What Are Cells, Really?

Here’s a strange thing about biology: you’re made of roughly 30 trillion cells, and each one of them is alive. You can see them under a microscope. You can poke them with needles. You can break them open and study the chemicals inside. But when philosophers ask the simple question “What is a cell?”, nobody has a completely satisfying answer.

Is a cell a tiny machine? Is it more like a chemical factory? Is it a kind of living organism—something as real and individual as you are? Or is it just a blob of special goo that happens to be organized in a complicated way? For about 350 years, scientists and philosophers have been arguing about this, and they still haven’t settled it.

The Strange History of a Word

When the scientist Robert Hooke first saw cells in 1665, he was looking at a thin slice of cork through a homemade microscope. What he saw looked like a honeycomb—row after row of little empty boxes. He called them “cells” because they reminded him of the small rooms (cellae in Latin) where monks slept in monasteries.

The problem is that Hooke was looking at dead plant cells. The living parts had dried up and disappeared. All he saw were the walls. So right from the start, the word “cell” pointed to the wrong thing. It suggested that cells were containers, when actually the important stuff—the living part—is what’s inside.

For the next two centuries, scientists argued about whether cells were even the basic units of life. Some thought the real substance was something else entirely: a gooey material they called protoplasm. These “protoplasm theorists” said that calling a living unit a “cell” was like calling a honeybee living in a honeycomb a “cell”—you’d be naming the bee after its house. The living stuff, they insisted, was what mattered. The cell wall was just packaging.

But other scientists pushed back. They argued that cells really were the fundamental units, and that protoplasm was just one part of a cell. A compromise eventually emerged: a cell is “a lump of protoplasm inside of which lies a nucleus.” This definition stuck, but it didn’t really settle the deeper question.

Three Metaphors for What a Cell Is

Philosophers have noticed that scientists never just describe cells neutrally. They always use metaphors—comparing cells to things we already understand. And the metaphor you choose changes how you think about cells, what questions you ask, and what experiments you design.

The Factory Metaphor

For much of the 20th century, the dominant way of thinking about cells was as factories or chemical laboratories. On this view, the cell is a building that contains different rooms (organelles), each with specialized machinery. The mitochondria are the power plant. The endoplasmic reticulum is the assembly line. The Golgi apparatus is the shipping department. The nucleus is the management office where instructions are stored.

This metaphor has been incredibly useful. It led scientists to ask: What chemical reactions happen in each “room”? What enzymes (the workers) carry out those reactions? How do raw materials get converted into products?

But the factory metaphor has limitations. Factories are designed by intelligent engineers. Cells weren’t—they evolved. Factories have a clear purpose (making things for sale). Cells don’t have a purpose in that sense; they just are. And factories don’t build themselves. Cells do.

The City Metaphor

More recently, cell biologists started noticing something that the factory metaphor missed. Cells aren’t static rooms with machinery bolted down. They’re full of movement. Proteins walk along molecular highways. Vesicles bud off from one organelle and travel to another. The whole cell is constantly rearranging itself.

This led to a new metaphor: the cell as a city with bustling traffic. Instead of assembly lines, you have roads and delivery trucks. Instead of fixed departments, you have neighborhoods with shifting populations. The “trucks” are molecular motors called kinesins and dyneins that carry cargo along microtubules (the roads). Things are constantly coming and going.

The city metaphor captures something real that the factory metaphor missed. But it also raises questions: Who directs the traffic? How does the city maintain itself? Who fixes the potholes?

The Organism Metaphor

A third tradition has always seen cells differently: as tiny organisms in their own right. Some single-celled creatures like amoebas are obviously organisms—they move, eat, reproduce, and die. But the cells in your body are also, in a sense, alive. They eat, they breathe, they make decisions (like whether to divide or die), and they communicate with each other.

This metaphor raises a deep puzzle: If each of your cells is a living organism, then what are you? Are you a society of trillions of tiny beings? Some philosophers have taken this idea seriously. They’ve compared multicellular organisms to cities or nations, where cells are the citizens. On this view, your cells are like you—they can sacrifice themselves for the good of the whole (this actually happens: cells sometimes commit programmed suicide to protect the rest of you).

But if cells are organisms, do they have their own interests? Can they “want” things? And what happens when cells stop cooperating—when they become cancer?

The Deepest Puzzle: What Makes a Cell Alive?

This gets complicated, but here’s what it’s about. For a long time, many scientists thought that to understand a cell, you just needed to take it apart, identify all the chemicals, and figure out what each one does. This is called the reductionist approach: understand the whole by understanding its parts.

But some biologists and philosophers pushed back. They pointed out something obvious that’s easy to miss: if you take a cell and grind it up into a chemical soup, you don’t get a living thing. You get a dead puddle of chemicals. The organization matters enormously. The parts have to be arranged in exactly the right way, in the right places, at the right times.

This is the holist position: the whole is not just the sum of its parts. The organization of the parts is what makes the difference between life and death.

This debate—reductionism vs. holism—goes back centuries. In the 19th century, some scientists (called vitalists) even argued that living things must contain some special non-physical “life force” that couldn’t be found in non-living matter. Most modern scientists reject this, but the puzzle remains: how do you explain the difference between a living cell and a dead one, when they contain exactly the same chemicals?

One promising answer comes from thinking about constraints. In physics, a constraint is something that limits how parts can move, thereby creating new possibilities. For example, the tracks on a train track constrain the train’s movement, but they also enable it to go much faster than it could off-road. In a cell, structures like microtubules constrain where molecular motors can walk, but they also enable those motors to carry cargo to distant parts of the cell.

This perspective shows that organization isn’t just about putting parts in the right places—it’s about creating systems where parts control and constrain each other. A cell is alive because its parts are organized into feedback loops where the outputs of one process become the inputs of another, and the whole system maintains itself, repairs itself, and reproduces itself.

How Do Scientists Actually Study Cells?

Philosophers don’t just ask what cells are. They also ask how scientists come to know about cells—and whether they can really trust what they see.

Here’s the problem. To see the inside of a cell, you need a microscope. But to prepare a cell for a microscope, you have to kill it, slice it incredibly thin, soak it in chemicals to preserve it, and stain it with heavy metals so that different parts show up. By the time you’re looking at it, the cell is dead, dried out, and thoroughly mangled.

How do you know that what you’re seeing is real and not just an artifact of your preparation method?

This is not an idle question. It has happened many times that scientists thought they’d discovered a new structure inside cells, only to find out later that it was just a blob created by the chemicals they used. In the 1950s, some of the very best cell biologists—including a future Nobel Prize winner—argued that the Golgi apparatus (an organelle now known to be real) was just an artifact of staining. For fifteen years, the leading scientist in the field didn’t allow his lab to even mention it. But eventually, better evidence showed that the Golgi apparatus was real after all—it’s a crucial part of the cell’s shipping system.

The opposite happened with something called the mesosome. In the 1950s, electron microscopists saw folded structures inside bacteria and named them mesosomes. For years, scientists thought these structures did important jobs like helping bacteria divide. But in the 1970s, better preparation methods showed that mesosomes were artifacts—they were created by the chemicals used to fix the cells for microscopy. The structures didn’t actually exist in living bacteria.

So how do scientists tell real structures from artifacts? Philosophers have argued about this for decades. Some say it’s about robustness—if you can see the same structure using completely different methods (different stains, different microscopes, different preparation techniques), it’s probably real. But other philosophers point out that this isn’t always enough: when a new technique reveals things that no existing technique can see, scientists have to use other criteria, like whether the structure fits into a plausible theory about how the cell works.

Why This Still Matters

You might think: okay, cells are complicated. So what? We’ve been studying them for hundreds of years. Don’t we basically understand them by now?

The answer is: not really. Every decade, cell biologists discover something that forces them to rethink what a cell is. They find new structures, new ways that cells communicate, new ways that cells make decisions. And the philosophical questions—Are cells machines? Are they organisms? What makes them alive?—are still wide open.

Here’s one way to see why it matters. Right now, scientists are trying to build synthetic cells—cells made from scratch in the lab using non-living chemicals. This isn’t science fiction. They’ve already made artificial cell-like structures that can grow and divide (sort of). But nobody has yet made a truly living cell from scratch. And one reason is that we still don’t fully understand what makes something alive.

Is life just a very complicated chemical machine? If so, then building a living cell is just an engineering problem—difficult but in principle solvable. But if life requires something else—a special kind of organization, or a certain history, or something we haven’t even thought of—then the problem might be much deeper.

That’s the strange thing about cells. They’re right there, under the microscope. They’re made of the same atoms as everything else. And yet, after 350 years, they still haven’t given up their deepest secret: what it means to be alive.

Appendices

Key Terms

Term	What it does in the debate
Cell	The basic unit of living things—but philosophers disagree about whether it’s a container, a machine, a city, or an organism
Organelle	A specialized structure inside a cell (like the nucleus or mitochondria) that does a particular job
Protoplasm	The living goo inside cells—some scientists thought this, not the cell wall, was the real basis of life
Metaphor	A comparison that shapes how scientists think about cells; different metaphors lead to different research questions
Reductionism	The approach of understanding cells by breaking them into their chemical parts—useful but may miss how organization matters
Holism	The idea that the whole is more than the sum of its parts—you can’t understand cells just by listing their chemicals
Constraint	A limitation that also creates new possibilities; in cells, structures like microtubules both restrict and enable movement
Artifact	Something that appears in a microscope image but is actually created by the preparation method, not present in the living cell
Robustness	A way of testing whether what you see is real—if different methods give the same result, it’s probably real

Key People

Robert Hooke (1635–1703): The scientist who first saw cells under a microscope and named them after monks’ rooms—starting 350 years of confusion.
Theodor Schwann (1810–1882): One of the founders of cell theory, who argued that all living things are made of cells and that cells are where metabolism happens.
Rudolf Virchow (1821–1902): A pathologist who famously said “all cells come from cells”—rejecting the idea that cells could form spontaneously.
George Palade (1912–2008): A pioneering cell biologist who helped develop methods for studying cells with electron microscopes; he initially thought the Golgi apparatus was an artifact.

Things to Think About

If each of your 30 trillion cells is technically alive, is your body a single living thing or a society of living things? Where’s the line?
When scientists use a metaphor like “the cell is a factory,” does that help them discover real things or does it risk making them see only what fits the metaphor?
The mesosome was believed to be real for 20 years by good scientists before it was shown to be an artifact. We now know the Golgi apparatus is real—but it too was once dismissed as an artifact. How do you decide when to trust new microscope images?
If you could build a synthetic cell that eats, grows, and reproduces, would it be alive? What if it was made entirely of non-living parts? Would that change your answer?

Where This Shows Up

Medicine: Cancer is what happens when cells stop cooperating. Understanding what a cell is—and what makes it behave—is crucial to treating diseases.
Artificial life: Scientists trying to create synthetic cells are wrestling with exactly the same philosophical questions about what life is.
Your own body: Every time you get sick, heal a cut, or grow taller, millions of cells are making decisions. Understanding how they “decide” things (like when to divide or die) could change medicine.
Everyday arguments: The debate about whether the whole is more than the sum of its parts—the holism vs. reductionism debate—shows up in arguments about everything from education to politics to designing better robots.