Everything in its place

Worlds shaped by home

Picture three ancient peoples: one on the edge of a desert, one in a river valley, one by the sea. Each would picture the universe differently, because each could see only its own corner of it. People explained the unseen world by extending what they saw every day.

Some pictures were widely shared. Heaven is round and the earth is square: the sky arches overhead like a bowl, and the ground stretches out flat. The ancient Hebrews imagined a solid dome, the firmament, with waters above it and the underworld below the earth. A picture often credited to ancient India, though mostly known from European retellings, set the earth on the backs of great elephants, standing in turn on a giant turtle. When the turtle stirred, the ground shook. Who else could carry such a weight?

Philosophers have noticed that people tend to explain nature in the image of their own society. Greek myth is a good example. A violent storm meant Zeus was in a temper. A poor harvest meant the gods were angry. Greece sits on the Mediterranean, so earthquakes came from Poseidon, god of the sea, striking the ground with his trident. The gods were people, with two differences: power over nature and very long lives. Those were exactly the powers humans wished for.

The first step

As Greek myths piled up, they began to contradict one another. The more they explained, the more tangled they became. Then someone proposed a different approach.

Thales came from a wealthy family in Miletus, a busy port that let him travel and learn. According to ancient tradition, he studied geometry in Egypt, where surveyors measured out farmland each year after the Nile flooded. He brought geometry home and joined it to a new habit of mind. Thales refused to explain nature with anything supernatural. He asked what everything is made of, and answered: water.

Earthquakes, in his view, were not Poseidon’s work. The earth floats on water like a ship, and when a big wave rocks it, the ground shakes. The idea is wrong, but look at its logic. Thales could watch ships rocking in the harbor. He assumed that the part of the world he couldn’t see obeys the same rules as the part he could. There are no special divine causes out there, only more nature. This was physics’ first step. It was more of a crawl, like a baby’s first movement.

Aristotle’s cosmos

Three centuries later, Aristotle (384–322 BCE) gathered everything before him into a single system. His picture of the universe shaped Western thought until the 1600s, about two thousand years. One might compare his influence to that of Confucius. Aristotle criticized earlier writers for letting myth into the study of nature and wanted only natural, observable causes. Yet even he couldn’t tell which of his assumptions came from experience and which he had simply inherited.

At the center of Aristotle’s cosmos sat the earth, completely still. That fits experience. Lie outside on a summer night and look up at the stars. Do you feel the earth moving? Around it turned seven wandering lights, which the Greeks counted as planets. They were the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. They shift from night to night against the fixed stars, which seemed set in one great outer sphere.

The Moon’s sphere divided two realms. Below it lay our world, where everything is made of four elements: earth, water, air, and fire. Each element has a nature that tells it where to go. Earth moves toward the center of the universe, which is also the center of the earth. Water moves the same way, less strongly, so a stone sinks in a pond. Air rises above water, and fire leaps upward, away from the center. Every object behaves according to its mixture of elements. Drop a rock and it falls. Boil water and the steam rises.

Above the Moon was a fifth element, aether, found nowhere on earth. The heavens were perfect, and a perfect thing moves in the most perfect figure, a circle, at a perfectly steady speed. The universe was completely full, because, as the saying goes, nature abhors a vacuum. Whenever later physicists met something they couldn’t explain, the aether tended to come back. The early computer network called Ethernet was named after it.

Why Aristotle seemed right

Aristotle’s ideas about motion matter most for the rest of this series. For him, rest was the natural state of things. A moving object on its own slows down and stops. To keep something moving, something has to keep pushing it. Push a cart and it rolls; stop pushing and it stops. Motion toward an element’s natural place, like a stone falling, needs no explanation. Any other motion is “violent,” against nature, and needs a force to cause it and keep it going.

This is completely true to everyday life. Look for a counterexample. Can you push a cart, stop pushing, and watch it roll on forever? The error was tiny, and it took two thousand years and some of the cleverest people in history to find it. Try it yourself.

A place for everything

Aristotle’s physics explains things by their purposes. Historians call this teleology, from the Greek telos, “end” or “goal.” Every object has a nature and a natural place, and it moves to fulfill its nature. Earth belongs below water, water below air, air below fire. So a naturalist’s job was to sort things into kinds, then work out each kind’s nature and purpose. There were no laws to discover beyond that.

In this reading, the picture lasted so long partly because it matched society as well as nature. If every element has its natural place, so does every person. Aristotle argued that some people are slaves by nature, while others are free by nature and nobles belong where they are. His physics justified the social order, and the social order made his physics feel obvious. It explained everything people saw, in the heavens and in the streets.

But it predicted very little. Ancient physics can be summed up in a formula: observation plus speculation. The observations were rough, made with the naked eye. The speculation carried assumptions nobody had tested, like the perfect circles of the heavens. Why must the stars move in perfect circles? Nobody could see that they did. It was a philosophical “fact,” not an observed one, inherited from older ideas of perfection. Greek engineers like Archimedes discovered the law of the lever and the principle of buoyancy, but these never grew into a full science of motion.

Now turn the lesson on yourself. Which of your beliefs feel like facts, but were never tested?