Aristotle: Physics

Aristotle was a Greek thinker who lived about 2,350 years ago. He wanted to understand everything: animals, the stars, poems, governments, and why things move and change.

He asked a simple-sounding question: why? Why is a statue the way it is? He said there are four different kinds of answer. Take a bronze statue: it is made of bronze; it has the shape of a horse; a sculptor made it; and it was made to honor a winner of a race. Each is a different kind of "because".

Aristotle also thought heavy things fall faster than light things, in proportion to their weight. That sounds sensible: a stone drops faster than a leaf. About 1,900 years later, Galileo showed that if you take away the air, a heavy ball and a light ball fall together. Aristotle wasn't foolish. He was describing what we usually see in air and water. But he didn't test the rule carefully with balls of the same size.

Who, when and why

Aristotle (384–322 BC) was born in Stagira in northern Greece, the son of a doctor. At seventeen he went to Athens and studied in Plato's Academy for about twenty years. After Plato died he lived on the island of Lesbos and elsewhere, tutored the young Alexander the Great for a few years, and in 335 BC returned to Athens and founded his own school, the Lyceum. The Physics is a set of lectures from his teaching, written down by him and edited by later scholars. "Physics" here means the study of physis, nature: everything that changes.

The problem he faced came from earlier thinkers. Parmenides had argued that change is impossible, since nothing can come from what is not. Zeno had offered puzzles "proving" that motion can't happen. Others, like Democritus, said everything is atoms bumping in a void, with no purpose. Aristotle wanted a science of change that accepted what our senses show: things really do move, grow and change.

The four causes

The final cause is the boldest. Aristotle saw purpose throughout nature: teeth grow sharp in front for biting, flat behind for grinding, and this, he argued, can't be mere luck because it happens always or for the most part. Modern biology explains such fit by natural selection instead (Darwin, 1859), but it still asks Aristotle's question: what is this part for?

Motion, the void and a surprise

Aristotle held that a thing moving through a medium like air or water goes faster when it is heavier and slower when the medium is thicker. In a void, with no medium at all, the speed would have to be infinite, which he took as absurd. So, he concluded, there is no void. In the middle of this argument he says something remarkable:

That is very close to Newton's first law, the law of inertia. But Aristotle meant it as an absurd consequence: if a void could exist, things in it would never stop, which he thought couldn't be. Newton, 2,000 years later, accepted exactly that conclusion.

Time, the infinite and Zeno

• Time is the number of motion with respect to before and after (IV.11): time is how we count change.
• The infinite exists only potentially: you can always divide a line again, but it is never divided into infinitely many pieces all at once.
• Zeno's runner must first cover half the track, then half the rest, and so on. Aristotle answers that the time can be divided just as the distance can, so the runner crosses infinitely many "halves" in finite time. Mathematicians later showed that $\tfrac12 + \tfrac14 + \tfrac18 + \dots = 1$.

What later science kept, and what it changed

• Changed: the laws of motion. Galileo and Newton replaced Aristotle's rules with inertia and constant acceleration; the void he denied is real (Torricelli and Pascal made one in a tube of mercury).
• Kept: careful definitions of motion, place, time and continuity; the potential infinite; and the habit of asking what causes change.
• Debated still: whether there is purpose in nature. Physics dropped final causes; biology rephrased them; philosophers still argue about them.

Nature as a source of change

Aristotle defines a natural thing as one that has in itself "a source or cause of being moved and of being at rest" (II.1). A stone falls because of what it is; an acorn grows into an oak because of what it is. A bed, by contrast, changes only because someone makes or moves it. This contrast between nature and craft runs through the whole book.

Book VIII argues that motion has always existed and needs a first source that is itself unmoved, the unmoved mover. Medieval readers such as Aquinas (later on this ladder) used this argument in their own reasoning about God; Aristotle develops it further in the Metaphysics. Readers have long disagreed about how far the argument succeeds.

Why it lasted

Aristotle's physics lasted nearly two thousand years because it fit everyday experience well. Things do stop when you stop pushing them; heavy things do usually fall faster in air. To get past it, Galileo had to imagine motion without friction or air, which nobody sees, and then test the idea with ramps and pendulums. Sometimes science moves forward by imagining what is not seen.

Big questions

• Is it a good argument to say "this happens always or for the most part, so it can't be chance"?
• Aristotle rejected the void because it led to a strange conclusion. When should a strange conclusion make us reject an idea, and when should we accept it?
• Can you think of a change that has no final cause? A change that seems to have one?