5.2. The Medieval Version of Here to There: A Little Bit of Aristotle

Look around you – everything is in flux. Things fall, birds fly, you move across a room, you throw a ball, the Sun is in a different spot in the sky during the day, and the stars appear to move from east to west during the night. Why? How? Our experience shows that all events but those in the sky are individual events. Throw a ball, and its motion will be slightly different each time. Regular, reliable motions appear to be reserved for the celestial bodies.

The correct understanding of everyday motion was long in coming and by Galileo’s time, everyone thought that Aristotle had nailed it, albeit with some embarrassments. Classical works had been out of reach of Europe until Greek philosophy and science essentially fell into Europe’s lap in the form of hundreds of conflicting Arabic translations in the 1300s. But translation of Aristotle’s Physics from original Greek, to Arabic, and then to Latin did not make his ideas any less confused than they originally were.

Aristotle, revered as “The Philosopher,” invented formal logic that taught people how to evaluate arguments. But while the Philosopher’s methods were refreshing when applied against church authority by Parisian college students, his ideas about motion were confused at best – but nonetheless became firmly stuck in the academic and Church communities where they were protected as philosophy, not as science.

Not until the end of the 16th century did Galileo shed Aristotle and lay the groundwork for the first systematic understanding of what it means for something to move. It took three centuries! Let’s lay out the basics of The Philosopher’s mechanics so we’ll have something to throw logical darts at.

5.2.1. Aristotle’s Mechanics and Astronomy

For Aristotle there were three kinds of motion: two “natural” and one “unnatural.”

Natural motion near the Earth was in a straight line, either down or up relative to the center of the Universe, which he located at the center of the Earth. The speed of downward motion depended on the amount of “earthy” composition of the object, and so its weight. If there was a lot of earthiness – think a rock – the object went down. If it had the property of “lightness” (a quality that’s foreign to us) it went up – think fire. So the apparent fact that every event we observe is a little different from every other event can be ascribed to the variations in the material composition of real things.

Natural motion beyond the orbit of the Moon was to be circular with every extraterrestrial body attached to its own rotating crystalline sphere. These spheres were all nested with common centers and rotated around the Earth to account for the apparently circular orbits that we see from the Earth. They included all of the known planets, the Sun, and the Moon, and even the stars all together in the outermost shell.

Unnatural motion was different. It only happens on Earth and it required a pusher, an active force that was in physical contact with the object throughout the motion. Therein lay one of the most obvious flaws in his model.

Rather than one consistent set of rules, Aristotle insisted that there were different physical laws for different parts of the universe.

It’s child’s play to make fun of these ideas. And I will. But Aristotle was describing what he saw in his everyday life and trying to fit it together into his much bigger, intricately stitched-together philosophical structure. Pull at one piece of that system and the whole thing tumbles apart like a game of Jenga. So while his physics was bizarre and wrong, it stuck. It had to stick because following the genius of St. Thomas Aquinas, Catholicism had hitched its wagon to the hard-to-ignore Aristotelian philosophy. But people knew for centuries that it had problems.

There were a number of other issues with his physics, like his “four causes” which made things move because they had some desire to find a natural place. But we’ll not go there. Okay. I’ll go there a little. Motion was not only for moving objects in space. A cat growing into a kitten was a form of motion. A seed growing into a plant was a form of motion and the seed’s desire was to reach that plant-ness as a matter of fulfillment. Now take this into the kind of motion that we care about – is Locomotion – and you’ll see why I didn’t go there. Much.

Galileo uncovered the modern notion of how things move and persuasively rid the intellectual community of Aristotle’s baggage.

He began to rebel while at Pisa, where he wrote an unpublished manuscript, de Motu (“On Motion”). These were immature ideas, but he was onto something. One of his conclusions was right on: all objects fall at the same rate, contrary to Aristotle’s insistence that heavier objects fell faster. His data? Not the Leaning Tower of Pisa. That’s a myth. He too looked around him. And saw things differently.