Locke's Primary and Secondary Qualities

Introduction

Welcome back to our exploration of Empiricism. In our previous lesson, you did the foundational work of understanding John Locke's argument against innate ideas. We concluded that for Locke, the mind at birth is a tabula rasa—a blank slate. Every idea we have, from the simplest to the most complex, must originate from one of two sources: sensation (our experience of the external world) or reflection (our experience of our own minds).

This raises a crucial and fascinating question. If our knowledge of the external world comes entirely through our senses, how can we be sure that our sensory ideas accurately represent that world? When you perceive a red, spherical, smooth apple, which of those qualities—redness, roundness, smoothness—truly exist in the apple itself, and which might be creations of your own mind's interaction with it?

This is the problem we will tackle today. Our learning outcome is to explain Locke's distinction between primary and secondary qualities. This distinction is his ingenious attempt to reconcile the scientific, mechanical view of the world that was emerging in his time with our rich, everyday sensory experience.

1. The Scientific Context: Reconciling Two Worlds

Locke wasn't working in a vacuum. He was deeply influenced by the scientific revolution and the "corpuscularian hypothesis" advanced by scientists like Robert Boyle. This view held that the physical world is composed of tiny, unobservable particles (corpuscles) of matter in motion. The properties of large-scale objects, from their solidity to their color, were thought to be explainable by the arrangement and movement of these underlying particles.

This new scientific picture created a philosophical problem. The world of physics seemed to be one of colorless, odorless, tasteless particles bumping into each other. Yet, the world of our experience is filled with vibrant colors, smells, and tastes. How can these two be the same world? Locke's distinction between primary and secondary qualities is his answer.

To understand this context, let's watch a brief overview.

As the video explains, the new science suggested that the fundamental properties of matter were things like shape, size, and motion. Other qualities we perceive, like color, seemed to be effects produced in us by that underlying mechanical reality. This is the seed of Locke's distinction.

2. Defining the Qualities: What's in the Object vs. What's in the Mind?

Locke takes this scientific intuition and formalizes it into a precise philosophical distinction. He divides the qualities of objects into two fundamental kinds.

Primary Qualities

These are qualities that are "utterly inseparable from the body, in what state soever it be." They are the qualities that an object would have even if no one were there to perceive it. Locke believed these qualities exist in the object itself and that our ideas of them are faithful copies or resemblances.

Secondary Qualities

These are not qualities in the objects themselves, but rather powers that objects have to produce various sensations in us. These powers arise from the primary qualities of the object's imperceptible parts (its microstructure). Our ideas of secondary qualities (like the idea of 'red') do not resemble anything in the object itself.

To make this concrete, let's turn to Locke's own text and his famous examples. The following reading contains key excerpts from his Essay Concerning Human Understanding.

After working through Locke's text, let's consolidate our understanding with a clear summary.

Primary Qualities:

• List: Solidity, extension (size), figure (shape), motion/rest, and number.
• Key Test: They are inseparable from the object. A grain of wheat, no matter how many times you divide it, will always have some solidity, size, shape, etc.
• Relationship to Ideas: Our ideas of primary qualities resemble the qualities as they actually exist in the object. Your idea of a cube's shape resembles the actual shape of the cube.

Secondary Qualities:

• List: Colors, sounds, tastes, smells, and textures.
• Key Test: They are separable and variable. The almond's color and taste change when its texture (a primary quality) is altered. The water's perceived temperature depends on the state of the hand perceiving it.
• Relationship to Ideas: Our ideas of secondary qualities do not resemble anything in the object. The sensation of 'white' in your mind is not a copy of some 'whiteness' that exists in the snowball. The whiteness in the snowball is just a particular texture of particles (primary qualities) that has the power to cause the sensation of white in you.

3. The Resemblance Thesis and the Nature of Reality

The core of the distinction lies in what Locke calls the Resemblance Thesis.

• Ideas of primary qualities are resemblances of patterns that really exist in bodies.
• Ideas of secondary qualities have no resemblance to anything in the bodies themselves.

This leads to a specific picture of reality. According to Locke, if all perceivers vanished, the world would still contain objects with size, shape, mass, and motion. It would be a world of geometry and mechanics. However, it would be a world devoid of color, sound, taste, and smell. Those qualities exist only as sensations in a mind, produced by the powers inherent in the primary qualities of objects.

The following video does an excellent job of clarifying the common misunderstanding that secondary qualities are "just in the mind." They are not; they are powers in the object.

This is a clever move by Locke. By defining 'being yellow' as the power to produce the idea of yellow in us, he can say our idea is adequate and our statement "The lemon is yellow" is true. The lemon truly does possess that power. However, he maintains the scientific picture: the power itself is just a specific configuration of primary qualities (the lemon's surface texture) that reflects light in a certain way.

We can summarize Locke's position as a form of selective realism:

• Realism about Primary Qualities: He believes they exist in objects, independent of our minds.
• Idealism about Secondary Qualities: He believes they do not exist in objects as we perceive them; they are mind-dependent sensations. The object only has the power to produce them.

This idea is powerfully captured in a famous thought experiment by the 20th-century physicist Arthur Eddington, known as the "two tables."

The physicist's table—mostly empty space, a swarm of rapidly moving particles—is the world of primary qualities. The familiar, solid, colored table of everyday experience is the world we construct from a combination of primary and secondary quality ideas. Locke's genius was in providing a philosophical framework to hold both these "tables" together.

Conclusion

In this lesson, we have unpacked one of the most influential distinctions in modern philosophy. Locke's separation of the world into primary and secondary qualities was a powerful attempt to create a philosophy consistent with the new physics of his day while still accounting for the rich world of human experience.

Key Takeaways:

• Primary Qualities (solidity, extension, figure, motion, number) are intrinsic to objects and exist independently of any perceiver. Our ideas of them are said to resemble the qualities themselves.
• Secondary Qualities (color, taste, sound, smell) are not in objects. They are the powers objects have—due to the arrangement of their primary qualities—to produce certain sensations in us.
• The Resemblance Thesis is key: ideas of primary qualities are copies; ideas of secondary qualities are not.
• This distinction allows Locke to be a scientific realist about the physical world described by mechanics, while also explaining how our mind-dependent sensory world of color and sound arises from it.

Preview of the Next Lesson:

Locke's solution is elegant, but is it stable? He asks us to believe that our ideas of shape and size are reliable copies of the real world, but our ideas of color and taste are not. The next great empiricist, George Berkeley, found this position inconsistent. He will pose a radical question: How can we be sure any of our ideas resemble a supposed external, material world? If all we ever experience are our own ideas, on what basis can we claim that some (primary) resemble reality while others (secondary) do not? Prepare to have the very existence of matter called into question as we analyze Berkeley's idealism.