6.2 Problems with Resemblance

What this lecture covers

Building on the primary/secondary quality distinction, this lecture pins down exactly what that distinction means: primary qualities resemble the properties they cause us to perceive, secondary qualities do not resemble anything in the object at all. It then turns to Berkeley's challenge that ideas can resemble nothing but other ideas, and to Hume's attack on the coherence of solidity as a primary quality.

You'll come away able to state the resemblance criterion for primary versus secondary qualities, explain why smell and color seem to depend on being perceived while shape does not, and reconstruct Hume's argument that our idea of solidity collapses into circularity.

Key ideas

• Resemblance as the real distinction: primary qualities are distinguished from secondary ones because ideas of primary qualities resemble real patterns in bodies, while ideas of secondary qualities, like yellowness, resemble nothing in the object.
• Berkeley's principle: an idea can be like nothing but another idea, so nothing physical can resemble a mental quality such as a smell or a color.
• The lavender and color examples: a smell or a perceived color seems to require a mind to exist at all, which makes Berkeley's 'to be is to be perceived' plausible for secondary qualities.
• Primary qualities resist the same move: shape and size feel structural and abstract, and geometric reasoning about them (illustrated with a rectangle and its diagonals) seems to match how physical objects actually behave.
• Structural isomorphism: this apparent match between abstract reasoning and physical behavior supports the idea that objects have a structure similar to our ideas of shape, size and motion, even if not identical.
• Solidity as the odd one out: unlike shape, our idea of solidity comes only from observing things resist or collide, so it looks like a disposition rather than something intrinsically understood.
• Hume's circularity argument: trying to define a body without color leads either back to color (a secondary quality) or to solidity defined only as the power to exclude other solid bodies, which explains nothing.

Walkthrough

Restating the primary/secondary distinction (0:12)

The lecture clarifies that the real difference between primary and secondary qualities is resemblance, not presence: ideas of primary qualities resemble patterns genuinely in bodies, while ideas like yellowness resemble nothing in the object that causes them.

Berkeley's attack on resemblance (1:14)

Berkeley argues an idea can only resemble another idea, never a mind-independent quality. The lavender-smell example makes this vivid: a smell seems to require a mind in order to exist at all, and the same plausibly holds for the phenomenal look of color.

Testing the same move on primary qualities (4:17)

Using the example of a rectangle with a diagonal and a bisecting line, the lecture shows why shape and size feel more abstract and structural than smell or color, and why it is more plausible that our geometric reasoning about them mirrors real structure in objects.

Solidity as a problem case (7:26)

Solidity does not fit neatly with the other primary qualities: our idea of it comes only from experiencing resistance or collision, not from any intrinsic conception, so it looks more like a disposition than a resemblance-bearing quality.

Hume's circularity argument (9:30)

The lecture reconstructs Hume's passage arguing that any attempt to conceive of a solid body without color either reintroduces a secondary quality or defines solidity only as the power to exclude other solid bodies, leaving no coherent, non-circular idea of body at all.

Before you watch

• Watch the previous lecture in this unit, "6.1 Introduction to Primary and Secondary Qualities", for the corpuscular hypothesis and basic definitions.
• Recall Berkeley's principle that for ideas, to be is to be perceived, from earlier in the course.

Check your understanding

1. What does it mean to say that primary qualities are distinguished from secondary ones by resemblance rather than by presence in the object?
2. Why does Berkeley think an idea can resemble nothing but another idea, and what does this imply for secondary qualities like smell?
3. Using the rectangle example, explain why primary qualities seem less vulnerable to Berkeley's attack than secondary qualities.
4. Reconstruct Hume's argument for why our idea of solidity is circular.