Types of Substance: From Aristotle to Modern Metaphysics

What is the most basic “stuff” of reality? This deceptively simple question has occupied philosophers for over two thousand years, and the answer depends largely on what kind of substance you’re talking about. In metaphysics, substance is the foundational concept-the bedrock upon which everything else rests. But substance is not a single, uniform idea. Across centuries, philosophers have carved it into different types: primary and secondary, complete and incomplete, simple and composite. Each classification reveals something different about how we understand the building blocks of existence. Let’s walk through these distinctions, starting with Aristotle and moving into modern metaphysical thought.

What does “substance” mean in metaphysics? 🔗

Before diving into the types, it helps to clarify what philosophers mean by “substance.” In everyday language, “substance” is a synonym for “material” or “stuff.” But in philosophy, the term carries a much more precise meaning. It originates from the Greek word ousia, derived from the verb einai (to be). As the Stanford Encyclopedia of Philosophy explains, substances are those entities that best merit the title “beings”-the foundational things from which everything else derives. A substance is not a property like colour or weight, nor is it an event like a thunderstorm. It is the thing that has properties and undergoes events.

The Internet Encyclopedia of Philosophy identifies two core senses of the term. In the first sense, substance refers to object-like entities rather than property-like ones-an elephant is a substance, but the elephant’s colour is not. In the second sense, substance refers to the fundamental building blocks of reality, whatever those turn out to be. These two senses have guided metaphysical debate since Aristotle first laid them out in the Categories.

Primary and secondary substance 🔗

The most influential classification of substance comes from Aristotle’s Categories, where he draws a fundamental line between primary substances and secondary substances. This distinction forms the backbone of Western metaphysics and continues to shape philosophical discussion today.

Primary substance: the individual entity 🔗

For Aristotle, a primary substance is a particular, concrete individual-this specific horse, that particular human being, this individual tree. Primary substances are the things that exist in the most fundamental sense. They are not said of any subject, nor do they exist in any subject. In Aristotle’s framework, everything else-properties, relations, quantities-depends on primary substances for its existence. Without individual entities to bear properties, there would be no properties at all.

As scholars at the Stanford Encyclopedia of Philosophy note, Aristotle treats primary substances as the basic “things that there are” in the world of the Categories. A particular shade of grey cannot exist on its own-it must be the colour of something. But an individual horse simply exists. It does not need to belong to another entity in order to be real. This independence is the hallmark of primary substance.

Secondary substance: species and genera 🔗

Secondary substances, by contrast, are the species and genera to which primary substances belong. If Socrates is a primary substance, then “human being” (his species) and “animal” (his genus) are secondary substances. These are universals-general categories rather than concrete individuals. They tell us what kind of thing a primary substance is.

Aristotle gives secondary substances a rank, too. The species is closer to primary substance than the genus, because it identifies the individual more precisely. Saying Socrates is a “human being” tells you more about what he is than saying he is an “animal.” However, neither secondary substance can exist independently. There cannot be “humanity” floating around without any actual human beings to instantiate it.

Why does this distinction matter? 🔗

The primary-secondary distinction establishes a crucial metaphysical hierarchy. Primary substances are particulars; secondary substances are universals. Both are necessary for a complete understanding of reality. Without primary substances, there would be no individual instances for categories to describe. Without secondary substances, there would be no general framework for understanding what those individuals are. This interplay between the particular and the universal remains one of the central tensions in metaphysics.

It’s worth noting that Aristotle himself revisited this distinction in the Metaphysics, where he shifted emphasis from the concrete individual to its substantial form-the organising principle that makes a thing what it is. This move has puzzled commentators for centuries. In the Categories, the individual horse is the primary substance. In the Metaphysics, the form of the horse seems to take that role. Whether these two positions are compatible remains an open debate in Aristotle scholarship.

Complete and incomplete substance 🔗

A second major classification, developed primarily within the scholastic tradition, distinguishes between complete and incomplete substances. This distinction is less widely known than Aristotle’s primary-secondary division but is deeply important, especially in discussions about the relationship between the soul and the body.

Complete substance: self-sufficient entities 🔗

A complete substance is one that exists on its own and does not need to unite with another principle in order to be a fully realised entity. A table, a dog, a rock-these are complete substances. They possess everything they need to exist as the kinds of things they are. A dog does not require union with some additional metaphysical component to be a dog. It simply is one, fully and completely.

In the thought of Thomas Aquinas, a complete substance is what Aristotle would call a hoc aliquid-a “this something,” a particular individual that is complete in both being and kind. The composite of soul and body in a living human being, for instance, is a complete substance. Neither the soul nor the body alone qualifies as complete in the fullest sense.

Incomplete substance: requiring union with another principle 🔗

An incomplete substance is one that, while real, is not self-sufficient. It needs to unite with another principle to form a complete entity. The paradigmatic example in scholastic philosophy is the human soul. Aquinas held that the rational soul is the substantial form of the human being-it is what makes a body alive and human. But separated from the body (as at death), the soul exists in an unnatural, diminished state.

As scholars studying Aquinas’s metaphysics have noted, he concluded that the intellective soul is an incomplete substantial part of the human being that possesses a degree of independence from the body due to its capacity for intellectual operation. The soul can survive the body’s death, but it remains oriented toward matter. It is, in a real sense, incomplete without the body it was designed to animate. This is why, in Christian theology informed by Aquinas, the final resurrection of the body is not optional-it is the restoration of the soul to its proper, complete state.

The late scholastic philosopher Francisco Suárez pushed this idea further, arguing that both prime matter and the substantial form that shapes it are incomplete or partial substances, each capable of existing independently under certain conditions (such as divine conservation), but neither constituting a full substance on its own. This represented a significant development beyond Aquinas’s stricter position.

The philosophical significance 🔗

The complete-incomplete distinction highlights a key insight: not everything that exists is fully self-sufficient. Some entities are inherently relational-they exist in order to unite with something else. This challenges any metaphysics that treats all substances as isolated, independent atoms of being. It suggests that reality has a layered, interconnected structure where some things find their fullest expression only in union with others.

Simple and composite substance 🔗

The third major classification distinguishes between simple substances (those that have no parts and cannot be divided) and composite substances (those made up of distinct components). This distinction runs through the entire history of metaphysics, from ancient atomism to Leibniz’s monadology.

Simple substance: the indivisible foundation 🔗

A simple substance is one that lacks internal parts or composition. It cannot be broken down further. In Aristotle’s framework, God-the unmoved mover-is a simple substance: pure actuality with no potentiality, no matter, and no composition of any kind. In the scholastic tradition, Aquinas held that angels are immaterial beings, simple in the sense that they lack matter-form composition (though they still have a composition of essence and existence, since only God is absolutely simple).

But the most famous theory of simple substance belongs to Gottfried Wilhelm Leibniz. In his Monadology (1714), Leibniz argued that the ultimate building blocks of reality are monads-simple, indivisible, immaterial substances. As the Encyclopaedia Britannica explains, monads are basic substances that make up the universe but lack spatial extension and are therefore immaterial. Each monad is unique, indestructible, and dynamic, possessing properties that are functions of its perceptions and appetites.

Leibniz’s reasoning was straightforward: composite things exist, so the simpler parts from which they are composed must also exist. If you keep dividing, you must eventually arrive at something absolutely simple-something with no parts at all. These are monads. As the Internet Encyclopedia of Philosophy notes, Leibniz’s universe ultimately contains only God and these non-composite, immaterial, soul-like entities. Everything else-space, time, material objects-is either an aggregate of monads or a well-founded phenomenon arising from monadic activity.

Composite substance: made of distinct parts 🔗

A composite substance consists of two or more distinct principles united into a single entity. The most important example in Aristotelian-Thomistic philosophy is the hylomorphic composite-a substance composed of matter and form. Every material object in our experience is a composite in this sense. A bronze statue is composed of bronze (matter) and a particular shape (form). A human being is composed of a body (matter) and a soul (form).

The key question about composite substances is what accounts for their unity. Why is a human being one thing rather than just a pile of matter with a soul sitting alongside it? Aquinas’s answer is that the soul, as the substantial form, is the single organising principle that gives the body its life, structure, and identity. The composite is not an accidental unity (like a heap of stones) but a per se unity-something that is genuinely, intrinsically one.

Leibniz, by contrast, was deeply sceptical of composite substances. In his later philosophy, he argued that composites are ultimately just aggregates-collections of simple substances that appear unified but lack genuine per se unity. The only true substances are monads. What we perceive as a composite body is really an infinite collection of monads, coordinated by God’s pre-established harmony to behave as though they interact with one another.

Substance across philosophical traditions 🔗

These classifications of substance are not confined to a single tradition. They reflect a much broader philosophical conversation that spans cultures and centuries.

The rationalist tradition 🔗

René Descartes introduced a radically different approach to substance by dividing reality into two fundamentally distinct kinds: res cogitans (thinking substance, or mind) and res extensa (extended substance, or matter). This substance dualism broke sharply with Aristotelian hylomorphism, which saw mind and body as two principles of a single substance. For Descartes, mind and body are each complete substances in their own right-a position that generated the famous mind-body problem that philosophy has grappled with ever since.

Baruch Spinoza went in the opposite direction, arguing that there is only one substance-God or Nature-and that everything we experience is a mode or modification of that single, infinite substance. Leibniz, as we’ve seen, took yet another path, positing an infinite plurality of simple substances (monads) as the true foundation of reality.

Beyond the Western canon 🔗

The concept of substance also appears in classical Indian philosophy. The Nyaya-Vaisheshika school identifies substance (dravya) as one of several fundamental categories of being, listing nine types of substance including earth, water, fire, air, ether, time, space, soul, and mind. Meanwhile, Buddhist philosophy challenged the very idea of substance, arguing that what we take to be enduring things are really just impermanent bundles of properties and processes-a position that has interesting parallels with the bundle theory in Western metaphysics.

Why these distinctions still matter 🔗

These classifications of substance might seem like abstract exercises, but they have real consequences for how we understand the world. The primary-secondary distinction shapes how we think about the relationship between individuals and the kinds they belong to-a question that remains live in debates about natural kinds in the philosophy of science. The complete-incomplete distinction informs discussions about personal identity and the mind-body relationship. And the simple-composite distinction underlies fundamental questions in the philosophy of physics: are the basic entities of reality partless and indivisible, or are they complex structures all the way down?

What this survey reveals is that “substance” is not a single, fixed concept but a family of related ideas that philosophers have developed, challenged, and refined across millennia. The diversity of positions-from Aristotle’s hylomorphic composites to Leibniz’s windowless monads, from Aquinas’s incomplete souls to Spinoza’s single infinite substance-reflects the genuine complexity of asking what the world is ultimately made of.

What do you think? If you had to choose, would you side with Leibniz in thinking that only simple, indivisible substances are truly real, or with Aristotle in holding that composite beings like you and me deserve the title of “substance” in the fullest sense? And does the idea of an “incomplete substance”-something real but not self-sufficient-change how you think about the relationship between mind and body?

References

1. https://plato.stanford.edu/entries/substance/
2. https://iep.utm.edu/substance/
3. https://plato.stanford.edu/entries/aristotle-metaphysics/
4. https://plato.stanford.edu/entries/aquinas/
5. https://www.conimbricenses.org/encyclopedia/suarezs-hylomorphism-and-body-soul-composition/
6. https://www.britannica.com/topic/monad
7. https://iep.utm.edu/leib-met/