What Are Categories? A Puzzle About the Kinds of Things

The Puzzle of Too Many Lists

Imagine someone asked you: “What are the most basic kinds of things in the universe?” You might start listing. Animals, plants, rocks, people. But those are pretty specific. What about the really big buckets? Maybe you’d say: physical things, like tables and trees. Mental things, like thoughts and feelings. Abstract things, like numbers and shapes. Those seem like different kinds of things entirely.

Now imagine a philosopher asks: “Give me the absolute most basic kinds. The categories that everything else falls into. No more than ten.”

Tricky, right?

Now imagine three different philosophers each gives you a different list of ten. Then someone says there should be twelve. Someone else says only four. Someone else says there’s really just one basic kind, and everything else is a variation of it.

Who’s right? How would you even decide?

That’s the puzzle this article is about. It’s about what philosophers call categories — the most general kinds of things that exist, or that we can think about.

Where the Idea Came From: Aristotle’s Ten

The first person to try making a serious list of categories was Aristotle, a Greek philosopher who lived around 350 BCE. He came up with ten:

Substance — a man, a horse, a tree
Quantity — four feet long, five pounds
Quality — white, grammatical, brave
Relation — double, half, larger than
Place — in the marketplace, at school
Date — yesterday, last year
Posture — sitting, lying down
State — wearing shoes, having armor on
Action — cutting, burning
Passion — being cut, being burned

Some of these probably look a bit weird to you. “Posture”? “State” — like, wearing shoes? Really? But Aristotle had a reason for including them. He noticed that when you ask different questions about something — “What is it?” versus “How much?” versus “Where?” — you get very different kinds of answers. And those different kinds of answers, he thought, pointed to genuinely different kinds of things in the world.

Most important for Aristotle were substances — individual things like a particular person or a particular horse. Everything else, he thought, depends on substances. You can’t have a “whiteness” floating around without something white. You can’t have a “cutting” without something doing the cutting. Substances are the foundation.

This seems pretty reasonable, but it raises a question: how did Aristotle know there were exactly ten categories, and not nine or eleven? He never really proved it. And different places in his writings, he gives slightly different lists. That’s the first hint that this is harder than it looks.

The Kantian Turn: Maybe Categories Are in Our Minds

Fast forward about 2,100 years. A German philosopher named Immanuel Kant had a different idea. Maybe, Kant thought, we can never really know what the “ultimate categories of reality” are — because we’re stuck inside our own minds. We see the world through a kind of mental filter, and the categories we use might be features of how we think, not of how the world is in itself.

Kant’s approach was clever. He looked at the different kinds of judgments we can make — ways of putting thoughts together. He noticed that every judgment has a quantity (all swans are white, some swans are white, this swan is white), a quality (positive, negative, limited), a relation (categorical, hypothetical, disjunctive), and a modality (possible, actual, necessary). From these, he derived twelve categories:

Quantity: Unity, Plurality, Totality
Quality: Reality, Negation, Limitation
Relation: Substance-and-Accident, Cause-and-Effect, Community (reciprocity)
Modality: Possibility, Existence, Necessity

Kant wasn’t trying to list the kinds of things that exist independently of us. He was listing the concepts that our minds must use to make sense of anything at all. According to him, we can’t help but see the world in terms of substances, causes, and so on — because that’s how our minds are built.

This is a very different project from Aristotle’s. Aristotle wanted to describe the world. Kant wanted to describe the mind that does the describing. Both called what they were doing “categories,” but they meant different things.

A Different Route: Categories Through Nonsense

Here’s another approach that might seem strange at first. Instead of trying to guess what the most basic kinds of things are, what if we looked at what happens when you mix up the wrong kinds?

Consider a sentence like: “The number seven is green.” Almost everyone agrees: that’s odd. Numbers aren’t the kind of thing that can have colors. It’s not just false — it seems like a mistake of category. You’re treating a number as if it were a physical object.

Or consider: “Thursday is in bed with a cold.” Again, weird. Days of the week don’t have bodies or get sick.

The philosopher Gilbert Ryle made this idea famous (though he got it from an earlier philosopher named Edmund Husserl). Ryle argued that when you get nonsense like this, it’s a clue that you’re dealing with two different categories. The test is simple: take two expressions, and see if there’s a sentence-frame where you can substitute one for the other and get nonsense in one case but sense in the other. If so, they’re in different categories.

This test has problems, though. Another philosopher, J. J. C. Smart, pointed out that you could use it to claim that “chair” and “bed” are in different categories — because “the seat of the ___ is hard” works for “chair” but seems odd for “bed.” But chairs and beds are pretty similar kinds of things. So the test seems too strict, or maybe the idea of “nonsense” is too vague.

Identity Conditions: A More Precise Method

Here’s a different way to think about categories that many philosophers now prefer. It starts from a simple observation: for any kind of thing, there are rules about what counts as the same thing again.

Think about a cat. If you see a cat on Monday and the same cat on Tuesday, that’s one cat. But “the same cat” doesn’t mean “the same collection of atoms” — cats lose and gain atoms all the time. It means the same organism, the same individual cat.

Now think about a puddle. If you see a puddle on Monday and what looks like the same puddle on Tuesday, is it the same puddle? Not really — puddles don’t have clear identity conditions. It’s more like “there’s a puddle” than “this particular puddle persists.”

Philosophers call these identity conditions — the rules that tell you when something counts as the same thing over time. And they’re different for different kinds of things. The identity conditions for a person are different from the identity conditions for a rock, which are different from the identity conditions for a song, which are different from the identity conditions for a number.

The philosopher Michael Dummett and others have suggested that categories can be distinguished by their identity conditions. If two kinds of things have different identity conditions — different rules for what counts as “the same one” — then they belong to different categories.

This approach has some nice features. It doesn’t claim to give you one single list of all categories — because people can invent new kinds of things with new identity conditions. (Think about software, or fictional characters.) It guarantees that categories are mutually exclusive — one thing can’t have two different sets of identity conditions. And it helps explain certain philosophical confusions: when someone tries to identify persons with their bodies, they might be making a category mistake, because persons and bodies have different identity conditions.

The Skeptical Worry: Is There Really One Right Answer?

So far, we’ve seen different philosophers give very different lists of categories. Aristotle gave ten. Kant gave twelve. Others have given four, or eight, or one. Husserl thought there were two different dimensions of categories — formal and material — and maybe also existential categories (ways of existing). Some philosophers today think there might not be a single correct answer at all.

The philosopher Jan Westerhoff compares categories to mathematical axioms. There isn’t one “right” set of axioms for geometry — you can use Euclid’s or non-Euclidean systems depending on what you’re doing. Similarly, maybe there isn’t one right set of categories. Different category systems might be useful for different purposes. What matters is not that a category system is “true,” but that it’s helpful for organizing our thinking.

This might sound like a cop-out. But think about it: when you organize your music, you might sort by artist, by genre, by mood, by year, or by how much you like each song. None of these is the correct way. Each highlights different things. Maybe categories are like that — useful tools rather than discoveries about the deep structure of reality.

Why Categories Still Matter

Even if there’s no single right answer, categories are still important — maybe more important than you’d think.

First, categories help us avoid confusion. When people argue about whether the mind is the same thing as the brain, or whether numbers “really exist,” or whether you can have two things in the same place at the same time, those arguments often turn on category questions. Are minds and brains the same kind of thing? What kind of thing is a number? Can two objects of different categories share the same location?

Second, categories show up in how we actually talk and think. Linguists and cognitive scientists study how people naturally divide things into categories. Do children think about objects and events differently? Do speakers of different languages organize the world the same way? These are empirical questions, but they connect directly to the philosophical ones.

Third, categories matter for organizing information. When computer scientists build databases or artificial intelligence systems, they have to decide what kinds of things exist in their system. A medical database needs categories like “patient,” “symptom,” “disease,” “treatment.” A geography database needs categories like “river,” “mountain,” “city.” These “ontologies” — systems of categories — are built on philosophical ideas, even if the people building them don’t always realize it.

The Open Questions

Here are some questions that philosophers still argue about:

Are categories discovered or invented? Do we find the structure of reality, or do we impose it?
Could there be categories we can’t even think about, because our minds are limited?
When two philosophers disagree about categories, are they arguing about the world, or are they just using words differently?
Is there a best way to divide things into categories, or are there many equally good ways?

Nobody has settled answers to these questions. But that’s part of what makes categories interesting. They force us to think about what it means for two things to be “the same kind” — and whether that sameness is in the things themselves, or in how we see them.

Appendix

Key Terms

Term	What it does in this debate
Category	The most general kind that something can belong to; philosophers disagree about what these are and whether there’s one correct list
Substance	For Aristotle, the most basic kind of thing — an individual like a person or a horse, on which everything else depends
Identity conditions	The rules that tell you when something counts as “the same thing” again; different categories have different identity conditions
Category mistake	Putting something in the wrong category, leading to nonsense or confusion (like treating a number as if it could be green)
Descriptivist approach	Treating categories as describing our concepts or language rather than the world itself
Realist approach	Treating categories as real divisions in the world, independent of how we think or talk

Key People

Aristotle (384–322 BCE) — Greek philosopher who made the first famous list of ten categories, starting a debate that’s still going
Immanuel Kant (1724–1804) — German philosopher who argued that categories come from the structure of the human mind, not from the world itself
Gilbert Ryle (1900–1976) — British philosopher who argued that many philosophical problems come from “category mistakes,” like treating mental states as if they were physical things
Edmund Husserl (1859–1938) — Philosopher who developed a test for category differences based on what happens when you substitute words in sentences and get nonsense
Michael Dummett (1925–2011) — British philosopher who argued that categories can be distinguished by their identity conditions, making the idea more precise

Things to Think About

If you had to make your own list of the ten most basic kinds of things in the universe, what would you put on it? What would you leave out? How would you defend your choices?
Think about something like a song. What category would you put it in? Is it a physical thing (sound waves, a recording)? A mental thing (what you hear in your head)? An abstract thing (like a pattern or structure)? Could it be in more than one category at once?
Suppose someone says: “Numbers don’t really exist — they’re just useful fictions.” How would category thinking help you evaluate this claim? What would it mean to say numbers are a different kind of thing from rocks?
If categories are just tools for organizing our thinking (as some philosophers suggest), does that mean we can make up any categories we want? Are there limits? Could someone claim that “happy” and “Tuesday” belong to the same category?

Where This Shows Up

Every time you organize anything — your music library, your school supplies, your thoughts — you’re using categories. The question is whether some ways of categorizing are better than others.
In computer science and information science, “ontologies” (systems of categories) are used to organize data so that different systems can share information. A medical ontology needs to decide whether “patient” and “person” are the same category or different ones.
In debates about artificial intelligence, people argue about whether AI systems could have genuine thoughts or feelings — which is a question about whether minds and computers are the same kind of thing.
In arguments about identity — about what makes you the same person over time, whether a statue and the lump of clay it’s made of are one thing or two — these are category questions in disguise.