Everything we see is in motion. A seed becomes a tree, a student becomes a scholar, hot water cools, and empires crumble. This simple, inescapable observation of change presented early thinkers with a profound puzzle. If a green leaf withers and turns brown, is it still the same leaf? When the boy Socrates ages into the old philosopher, what is the continuous thread that lets us say he is the same person?
The Aristotelian Inheritance
Before Aristotle systematized the problem, thinkers had wrestled with this dizzying reality. Some saw only the river’s constant flow, concluding that stability was an illusion. Others, staring into the abyss of perpetual change, insisted that true being must be static and absolute, leaving the world of our senses a grand, unreliable fiction. Aristotle’s solution was less radical but far more architecturally robust. He did not dismiss change; he sought to explain it by anchoring it to something that persists through it. Something, he argued, must endure.
This enduring thing he called ousia—a term later chiseled into Latin as substantia, or substance. For Aristotle, a substance is the fundamental unit of reality, the primary subject of which all else is said. Socrates’s location, his mood, his pallor, his knowledge of geometry—all these are properties, or "accidents." They cling to him. They can alter or disappear entirely without Socrates himself ceasing to be. The substance, Socrates, is the stable entity upon which the chaotic drama of change unfolds. It was a powerfully intuitive idea (or at least, the most durable cognitive tool for organizing reality), and it would ripple through Western thought for the next two millennia.
The Dual Identity of Ousia: Matter's Canvas and Form's Imprint
But what is this bedrock? Here, Aristotle’s legacy becomes more complex, bequeathing to the medievals a concept with a crucial internal tension. On one hand, substance is the ultimate substratum—the underlying stuff that remains when all properties are stripped away. Imagine a bronze statue. You can change its colour, its temperature, its location. If you could somehow peel away every one of its qualities, you would be left with the pure, unqualified bronze-ness that underpins it all. This is substance as the ultimate subject, a kind of metaphysical canvas that is, in itself, almost entirely unknowable.
Yet this was not the whole story, nor the part that Aristotle ultimately prioritized. For him, in his most considered moments, true substance was less the inert stuff and more the active, defining principle—the form or essence (the eidos)—that sculpts that stuff into a recognizable, functioning individual. What makes Socrates a substance is not just the particular flesh and bone that constitutes his body, but the organizing principle of "humanness," his soul, that makes him what he is. It is this form that makes him a human and not a horse, a rational animal and not a stone. The true being of a thing was not its raw material but its intelligible structure.
Substance, then, arrived in the medieval world with a dual identity. It was both the fundamental subject of predication (the substratum) and the essential definition of a thing (the form). A single, individual thing—this horse, this man—was a composite of both. This very duality, this tension between substance as the ultimate subject and substance as the defining essence, became the fertile, and often fraught, ground upon which the great scholastic cathedrals of thought would be built.
When Categories Met Theology
The Arabic manuscripts arrived in Toledo during the twelfth century, carrying Aristotle's Metaphysics back into a Latin world that had forgotten it. Translators hunched over texts, rendering Arabic glosses on Greek arguments into serviceable Latin. The words emerged strange, technical, bristling with unfamiliar distinctions.
Christian theologians discovered a problem immediately.
God, according to Aristotle, counted as a substance. The highest substance, certainly—the unmoved mover, pure actuality without potentiality, form without matter. But still: a substance, one being among others, however exalted. The Christian God, by contrast, didn't simply exceed other substances in degree. He differed in kind, the creator standing outside the created order entirely. Placing God within Aristotelian categories felt like caging the infinite.
Aquinas threaded this needle with characteristic precision. God, he argued, transcends the category of substance while remaining its source. Created substances possess essence and existence as separate principles—a horse has horse-nature (essentia), and it also happens to exist (esse). God alone fuses these absolutely. His essence is existence itself (ipsum esse subsistens). Every other substance receives existence from outside; God simply is being, underived and necessary.
This move preserved Aristotle's categories for the created realm while exempting the divine. But it required splitting substance into analogical tiers. Substances don't all "be" in the same way. Angels lack matter entirely, pure intellects subsisting without bodies. Humans compound form and matter, souls wedded to flesh. Animals and plants possess vegetative and sensitive souls but no rational principle. The category "substance" now stretched across an ontological spectrum, unified only by family resemblance rather than univocal definition.
Meanwhile, other theological puzzles pressed harder. The Eucharist became a testing ground for medieval theories of substance. The bread looks like bread, tastes like bread, nourishes like bread. Every sensory quality screams "bread." Yet Catholic doctrine insisted the substance transforms into Christ's body while the accidents remain. How?
Duns Scotus proposed that accidents can exist without inhering in any substance, suspended miraculously by divine power. Ockham went further: perhaps we don't need the substance-accident distinction at all. Why multiply entities? The appearances themselves suffice. What we call "substance" might simply be a stable pattern of qualities, a convenient fiction rather than metaphysical bedrock.
(The Church, predictably, found this nominalist gambit threatening.)
The Fracturing of Consensus
By the fourteenth century, Aristotle's tidy framework had splintered into competing schools.
Realists insisted universals existed independently—not as Platonic forms in some celestial realm (See 'Plato'), but as real commonalities instantiated in particulars. "Horseness" wasn't merely a mental abstraction. It named something genuinely shared by all horses, a universal nature present in each individual. Substance, for realists, anchored these universals in concrete particulars while allowing the universals themselves metaphysical weight.
Nominalists denied this multiplication of entities. Only individuals exist. "Horse" is just a word (nomen), a label we apply to similar-looking individuals for convenience. Strip away our linguistic habits, and you find only Flicka, Secretariat, this gray mare, that black stallion. No universal horseness unites them except our decision to group them together. Substance collapses into the particular, and the particular needs no further metaphysical scaffolding.
Ockham's razor sliced through the realist ontology. Don't posit universals when particulars suffice. Don't assume substance underlies accidents when the accidents themselves explain our experience. The nominalist program emptied the medieval cosmos of its invisible furniture, leaving only the observable and the necessary.
Yet even as philosophers debated substance's nature, they relied on its vocabulary. Legal theorists asked whether corporations possessed substance beyond their individual members. Medical writers traced how substantial forms determined bodily health. Alchemists sought the prime matter underlying metallic transformations, hoping to transmute lead into gold by manipulating substantial principles.
The concept had escaped pure metaphysics and colonized adjacent disciplines. Substance became the lingua franca for discussing identity, persistence, and underlying nature across domains. When a peasant asked what made bread bread, or when a merchant wondered whether his scales measured true quantity, they trafficked—knowingly or not—in Aristotelian categories refined through centuries of scholastic disputation.
By the Renaissance, new empirical methods would challenge substance-based explanations. Mechanical philosophers would reduce qualities to quantities, forms to arrangements of particles. But the medieval conversation had already demonstrated what Aristotle couldn't have foreseen: that substance wasn't a fixed concept discovered in nature, but a flexible tool reshaped by each generation's questions.
The scholastics transformed Aristotle's categories not by abandoning them, but by stressing them past their original design parameters. They bent substance around theological paradoxes, stretched it across ontological hierarchies, tested it against empirical puzzles. What emerged wasn't Aristotle's substance anymore—it was something far stranger, far more supple, marked by the specific pressures of medieval thought.
The matter beneath form. The subject beneath predicates. The reality beneath appearances. Each age finds different weight in these prepositions, different emphasis in the "sub-" that gives substance its name. Medieval philosophers heard in that prefix both a promise and a problem: that something stands beneath, stable and real—but that we might never fully grasp what.
For more insight:
- Adamson, Peter. Medieval Philosophy. Oxford University Press, 2019.
- Feser, Edward. Scholastic Metaphysics: A Contemporary Introduction. Editiones Scholasticae, 2014.
- Gilson, Étienne. History of Christian Philosophy in the Middle Ages. Random House, 1955.
- Marenbon, John. Medieval Philosophy: An Historical and Philosophical Introduction. Routledge, 2007.
- Marenbon, John, editor. The Oxford Handbook of Medieval Philosophy. Oxford University Press, 2012.
- Pasnau, Robert, editor. The Cambridge History of Medieval Philosophy. Cambridge University Press, 2010.
- Pasnau, Robert. Metaphysical Themes 1274–1671. Oxford University Press, 2011.