6.1 Introduction to Primary and Secondary Qualities

What this lecture covers

The lecture opens a new unit on perception by explaining why the rise of modern mechanical science, in figures like Robert Boyle and John Locke, forced philosophers to rethink how perception works. Instead of objects transmitting their qualities directly to the mind, the new picture explains perception through causal intermediaries: particles or waves that carry information without resembling the objects that produced them.

You'll come away able to state the corpuscular hypothesis, define primary and secondary qualities, and explain Locke's argument that simple ideas like yellow are epistemically 'adequate' even though they do not resemble anything in the object.

Key ideas

• Corpuscular hypothesis: Boyle and Locke's conjecture that all matter is made of the same underlying stuff, shaped into corpuscles that differ in size, shape and arrangement, and that these differences explain the different properties of substances.
• Primary qualities: shape, size, motion, texture and what Locke called solidity, treated as belonging intrinsically to objects and used to explain how things appear to us.
• Secondary qualities: powers in objects, arising from their primary qualities, to produce sensations such as color, sound, taste and smell in an observer; nothing resembling the sensation exists in the object itself.
• The hot plate example: pain and felt warmth are clearly mental, shape is clearly in the object, but color sits uncomfortably in between, exposing the puzzle at the heart of the primary/secondary distinction.
• Locke's disputed sentence: a famous, ambiguously punctuated passage in the Essay has led some readers to think Locke denies secondary qualities are in objects at all, when he actually holds they are in objects, as powers.
• Berkeley's misreading: Berkeley read Locke as saying secondary qualities exist only in the mind, but Millican argues this misrepresents Locke's actual, more careful position.
• Adequate ideas: Locke argues a simple idea like yellow is automatically an adequate representation of the object, because being yellow just means having the power to produce that idea in a suitably placed observer.

Walkthrough

The shift to a mechanical picture of perception (1:12)

The lecture traces how the move from Aristotelian to mechanical science, starting with Galileo and Descartes, changed how perception was explained. Rather than something intrinsically similar to the object reaching the eye, mechanists explained perception through particles or waves striking the sense organs and being processed by the brain, with no resemblance required between the intermediary and the object.

Locke's corpuscular hypothesis (4:20)

Locke adopts Boyle's corpuscular hypothesis cautiously, treating it as the most intelligible available account rather than a certainty. The hypothesis explains differences between substances like gold and lead through differences in their microstructure, while all corpuscles are made of the same 'universal matter'.

Defining primary qualities (6:20)

Primary qualities, shape, size, motion, texture and solidity, are treated as belonging to matter itself and as the terms in which the appearance of things is to be explained. The hot-plate example is used to test intuitions: pain and warmth feel clearly mental, the circular shape feels clearly objective, and color sits in an uncomfortable middle ground.

Secondary qualities as powers (9:25)

Working through Locke's own text, the lecture untangles a notoriously ambiguous sentence about secondary qualities being 'nothing in the objects themselves but powers'. Millican argues Locke consistently holds that secondary qualities are real properties of objects, understood as powers to produce ideas, contrary to how Berkeley later read him.

Adequate ideas and the epistemological payoff (11:25)

The lecture explains Locke's argument that simple ideas, like the idea of yellow, are automatically adequate representations of their objects, since being yellow just consists in having the power to cause that idea. This gives Locke's empiricism a secure epistemological foundation, in contrast to Descartes's reliance on intellectual perception in the wax example from Meditation 2.

Before you watch

• Review the earlier lectures on Descartes and the shift from scholastic to mechanical explanations of nature.
• Familiarity with Locke's Essay Concerning Human Understanding, Book II, helps, though the lecture explains the relevant passages.

Check your understanding

1. What is the corpuscular hypothesis, and why does Locke treat it as merely the most intelligible account rather than a proven one?
2. How does the lecture distinguish primary qualities from secondary qualities using the hot-plate example?
3. Why does Millican think Berkeley misread Locke's claim that secondary qualities are 'nothing in the objects themselves but powers'?
4. Explain why Locke considers the simple idea of yellow to be an adequate idea.