What Makes Things Move? Medieval Ideas About Cause and Effect

Here’s a strange thing: when you throw a ball, why does it keep moving after it leaves your hand? You’re not pushing it anymore. So what is?

You probably already know what modern physics says: once something is moving, it keeps moving unless something stops it. That’s called inertia. It seems obvious now, but for about 2,000 years, the smartest people in the world thought something completely different. They thought things needed to be continuously pushed or they would stop instantly. They thought rest was the natural state of things, and motion required a constant cause.

This difference isn’t just about getting the science right. It’s about something deeper: what does it mean for one thing to cause another to happen? And once you start asking that, you find yourself in a tangle of questions that philosophers still argue about today.

The Problem of the Thrown Stone

Aristotle, a Greek philosopher whose ideas dominated European thinking for centuries, had a theory of motion. He divided all motion into two kinds: natural motion and violent motion.

Natural motion is easy. A rock falls because it’s trying to get to its natural place (the center of the universe). Fire rises because it’s trying to get to its natural place (the heavens). Things just do what comes naturally to them.

Violent motion is the hard case. When you throw a rock, you’re forcing it to do something unnatural. According to Aristotle, the rock keeps moving because the air behind it rushes around to fill the space it leaves, pushing it forward. The air itself becomes a kind of invisible hand that keeps pushing.

Medieval thinkers found this deeply unsatisfying. A 14th-century philosopher named John Buridan pointed out some obvious problems. If air is what pushes the rock, then a light object like a feather should be easier to throw than a heavy rock, since air could push it more easily. But that’s not what happens — the rock flies much farther. Also, what happens if two stones are thrown past each other from opposite directions? How is the air supposed to push in two directions at once?

Buridan and another philosopher, Nicole Oresme, proposed a different explanation. They said that when you throw something, you give it a kind of internal force they called impetus. This impetus lives inside the moving object and gradually wears down, which is why the rock eventually falls. The impetus theory was still thoroughly medieval — it treated motion as something that needs a continuous cause, just a different kind of cause than Aristotle’s. But it was getting closer to our modern idea of inertia.

Here’s where it gets really interesting. Oresme realized that if the earth were rotating while the heavens stayed still, everything would look exactly the same to us. You couldn’t tell the difference just by looking. This was a genuinely radical thought in the 14th century. But Oresme and Buridan weren’t quite ready to say the earth actually moves — they just said it could be, so far as we could observe.

Can the Will Cause Itself?

There’s another kind of motion that medieval philosophers worried about: the motion of the will. When you decide to do something, what causes that decision?

Aristotle thought actions are caused by beliefs and desires — you want something, you think a certain action will get it, so you act. This fits with Aristotle’s general rule that nothing can cause a change in itself. Something else has to do the pushing.

But a philosopher named John Duns Scotus disagreed. He thought the will was special. Unlike natural causes (like fire, which always burns if you put your hand in it), the will can choose otherwise even when everything else is the same. You can be presented with the most amazing thing imaginable and still say “no thanks.” Scotus thought this meant the will causes its own motions — it’s a self-mover.

William of Ockham took this even further. He said that we’re not even properly caused by our goals or desires. We’re influenced by them, but not necessitated. A free agent, for Ockham, is someone who, in exactly the same circumstances, could have chosen differently. This is a surprisingly modern-sounding view of free will, and it’s still being debated today.

How Do We See Things?

There’s a famous medieval debate about how perception works. The traditional view was that when you look at a tree, the tree sends out little copies of itself — called species — through the air, into your eyes, and eventually into your brain. These species were like tiny messengers that carried the tree’s form to your mind.

Ockham said this was nonsense. He used his famous “razor” — the principle that you shouldn’t multiply entities without necessity — to argue that species don’t exist. We can explain perception without them. Instead of species traveling through the air, Ockham thought the tree acts directly on our senses at a distance.

Most of Ockham’s contemporaries thought this was crazy. Action at a distance seemed impossible. But the debate shows something important: what counts as a good explanation depends on what you’re willing to count as real. Ockham’s razor didn’t just cut away unnecessary stuff — it forced him to accept something that seemed equally weird (action at a distance) rather than the traditional weird thing (species).

Could God Have Made Different Rules?

Here’s a question that might seem like a strange detour, but it’s essential: Are the causal laws we see in the world necessary? Could they have been different?

Medieval philosophers thought about this by asking about God’s power. They distinguished between God’s absolute power (what God could do, period) and God’s ordained power (what God actually chose to do, given that God is just and consistent).

In terms of absolute power, God could make fire cold, could make rocks float, could do literally anything that’s logically possible. But in terms of ordained power, God established a regular order. In this world, which God actually created, fire burns and rocks fall. Most medieval thinkers thought the causal connections in our world were necessary — not because they had to be, but because God’s choice to order things this way was stable and reliable.

This distinction gave medieval philosophers a powerful tool. They could explore possibilities (“what if fire were cold?”) without denying the actual regularities of nature. It’s a way of being open to alternative possibilities while still taking real causation seriously.

Do Things Act for the Sake of Ends?

This is the strangest part of medieval causation for us. Aristotle and many medieval thinkers believed in something called final causes — purposes or ends that explain why things happen. A tree grows because its end is to be a full-grown tree. A rock falls because its end is to be at the center of the universe. Everything in nature, not just conscious beings, acts for the sake of some goal.

Ockham was suspicious of this. He was happy to talk about efficient causes (what we now just call causes) but he was never quite sure what final causes added. If a tree grows because of its nature (the efficient cause of its growth), do we really need to say it’s also growing for the sake of being a tree? Ockham never fully settled this question, and philosophers still argue about whether purpose belongs only in minds or also in nature itself.

Why This Still Matters

You might think these debates are just old wrong science. But they’re not really about science — they’re about what causation itself is. When you say “X caused Y,” what are you actually saying? Do causes push? Do they pull? Do they just happen to go together regularly?

Modern physics has moved beyond medieval impetus theory. But the philosophical questions haven’t gone away. When you decide to raise your hand, what caused that? Were you free to do otherwise? When you see a tree, is it the tree itself you’re seeing, or some representation of it in your mind? When scientists discover that one thing regularly follows another, does that prove the first caused the second?

These are the questions medieval philosophers were working on. Their answers are often weird and sometimes wrong. But the questions themselves are still very much alive.

Appendix: Key Terms

Term	What it does in this debate
Impetus	An internal force that medieval philosophers said keeps a thrown object moving until it runs out
Natural motion	Motion that happens because a thing is seeking its natural place (like a rock falling)
Violent motion	Motion that’s forced on something against its nature (like a thrown rock)
Species	Hypothetical “little copies” of objects that were supposed to travel through the air to our senses
Final cause	The purpose or end that something is aiming for (like a tree growing toward being a full tree)
Efficient cause	What we normally mean by “cause” — the thing that makes something happen
Absolute power	What God could do if unlimited by any choices already made
Ordained power	What God actually does, given the orderly world God chose to create
Self-motion	The idea that something can cause changes in itself, especially the will

Key People

Aristotle — Ancient Greek philosopher whose ideas about causation dominated medieval thinking; thought nothing can cause a change in itself and that everything in nature acts for an end
John Buridan (14th century) — Philosopher who proposed the impetus theory of projectile motion and realized we couldn’t tell if the earth rotates
Nicole Oresme (14th century) — Another philosopher who developed impetus theory and argued that earth’s rotation is consistent with what we observe
John Duns Scotus (13th-14th century) — Philosopher who argued that the will can cause its own motions and is not determined by desires
William of Ockham (14th century) — Radical thinker who used his “razor” to argue against species and other unnecessary entities; thought free agents can always choose otherwise
Thomas Aquinas (13th century) — Influential philosopher who helped develop the distinction between God’s absolute and ordained power

Things to Think About

If you had to choose between Aristotle’s air-pushing theory and Buridan’s impetus theory to explain a thrown ball, which would you pick? What kind of evidence would settle the question? (Remember, you’re a medieval philosopher without access to modern physics.)
Ockham said the will can always choose otherwise, even when everything else is the same. Is that what freedom means to you? Or could you be free even if your choices were determined by your character and desires?
If you think about your own perception — when you look at this page, are you seeing the page itself, or are you seeing some kind of representation of it in your mind? How would you tell the difference?
The medieval philosophers thought about “what if God made different causal rules” as a way of testing whether causal connections are necessary. Can you think of a modern version of this? What would it mean if the laws of physics could have been different?

Where This Shows Up

Science fiction often explores “what if the laws of nature were different” — medieval debates about God’s absolute power are doing a version of the same thing
Criminal law depends on whether we think people could have chosen otherwise — if Ockham is right about the will, punishment might make more sense than if everything is determined by prior causes
Artificial intelligence debates about whether machines can really “decide” anything echo medieval debates about whether the will is special or just another kind of cause
Everyday arguments about whether someone “caused” something to happen touch all the same puzzles medieval philosophers were wrestling with