What Is Stuff Made Of? Medieval Islamic Debates About Atoms, Motion, and the Nature of Reality

Imagine an ant trying to cross a sandal. Before it can get to the other side, it first has to reach the halfway point. But before it can reach the halfway point, it has to reach the halfway point of that distance. And before that, the halfway point of that. There are always more halfway points. So the ant can never actually cross the sandal. And yet ants cross sandals all the time. How is that possible?

This puzzle is more than two thousand years old. It was first posed by the Greek philosopher Zeno, and it bothered people long after him. But in the medieval Islamic world, between roughly 800 and 1300 CE, philosophers and theologians took it especially seriously. They realized that how you solve this puzzle tells you something deep about what the world is actually made of.

Two Competing Visions

In the cities of Baghdad, Cairo, and Cordoba, two very different groups of thinkers were arguing about the nature of reality. One group, called the mutakallimun (theologians), believed that the world was made of tiny indivisible particles—atoms. The other group, called the falasifa (philosophers), followed Aristotle and believed that matter was continuous, like a smooth sheet that could be divided forever in theory, even if you couldn’t actually do it.

Both groups were smart, and both thought the other group was making a terrible mistake.

The Atomists’ Case

The theologians who believed in atoms had a simple but powerful argument. If you look at any physical object—a rock, a cup of water, a human body—you can imagine cutting it in half. Then cut one of the halves in half again. Can you keep doing this forever? The theologians said no. Eventually, you’d have to reach something so small that it couldn’t be divided any further. That’s an atom.

Why couldn’t you keep dividing forever? Because, they said, if you could divide forever, then the object would have an actually infinite number of parts. But an actual infinite is impossible. (Nobody, they thought, has ever actually counted to infinity or seen an infinite number of things exist all at once.) So there must be a stopping point—a smallest possible piece of stuff.

This wasn’t just an abstract claim. It connected to something you can observe. If bodies were made of an infinite number of parts, then a mustard seed and a mountain would both have exactly the same number of parts: infinity. But we know a mountain is bigger than a mustard seed. So the number of parts must be finite, which means the parts themselves must have a smallest size.

The atomists also had to explain something else. If everything is just atoms, why do different things look and feel different? Why is fire hot and ice cold? They said that atoms themselves are all the same. The differences come from “accidents”—properties that cling to atoms like paint on a wall. Color, taste, smell, temperature, even life itself—these are all accidents that can come and go while the atoms stay the same. A fire atom is just an atom that happens to have the accident of “heat” stuck to it.

The Problem with Atoms

But the philosophers who followed Aristotle had some devastating objections. Their most powerful argument involved something you probably learned in school: the Pythagorean theorem.

Imagine space is made of little squares, like a chessboard. (This is how the atomists pictured it.) Now draw a right triangle whose two short sides are each 4 atoms long. According to the Pythagorean theorem, the long side should be about 5.66 units long. But if space is made of atoms, the diagonal can only go from one atom to the next in a zigzag. The longest it can be is 4 atoms—the same as the short sides. That means 4² + 4² = 4², which is 32 = 16, which is obviously false.

The philosophers concluded: if the Pythagorean theorem is true (and it is), then space cannot be made of atomic squares. Atoms don’t work mathematically.

They also had a physical objection. Imagine three atoms in a row: A, B, and C. For A and C to be separate, there has to be something about B that keeps them apart. But if B is truly indivisible—if it has no parts at all—how can one side of B touch A and a different side touch C? If you can point to the “A-side” of B and the “C-side” of B, then B has parts after all. This was a serious problem for the claim that atoms couldn’t even be conceptually divided.

The Anti-Atomist Solution: The Continuous

For the philosophers, matter wasn’t made of tiny bricks. It was continuous, like water or like the smooth surface of a table. You could divide it forever, but only in your mind. The divisions aren’t actually there until you make them. It’s like thinking about the number line: you can always think of a number halfway between 0 and 1 (0.5), and then halfway between that and 0 (0.25), and so on forever. But that doesn’t mean there are an infinite number of numbers actually sitting there waiting to be discovered. They’re just possibilities.

This solved the ant puzzle too. The ant doesn’t have to actually go through each of the infinite halfway points. Those halfway points exist only as possibilities, not as real places it has to visit. The ant just moves smoothly from one side of the sandal to the other.

But Wait—What Is Motion, Really?

This is where things get really strange. If you think about it, what does it mean for something to be moving at a single instant? At exactly 3:00 PM, is a moving arrow at rest? If you freeze time, the arrow is in one place. But if it’s in one place, how is it moving? And yet clearly the arrow does move between 3:00 and 3:01.

One of the most brilliant philosophers, Avicenna (980–1037), tried to solve this. He said there are really two meanings of “motion.” One is the motion you think about when you look at where something started and where it ended—you might call that “traversal motion.” That kind exists only in your mind, as a comparison between two different moments.

The other kind is the motion happening right now, in the present instant. Avicenna argued that at any given instant, a moving object is at a point, but it’s only at that point for that instant. If it stayed there for longer than an instant, it would be at rest. The motion isn’t something you can catch and hold still—it’s the fact that the object is at that point for precisely no time at all.

This is incredibly hard to wrap your head around, and Avicenna knew that. Later philosophers argued with him about it for centuries.

A Rebel: The Theory of Leaps

Not everyone took sides neatly. One thinker named al-Nazzam (died around 840) rejected atoms but also rejected the standard philosophers’ view of continuous motion. His solution to Zeno’s puzzle was startling: objects don’t actually move through every point in space. They leap. An ant crossing a sandal makes a finite number of jumps, skipping over the intervening spaces entirely.

This sounds crazy, but al-Nazzam had a clever argument. Imagine a millstone spinning. The outer edge has to travel a much longer distance than the inner hub, but both complete one rotation in the same time. So the outer edge is moving faster. How? The atomists said that slower objects rest more often—so the hub atoms stop for a moment while the outer atoms keep going. But al-Nazzam pointed out that if that were true, the millstone would fly apart, because some atoms would be moving while others next to them were stopped. Since the millstone stays together, the only explanation is that the outer atoms leap over some spaces.

Most people thought al-Nazzam was wrong, but nobody could completely refute him.

A Third Way: Minima Naturalia

The philosophers had another idea that split the difference. While they rejected indivisible atoms, they accepted something called minima naturalia—“natural minimums.” The idea is that for any specific kind of stuff, there’s a smallest amount that can still be that stuff.

Think about a cup of water. If you keep dividing it into smaller and smaller drops, eventually you’ll get a drop so tiny that it evaporates instantly when exposed to air. At that point, it’s no longer water—it’s become air (or water vapor). So there’s a smallest amount of water that can actually be water in normal conditions. Any smaller, and the stuff changes into something else.

This wasn’t the same as atoms. You could still theoretically divide that tiny water droplet into smaller parts—they just wouldn’t be water anymore. The “minimum” wasn’t about the smallest possible piece of matter, but about the smallest piece that could still be a certain kind of thing.

What About Space and Time?

The atomists and philosophers also argued about whether space itself was made of little cells or was continuous. And about time too. Was time made of tiny moments, like frames in a movie? Or was it smooth and continuous like flowing water?

Most atomists thought space was atomic too—a kind of grid of tiny cells. But some philosophers (like Avicenna) argued that this led to contradictions. And a few thinkers, like the physician-philosopher al-Razi (864–925), argued for something even stranger: absolute space and absolute time that exist independently of anything in them. Even if you destroyed the entire universe, he said, there would still be empty space and empty time where the universe used to be. Most others disagreed, saying that space and time only make sense as relationships between things.

The Bigger Question: Who Causes Change?

Underneath all these debates about atoms and motion was an even bigger question: what actually causes things to change?

The later theologians developed a radical answer called occasionalism. They said that God recreates the entire universe at every single moment. The apple that was on the table a moment ago doesn’t continue existing on its own—God creates it again, in a slightly different position. The cause of the apple falling isn’t gravity or any natural force. It’s God choosing to create the apple in a new place at each moment.

This means that what we call “causation” is really just God’s habit of doing things consistently. Fire doesn’t actually burn cotton. God creates fire and creates the cotton being burned at the same time. There’s no real connection between them except that God always does them together.

This is a very strange idea, but it solved some problems. If everything happens directly by God’s power, you don’t need to explain how one thing could affect another—which is actually very hard to explain. The philosophers thought this was ridiculous. They believed that things have real powers: fire really does have the power to burn, and a rock really does have the power to fall. But they struggled to explain how these powers work.

Why This Still Matters

These debates from a thousand years ago aren’t just history. They’re still alive. When physicists talk about whether space and time are continuous or made of tiny units (Planck length and Planck time), they’re wrestling with the same questions. When people argue about whether the universe had a beginning or has always existed, they’re using arguments that medieval Islamic thinkers refined. And when philosophers debate whether causation is real or just a pattern we project onto the world, they’re echoing the occasionalists.

The medieval Islamic thinkers were trying to understand what the world is made of at its deepest level. They didn’t solve it—we still haven’t. But they figured out what some of the hardest questions are, and they left us tools for thinking about them. The ant crossing the sandal is still crossing it, and we’re still trying to figure out how.

Key Terms

Term	What it does in the debate
Atom	The smallest possible piece of matter, which cannot be divided any further
Accident	A property (like color, temperature, or life) that can exist on an atom but isn’t part of the atom itself
Continuous	Something that can be divided forever in principle, without ever reaching a smallest part
Occasionalism	The view that God (or a divine power) is the only real cause of everything, recreating the world at each moment
Minima naturalia	The smallest amount of a specific kind of stuff (like water or flesh) that can still be that kind of stuff
Traversal motion	Motion considered as a comparison between where something started and where it ended (exists only in the mind)
Intermedial motion	The actual motion happening to an object at a single instant (exists in the world)

Key People

Avicenna (980–1037): A Persian philosopher and doctor who wrote massively influential works on almost every topic. He developed the “motion at an instant” theory and argued that space and time are relative, not absolute.
Al-Nazzam (died ~840): A rebel theologian who denied atoms and instead claimed that moving objects “leap” over intermediate spaces.
Al-Razi (864–925 or 930): A famous doctor and philosopher who broke from Aristotle, arguing that absolute space and absolute time exist independently of anything in them.
Averroes (1126–1198): A Spanish-Arab philosopher who wrote detailed commentaries on Aristotle and argued that time is circular, measured by the motion of the heavens.

Things to Think About

If you could travel back in time and ask both an atomist and a continuous-theorist to look at a piece of wood through a modern electron microscope, would that settle their argument? Or would they interpret what they saw differently?
The occasionalists said that fire doesn’t really burn cotton—God just creates fire and burned cotton at the same time. If that’s true, could God ever break the pattern and create fire without burned cotton? If God did that, what would you see? Is there any way to prove that God couldn’t do that?
Al-Nazzam said objects leap over space without passing through the middle. Can you think of a real-world observation that would prove him wrong? Or is there something about your own experience of moving that makes his theory impossible?
If you accept Avicenna’s idea that at any instant a moving object is at exactly one point but only for that instant, does that mean motion is made of tiny “snapshots” like frames in a video? Or is it something else?

Where This Shows Up

Modern physics debates whether spacetime is continuous or made of discrete units (Planck scale). String theory and loop quantum gravity take opposite sides, echoing the medieval disagreement.
Video games and animation actually use a form of atomic time—they update the world in discrete “ticks” or frames. But our own experience of motion feels continuous. Which one is real?
The problem of causation shows up in arguments about free will, artificial intelligence, and whether “laws of nature” are real things or just descriptions of patterns.
Debates about the beginning of the universe (did it have a first moment, or has it always existed?) reuse arguments from these medieval thinkers about whether time can be infinite.