1.3 From Aristotle to Galileo

What this lecture covers

This lecture explains why Aristotelian science, which had dominated Western thought for centuries, broke down in the early modern period. It follows the previous lecture's account of the crisis of authority, showing how that crisis played out specifically in physics and astronomy.

Millican first sets out Aristotle's theory of four terrestrial elements plus a fifth heavenly element (ether), each with a natural motion, and shows why this kind of 'striving' explanation is empty: saying a stone falls because it strives toward the center of the earth just redescribes the fact rather than explaining it. He then covers Galileo's alternative view of motion and his telescopic discoveries, which directly contradicted Aristotelian astronomy.

Key ideas

• Four (or five) elements: Aristotle held that earth, water, air and fire have natural motions toward their proper place, while heavenly bodies are made of a fifth element, ether, whose natural motion is circular.
• Empty explanation: 'nature abhors a vacuum' and similar explanations, parodied in Molière's line about a 'dormative virtue', merely redescribe a phenomenon instead of explaining it.
• Inertia: Galileo argued that what needs explaining is not why a moving object keeps moving, but why it stops; objects naturally continue in uniform motion unless a force acts on them.
• Falling bodies: objects of different weight fall at close to the same rate, contradicting Aristotle's prediction that heavier objects fall proportionally faster.
• Telescopic evidence: Galileo's telescope revealed mountains on the Moon, moons orbiting Jupiter, countless additional stars, and the phases of Venus.
• Phases of Venus: the fact that Venus can appear nearly fully illuminated is only possible if it orbits the Sun rather than the Earth, undermining the geocentric model even with its circles-on-circles adjustments.
• Mechanical philosophy: Galileo favored efficient causation (things acting on each other by contact) over Aristotle's final causation (things striving toward a purpose).

Walkthrough

Aristotle's physics and its empty explanations (0:10)

The lecture opens with Aristotle's theory of natural motion based on four elements, then shows how explanations like 'nature abhors a vacuum' fail to explain anything, using the siphon example and the Molière parody of the 'dormative virtue'.

Problems with vacuums and falling bodies (3:30)

Specific failures of Aristotelian physics are examined: the trajectory of cannonballs, a sledge that keeps sliding on ice without a mover, and the legendary Leaning Tower of Pisa experiment (echoed by the Apollo hammer-and-feather demonstration on the Moon).

Galileo's new mechanics (6:33)

Galileo's alternative is introduced: objects naturally continue moving in a straight line at constant speed unless a force acts on them, shifting the explanatory question from 'why does it move' to 'why does it stop'.

Telescopic astronomy and the collapse of geocentrism (9:14)

Galileo's telescope revealed the Moon's mountains, Jupiter's moons, and innumerable additional stars. The lecture explains how astronomers had used circles-on-circles to fit planetary motion to a geocentric model, and how the observed phases of Venus made that model untenable.

Mechanical philosophy (16:04)

The lecture closes by contrasting Aristotle's final causation (things striving for an end) with the mechanical philosophy's efficient causation (things acted on by contact), the model that would come to define early modern science.

Before you watch

• Watch lectures 1.1 and 1.2 first for the course's historical framing and the crisis of authority that preceded this scientific shift.

Check your understanding

1. Why does Millican say that 'nature abhors a vacuum' fails as an explanation?
2. How does Galileo's account of motion differ from Aristotle's?
3. Why did the phases of Venus pose such a serious problem for the geocentric model?
4. What is the difference between final causation and efficient causation?