Three red apples sit on a table. They are clearly different — different sizes, slightly different shades, sitting in different places. And yet you have no trouble seeing that all three are red, all three are apples, and all three are fruit.
What is going on when you do this? Is "redness" something that exists in the world, the way the apples themselves exist? Or is it just a label your mind sticks onto a bundle of slightly different things? When you say two apples share a color, are you claiming that they share an actual thing — or merely noticing a family resemblance?
This deceptively simple question is the problem of universals, and it has occupied serious philosophers from Plato to the present day. The answers people give to it turn out to determine, quietly, what they think about science, language, mathematics, morality, and the nature of reality itself.
The Three Big Positions
The medieval philosophers who fought over this question for several centuries identified three main camps. Each is still defended by serious thinkers today.
Realism says that universals are real things, existing independently of any particular instances of them. Redness, triangularity, and humanity are genuine features of reality that the apples and triangles and humans all participate in. Plato, the original realist, thought universals existed in a separate realm of Forms — the Form of Redness was the perfect, eternal blueprint of which actual red things were imperfect copies. Aristotle modified this: universals are real, but they exist in the things themselves, not in some separate realm.
Nominalism says the opposite. Only particular individual things exist — this apple, that apple, the third apple. "Redness" is just a name (Latin nomen) we attach to a class of similar particulars. There is no extra entity called Redness floating around. The medieval nominalist William of Ockham would put it this way: stop multiplying entities beyond necessity. The shared word does the work; we do not need a shared thing.
Conceptualism is a middle path. Universals are not "out there" in the world the way realists claim, but they are not just names either. They are mental concepts — the mind's way of organizing the similarities it perceives. Locke held a version of this. So, in a different way, did Kant.
Why It Mattered to the Medievals
The dispute was not academic. It shaped some of the most significant theological battles in Western history.
If universals are real (realism), then there is a real thing called humanity that all human beings share. This makes the doctrine of original sin — that all humanity participated in Adam's fall — more philosophically tractable. It also gives traction to the idea that species and natural kinds have stable essences that science can discover.
If only particulars exist (nominalism), then "humanity" is just a label we apply to a collection of individuals. Original sin becomes harder to make sense of as a metaphysical reality. So, the medievals worried, does the Trinity, the Eucharist, and the very notion of natural law.
The historian Richard Weaver argued in his 1948 book Ideas Have Consequences that the slow victory of nominalism in the late medieval and early modern periods — a victory often associated with Ockham himself — was a turning point in Western intellectual history. Once you stop believing in real essences, he argued, every subsequent philosophical movement (empiricism, scientism, postmodernism) becomes thinkable. Whatever you make of Weaver's thesis, the influence of the dispute is hard to overstate.
The Problem Hasn't Gone Away
You might assume that modern science has settled the matter. It has not.
Consider the laws of physics. When we say F = ma, we are claiming that some abstract relationship — call it "Newton's Second Law" — holds across every massive object in the universe. Is the law a thing that exists in nature, governing how particulars behave? That is a realist position. Or is it just a generalization we infer from many particular events? That is a nominalist position. Working physicists are usually too busy to take a side, but their best account of what their equations describe turns on it.
Or take mathematics. The number 7 is not anywhere in space and time. You cannot find it under a microscope. But mathematicians treat it as an object — they prove things about it, discover its properties, find that it behaves the same way for every mathematician everywhere. Are numbers real, abstract entities that mathematicians discover? (Realism, in this domain often called "mathematical Platonism.") Or are they conceptual constructs that humans invent? (Nominalism.) The question splits even contemporary mathematicians.
The same question shows up in arguments about morality. Are values like justice and cruelty real features of the world, picked out by our moral language? Or are they names we attach to behaviors we approve or disapprove? Whether you find moral realism plausible may depend, more than you realize, on what you think about universals in general.
Why the Question Is Worth Asking
Most people never explicitly choose a position on universals, but everyone implicitly has one — and the choice shapes how you think about almost everything else.
If you believe there are real natural kinds, you will trust certain kinds of science (taxonomy, biology, physics) to be tracking real features of reality. If you do not, you will see those sciences as practical organizational schemes whose categories could in principle be drawn up differently.
If you believe morality picks out real properties, you will think there is something to get right about ethics. If you do not, you will think ethics is mostly about coordinating preferences and behaviors.
If you believe abstract objects like numbers are real, you will be more sympathetic to arguments that the universe is mathematically structured at its core, and perhaps to certain arguments for theism. If you do not, you will treat mathematics as a powerful but invented tool.
The problem of universals does not have a single answer that all serious thinkers now accept. But it has the unusual virtue of being a question whose answer matters — quietly, structurally — for almost everything else you believe. Spending an afternoon thinking about whether redness is a real thing or just a useful word turns out to be one of the most consequential afternoons you can spend.
That is the strange beauty of philosophy at its best. The question that looks like it should have a quick answer ends up reshaping the floor you were standing on.
