Quick: look around the room you're in. You see a chair. A phone. Maybe a coffee mug. Those are things. But you also see the wood grain of the table, the coffee in the mug, the air itself (well, sort of). Those are stuff. Congratulations, your brain just performed a categorization task that philosophers have been arguing about for centuries, and you didn't even break a sweat.
A new commentary in Trends in Cognitive Sciences takes a deep look at this thing/stuff distinction and asks a question that seems obvious until you actually try to answer it: how the heck does your visual system know the difference?
The "One Fish, Two Fish" Problem
On the surface, the difference seems trivially easy. Things are discrete, countable entities with boundaries. One cup. Two cups. Three cups. You can point at a thing and say "that one" and everyone knows what you mean.
Stuff, on the other hand, is continuous material that you can divide without fundamentally changing what it is. Half a glass of water is still water. A single drop of water is still water. You can't really say "one water" the same way you can say "one cup." (Well, you can if you're ordering at a restaurant, but that's cheating because you're really talking about "one glass of water," and the glass is the thing.)
Simple enough, right? A rock is a thing. Water is stuff. We can all go home now.
Except no, we really can't, because this distinction starts falling apart the moment you look at it closely.
When Your Categories Have a Meltdown
Consider a pile of sand. Is that a thing or stuff? Most people would say stuff. But what about one grain of sand? That seems more thing-like. So somewhere between one grain and a pile, your brain made a judgment call. When exactly did sand transition from things to stuff? Was it at ten grains? A hundred? A thousand?
Or take a cloud. Is a cloud a thing? It has boundaries, sort of. You can point at it. But it's also constantly shifting, not really discrete, and made of continuous water vapor that you can't exactly count. It's weirdly both.
Here's the real brain-bender: the same exact material can switch categories based on context. Sugar in a bowl? That's stuff. Sugar cubes in a box? Now they're things. The material is identical. The only difference is how it's been packaged, and that's enough for your perception to completely flip the script.
Is a forest "stuff" made of trees? Or is each tree a discrete "thing" and the forest is just a collection? Honestly, it depends on why you're asking. If you're a park ranger thinking about ecosystem management, the forest might feel like stuff. If you're a logger counting inventory, every tree is very much a thing.
Your Visual System Is Running Two Different Programs
This is where it gets interesting from a neuroscience perspective. Perceiving things requires your visual system to detect boundaries, track objects through space and time, and maintain object identity. That coffee mug on your desk this morning? Your brain needs to know it's the same mug as yesterday, even though you moved it, the lighting changed, and you're looking at it from a different angle. That's a lot of computational work happening behind the scenes.
Perceiving stuff requires completely different operations. Here, your brain is processing texture, material properties, color gradients, and quantity estimation without worrying about individuation. When you look at water in a glass, you're not tracking individual water molecules. You're just registering "water" and maybe estimating "about half full."
These are fundamentally different modes of perception, and yet we do them simultaneously without any apparent effort. Walk onto a beach and you instantly see both individual shells (things) scattered across the sand (stuff), with waves (stuff? things? honestly unclear) rolling in from the ocean (definitely stuff). Your brain handles all of this seamlessly while you're just thinking about whether you put on enough sunscreen.
Why This Matters Beyond Philosophy Class
Understanding the thing/stuff distinction isn't just intellectual navel-gazing. It connects to real questions across multiple fields.
Developmental psychologists have noticed that children learn count nouns ("a dog," "two dogs") differently from mass nouns ("some water," "more sand"). The grammatical distinction maps onto something in how kids naturally parse the world, suggesting the thing/stuff boundary is built into how we develop language.
Linguists point out that different languages handle this split differently. Some languages make the distinction mandatory in their grammar. Others are more flexible. Does this mean speakers of different languages literally perceive the world differently? That's still debated, but it's a real question with real implications.
And for computer vision researchers trying to build AI that can understand scenes the way humans do, the thing/stuff problem is a genuine obstacle. How do you program a system to know when something should be segmented as an object versus processed as background material?
Your brain, it turns out, has been solving a surprisingly sophisticated categorization problem since you first opened your eyes. It just never bothered to tell you how hard the problem actually was.
Reference: Firestone C. (2025). A world of things and stuff. Trends in Cognitive Sciences. doi: 10.1016/j.tics.2025.09.019 | PMID: 41062359
Disclaimer: The image accompanying this article is for illustrative purposes only and does not depict actual experimental results, data, or biological mechanisms.