example, or questioning his teleological view of nature-his fundamental questions persist. What makes a thing the thing it is? What underlies change? What is the relationship between possibility and reality? These are not antiquated puzzles. They are live questions in contemporary metaphysics, philosophy of mind, and philosophy of science.

Aristotle’s insistence that form, matter, and purpose are all essential to understanding reality offers a rich alternative to reductive approaches that try to explain everything in terms of physical particles and forces alone. Whether or not one accepts his specific answers, the framework he built remains one of the most comprehensive and internally coherent attempts to make sense of what it means for anything to exist.

What do you think? Does Aristotle’s idea that everything strives toward its full actualization still hold up in a world shaped by modern science? And can the concept of an Unmoved Mover-a being of pure actuality-offer a meaningful account of why the universe exists at all?

References

1. https://plato.stanford.edu/entries/aristotle-metaphysics/
2. https://plato.stanford.edu/entries/substance/
3. https://en.wikipedia.org/wiki/Potentiality_and_actuality
4. https://iep.utm.edu/aristotle-motion/
5. https://www.britannica.com/biography/Aristotle/The-unmoved-mover
6. https://credomag.com/article/thomas-aquinas-on-the-unmoved-mover/