2.4 Isaac Newton and Instrumentalism

What this lecture covers

This lecture explains how Isaac Newton built on Kepler, Galileo, Descartes and Boyle to produce a theory of gravity that explained planetary motion with unprecedented simplicity and accuracy. It then examines the philosophical controversy this caused and Newton's own response to it.

Newton showed mathematically that a single force, proportional to each body's mass and inversely proportional to the square of the distance between them, produces exactly the elliptical orbits Kepler had observed, and that the same law explains falling objects on Earth. This discredited Descartes's vortex theory, which Newton proved could not generate elliptical motion. But gravity's action at a distance seemed to violate the mechanistic ideal that had discredited Aristotle, since it required no contact between bodies, and Newton refused to speculate about its underlying cause.

Key ideas

• Newton's law of gravity: force is proportional to the product of the two masses and inversely proportional to the square of the distance between them.
• Unifying terrestrial and celestial motion: the same equations explain both the orbits of planets and the fall of objects like cannonballs on Earth.
• Refutation of vortices: Newton showed mathematically that vortices cannot produce elliptical orbits, discrediting Descartes's physics, though it lingered longer in France than in Britain.
• The problem of action at a distance: gravity acting between bodies with no contact seemed to violate the mechanical ideal that had replaced Aristotelian 'striving'.
• 'Hypotheses non fingo': Newton's famous refusal to speculate about the cause of gravity, stating only that his equations matched observation.
• Instrumentalism: the view that a scientific theory only needs to deliver correct predictions, regardless of whether its underlying entities are real.

Before you watch

• Watch lectures 2.2 and 2.3 for Descartes's vortex theory and Boyle's corpuscularianism, and note Kepler's discovery of elliptical orbits from lecture 2.3.

Check your understanding

1. How does Newton's single law of gravity explain both planetary orbits and falling objects on Earth?
2. Why did Newton's theory of gravity trouble people committed to the mechanical philosophy?
3. What does 'hypotheses non fingo' mean, and what does it reveal about Newton's attitude to explanation?
4. How does instrumentalism differ from a view that insists a scientific theory's entities must be real?