1.4 From Galileo to Descartes

What this lecture covers

This lecture picks up the unresolved problem left by Galileo's physics: if heavenly bodies are made of ordinary matter like the Earth, why do the Moon and planets orbit rather than fly off in a straight line? It introduces René Descartes as the philosopher who took up this problem and built a comprehensive mechanical philosophy in response.

Millican shows how Descartes used the ancient sceptical 'problem of the criterion' as a weapon against Aristotelian tradition, arguing that only a science able to withstand sceptical doubt is worth believing. From that starting point, Descartes develops his famous 'I think therefore I am', an argument for God's existence, and ultimately a physics in which matter is pure extension in space and all motion takes the form of vortices.

Key ideas

• The orbit problem: if the Moon is ordinary matter subject to inertia, something must explain why it curves around the Earth instead of moving in a straight line.
• Descartes and scepticism: Descartes uses sceptical arguments deliberately to discredit appeals to Aristotelian tradition, demanding a foundation that survives doubt.
• 'I think therefore I am': Descartes's starting point of certainty, grounded in clear and distinct perception.
• Argument for God and reliable faculties: Descartes argues God exists, is not a deceiver, and therefore our properly used faculties are reliable.
• Matter as extension: for Descartes, the essence of matter is simply spatial extension; qualities like color are secondary, arising from interaction with our senses.
• The plenum and vortices: because matter just is extension, empty space is impossible, so all motion must take the form of circulating vortices, which Descartes uses to explain planetary and lunar orbits.
• Mind-matter dualism: Descartes holds that mind, whose essence is thinking, is entirely distinct from matter, whose essence is extension.

Walkthrough

The unsolved orbit problem (0:11)

The lecture opens by restating Galileo's inertia and the puzzle it creates: if the Moon is ordinary matter, why does it orbit the Earth instead of traveling in a straight line?

Descartes enters (2:32)

Descartes is introduced as 'the father of modern philosophy', noted for the readability of his Meditations. He uses sceptical arguments to reject reliance on Aristotelian tradition, insisting that only knowledge able to withstand scepticism counts as genuine science.

An epistemological foundation (4:01)

Descartes's method is summarized: certainty in his own existence and thought ('I think therefore I am'), grounded in clear and distinct perception, followed by an argument for a non-deceiving God who guarantees the reliability of properly used faculties. His concrete scientific achievements, including coordinate geometry and an account of the rainbow, are also noted.

Matter as extension (5:17)

The lecture explains Descartes's view that matter's essence is pure spatial extension, from which properties like impenetrability and inertia can supposedly be derived, while sensory qualities like color are secondary and not truly properties of matter itself.

Vortices and the laws of motion (6:33)

Because extension implies no empty space is possible, the physical world is a plenum, and all motion must take the form of vortices. This lets Descartes explain planetary and lunar orbits as bodies carried around within nested whirlpools of matter.

Before you watch

• Watch lectures 1.2 and 1.3 first: this lecture continues directly from Galileo's account of inertia and the crisis of Aristotelian astronomy.

Check your understanding

1. Why does the Moon's orbit pose a problem for Galileo's account of inertia?
2. How does Descartes use scepticism as a tool against Aristotelian tradition?
3. What is the relationship between Descartes's claim that matter is extension and his idea of vortices?
4. How does Descartes's view of mind differ from his view of matter?