What Makes Things Move? Aristotle's Answer (and Why It Still Matters)

Imagine you drop a rock. It falls. You let go of a balloon filled with helium. It rises. Why? Most people today would say “gravity” for the rock and “buoyancy” for the balloon. But those are just names we’ve given to the patterns we observe—they don’t really tell us what’s going on inside the rock or the balloon that makes them behave that way.

Aristotle, a philosopher who lived in Greece about 2,400 years ago, thought about this problem differently. Instead of just naming the pattern, he asked: what is a rock, such that falling is what it does when nothing stops it? And what is air, such that rising is what it does? His answers shaped science and philosophy for nearly 2,000 years. Some of them turned out to be wrong—but the questions he asked are still worth thinking about.

Natures: The Engine Inside

Aristotle’s central idea was that every physical thing has a nature—an inner principle that explains why it moves and rests the way it does. “Nature” here doesn’t mean “trees and mountains.” It means the built-in tendency that makes a thing behave like itself.

A rock’s nature makes it fall toward the center of the universe (which Aristotle thought was the Earth’s center). Fire’s nature makes it rise toward the heavens. A dog’s nature makes it grow, move around, and—if it’s healthy—act like a dog. When nothing interferes, things just do what their natures tell them to do.

But here’s where it gets interesting. If you throw a rock upward, you’re forcing it to do something against its nature. As soon as your hand stops pushing, the rock’s nature takes over, and it falls. So for Aristotle, every motion is either natural (following the thing’s nature) or forced (going against it). There’s no neutral motion—no case where the thing just doesn’t care one way or the other.

This already raises a puzzle. If the Sun and planets are moving in circles up in the sky, what’s their nature? Do they want to move in circles? Or is something forcing them? Aristotle thought circular motion was special—it had no opposite, so it couldn’t be forced. The heavenly bodies must have a nature that makes them go in circles forever. But then he noticed another problem: if they’re moving forever, something must be making them move. Something that itself doesn’t move.

We’ll get back to that.

The Four Causes: Why Things Happen

When you try to explain why something happens, Aristotle said you can give four different kinds of answers. He called them the four causes. They’re not really “causes” the way we use the word today—they’re more like different ways of being responsible for something.

Here’s an example using a house. Imagine trying to explain why a house exists.

Material cause: The bricks, wood, and mortar. The stuff it’s made of.
Formal cause: The shape or design—the plan that makes it a house rather than a pile of bricks.
Efficient cause: The builder who actually put it together. (This is closest to what we call “cause” today.)
Final cause: The purpose—what it’s for. Sheltering people.

Now, here’s what Aristotle thought about how these fit together. The material cause provides the potential to be a house. Bricks could become a house. The formal cause (the plan) actualizes that potential—it makes the bricks into an actual house. The efficient cause (the builder) is what does the actualizing. And the final cause (shelter) is the reason the whole thing happened at all.

For natural things—not houses but rocks, plants, animals—Aristotle said the formal cause, efficient cause, and final cause are often the same thing. A dog’s “form” (what makes it a dog) is also what makes it grow and move like a dog, and it’s also what the dog’s activities are for. That “form” is the dog’s nature. And for living things, Aristotle called that form the soul.

This is important: for Aristotle, the soul wasn’t a ghost inside the machine. It was the principle of life itself—the thing that made a living body be alive rather than dead. A dead dog has the same material as a live one, but it’s lost its form. It doesn’t have a nature anymore in the same way.

The Strange Principle: Like Causes Like

Aristotle believed something that sounds weird to us now: that in order to cause a change, the thing doing the causing has to already have the quality it’s producing. He called this the principle of causational synonymy.

So: only something actually hot can make something else hot. Only a human can make another human. Only something that is in motion can make something else move. This might seem obvious for some cases—of course a cold thing can’t heat anything up—but it gets tricky.

Consider: a sleeping pill makes you sleep. But the pill itself isn’t sleeping. So how does the principle work? Aristotle would say the pill contains the power to induce sleep, but it doesn’t exemplify sleep. His principle applies most strictly to the first cause in a chain. In the case of making a baby, the father has the human form, and that form is transmitted through the semen—which doesn’t look like a tiny human. The principle only requires that somewhere up the chain, there’s something that actually has the form.

But here’s where it really matters: Aristotle thought this principle ruled out the idea that a uniform object could move itself. If a rock is all the same stuff, you can’t say one part moves another—because the moving part would have to have the quality it’s giving, and there’s no difference between the parts. This meant that for Aristotle, nothing made of a single material could be self-moving. Living things could move themselves because they had souls—different parts with different roles. But rocks? Something outside them had to make them fall.

This leads to a big question: if nothing moves unless something else moves it, and that thing was moved by something else, and so on backward… where does it end?

The Unmoved Mover

Aristotle argued that the chain can’t go backward forever. There has to be something that causes motion without itself being moved. This is his famous unmoved mover.

But what is it? How can something cause motion without moving?

Aristotle thought about the heavenly spheres—the perfect circular motions of the planets and stars. These motions are eternal (they’ve always been happening) and they’re what keep the whole system going. The Sun’s motion along its yearly path causes the seasons, which causes plants to grow and die, which causes animals to live and eat each other, and so on. If the Sun stopped, everything on Earth would eventually stop too.

Now, the spheres themselves are in motion. Something must be moving them. But whatever moves them can’t be moving itself (because it’s a uniform body, and we already said those can’t self-move). And it can’t be moved by something else that’s also moving, because then we’d need an infinite chain.

So Aristotle concluded there must be something that moves without being moved. It has to be completely actual—no potential at all, because anything with potential could change, and if it could change, it wouldn’t be eternal and unmoved. It has to be simple (not made of parts). It has to have infinite power. And it has to be located at the edge of the universe, where motion is fastest.

But how does it cause motion? Aristotle’s answer in one of his books is strange: it moves “as an object of love.” The spheres desire the unmoved mover, and in striving toward it, they move. The unmoved mover itself just is—perfect, eternal, thinking about thinking—and its very existence draws the universe toward it.

If this sounds like something between science and poetry, that’s because it is. Aristotle was trying to explain the physical universe, but he found that at the very bottom of explanation, you hit something that can’t be described in physical terms.

What’s Still Alive Here?

Aristotle got a lot wrong. We know now that rocks don’t have an inner drive to reach the center of the universe. The Earth isn’t the center of anything special. The planets aren’t embedded in crystalline spheres. And something can affect something else without “having” the quality it produces—a magnet moves iron without being iron, and a sleeping pill works without sleeping.

But the questions Aristotle asked are still worth asking. What does it mean for something to “behave naturally”? When we say a rock “wants” to fall, are we just using a metaphor, or is there something real about the way things have tendencies built into them? And if everything that happens is caused by something else that happened before, can we ever reach a final explanation—or do we just keep pushing the question back?

Aristotle thought you could reach bottom. Some philosophers today think you can’t. Others think the question doesn’t make sense. But the pattern of his thinking—looking for what’s inside a thing that makes it behave the way it does, rather than just describing the behavior—still influences how scientists and philosophers think about biology, psychology, and the nature of the universe.

When you drop that rock tomorrow, you’ll probably think “gravity.” But you might also wonder: is gravity something the rock has—a real feature of what it is—or just a name for the pattern we see when rocks and Earth get close together? That’s an Aristotelian question. And nobody has completely settled it.

Appendix

Key Terms

Term	What it does in the debate
Nature	The inner principle that makes a thing move and rest in its characteristic way
Four causes	Four different kinds of answers to “why did this happen?”—material, formal, efficient, final
Form	The pattern or structure that makes something what it is (as opposed to the matter it’s made of)
Potentiality	A capacity to become or do something that isn’t yet actual
Actuality	The state of being fully what something is, or of having realized a potential
Unmoved mover	A cause of motion that isn’t itself in motion; the ultimate source of change in the universe
Natural motion	Motion that follows a thing’s nature, without force
Forced motion	Motion that goes against a thing’s nature
Principle of causational synonymy	The idea that a cause must have the same quality it produces (only hot things make other things hot)

Key People

Aristotle (384–322 BCE): A Greek philosopher who studied under Plato and later founded his own school, the Lyceum. He wrote about everything from biology to poetry to politics, and his ideas about nature dominated science for almost 2,000 years.

Things to Think About

Do things have “natures” in Aristotle’s sense, or is it just that we observe patterns and give them names? How would you test the difference?
If you explain a falling rock by saying “gravity,” have you actually explained anything—or just given the phenomenon a name? What would count as a real explanation?
Aristotle thought different kinds of things have different kinds of natures—rocks don’t have souls, dogs do, and humans have a special kind of soul with reason. Does this seem right to you, or is it just the way we talk?
The unmoved mover “moves as an object of love.” Does wanting something cause motion? Is desire a kind of cause? Where’s the force in wanting?

Where This Shows Up

Arguments about whether life is just chemistry or something more often appeal to ideas that look Aristotelian—that living things have a “form” that isn’t just their physical parts.
When people say something “just isn’t in my nature” to do, they’re using Aristotle’s framework without knowing it.
The question of whether the universe needs a first cause (a “prime mover”) is still debated by philosophers and physicists—Aristotle’s argument is one of the oldest versions.
In biology, the idea that organisms have built-in purposes (a heart is for pumping blood) is called “teleology,” and it comes straight from Aristotle’s final cause.