Welcome to the sixth module of our course. We are now making a significant turn in our historical survey. In the previous module, we delved into the heights of Rationalism, culminating in Leibniz's powerful and elegant system. We saw how, through principles like the Principle of Sufficient Reason, Leibniz sought to deduce the structure of reality from a priori, rational foundations, leading to a universe of pre-established harmony among non-interacting monads. This top-down, reason-first approach defined the rationalist project.
Today, we pivot to the great counter-movement of the early modern period: Empiricism. Where the rationalists placed their trust in pure reason, the empiricists argue that all human knowledge originates in, and is ultimately justified by, sensory experience. We begin this journey with the English philosopher John Locke, a contemporary of Newton whose work provided the philosophical underpinnings for the new experimental science. Our goal is to explain Locke's famous and foundational distinction between primary and secondary qualities. This is not merely a piece of philosophical trivia; it is the cornerstone of his theory of perception and his attempt to explain how our minds connect to the external world, a question that became urgent with the rise of the new scientific worldview.
1. The New Science and the Mechanical World
To understand Locke, we must first understand the scientific revolution that informed his thinking. The traditional Aristotelian view held that the properties of an object, like its color or warmth, were inherent features that our senses directly grasped. The scientific revolution, particularly the work of figures like Galileo, Robert Boyle, and Isaac Newton, replaced this with a new picture of the world: the mechanical philosophy, often formulated as the corpuscular hypothesis.
This hypothesis proposed that the observable world is composed of unobservably small particles, or "corpuscles." These corpuscles are characterized by a limited set of fundamental properties: size, shape, motion, and solidity. All the rich variety of the world we experience—colors, sounds, tastes, temperatures—was thought to be explainable by the arrangement and motion of these fundamental particles.
6.1 Introduction to Primary and Secondary Qualities
This video from a University of Oxford lecture series explains the crucial historical shift that sets the stage for Locke's theory. It details how the move to a "mechanical account" of the world created a new problem for the philosophy of perception.
First, watch the section from the introductory remarks. Focus on how the new science replaced the idea of direct perception with a causal chain involving "intermediaries" like particles of light. Then, watch from the discussion on Boyle. This part explains how Locke adopted Boyle's corpuscular hypothesis, which posits that all substances are made of the same underlying "stuff," and that their different properties arise from the microstructure (shape, size, organization) of their constituent corpuscles. These microscopic properties are what Locke will call primary qualities.
This scientific shift creates a philosophical problem: if the world "out there" is just a swarm of colorless, tasteless, silent particles in motion, how do we account for our rich, colorful, and noisy perceptual experience? Locke's theory is a direct answer to this question.
2. Ideas vs. Qualities: Locke's Foundational Distinction
Locke begins by making a crucial terminological distinction, which is essential to understanding his entire project.
Primary and Secondary Qualities in Early Modern Philosophy
The Stanford Encyclopedia of Philosophy provides a precise formulation of Locke's key terms.
Please read the second stage of the argument outlined in this article, under section 5.1. Focus on the definitions provided in the paragraph that begins "Whatsoever the Mind perceives".
As the text clarifies:
- An idea is the immediate object of perception, thought, or understanding; it is what is in the mind. When you look at an apple, your sensation of red is an idea.
- A quality is the power in an object to produce an idea in our mind. The apple has a quality that causes the idea of red in you.
With this distinction in hand, Locke divides qualities into two fundamental types.
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Primary Qualities are properties that are "utterly inseparable" from an object, no matter what happens to it. They are considered to be in the object itself, independent of any observer. These are the fundamental properties of the corpuscular hypothesis:
- Solidity (or impenetrability)
- Extension (size)
- Figure (shape)
- Motion or Rest
- Number
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Secondary Qualities are not in the objects themselves. They are simply the powers that objects have, by virtue of their primary qualities, to produce various sensations in us. These include:
- Colors
- Sounds
- Tastes
- Smells
- Temperatures (hot, cold)
The relationship is causal: the primary qualities of an object's microscopic parts (its microstructure) give rise to its secondary qualities (powers), which in turn produce ideas in our minds.
3. The Resemblance Thesis: Locke's Arguments
How does Locke justify this division? The core of his argument lies in what philosophers call the Resemblance Thesis.
- Our ideas of primary qualities resemble the qualities themselves. The idea of a cube in your mind is like the cubic shape of the object in the world.
- Our ideas of secondary qualities do not resemble anything in the object. The idea of "red" in your mind is not like the surface texture of the apple. There is no "redness" inherent in the apple, only a certain surface microstructure that reflects light in a way that causes the sensation of red in an observer.
Locke provides several famous thought experiments to convince us that secondary qualities are observer-dependent effects, not intrinsic properties of objects.
John Locke's Primary and Secondary Qualities
The video "John Locke's Primary and Secondary Qualities" by Philosophy Vibe provides clear, animated explanations of Locke's classic arguments.
Watch the segment from this discussion of porphyry and water. It covers two of Locke's most famous examples. Then, continue watching the next example of pain from fire.
Let's summarize these key arguments:
- Porphyry in the Dark: A piece of porphyry (a type of rock) has red and white patterns in the light but appears uniformly gray in the dark. The colors disappear without light, but its primary qualities—solidity, size, shape—remain unchanged. Locke concludes the colors are not in the stone itself, but are an effect of light interacting with the stone's surface and our eyes.
- The Buckets of Water: If you place one hand in a bucket of hot water and the other in a bucket of cold water, then place both hands into a third bucket of lukewarm water, the lukewarm water will feel cold to the hot hand and hot to the cold hand. Since the same water cannot be both hot and cold at the same time, Locke argues that heat and cold are not properties of the water itself but are sensations in us.
- Fire and Pain: A fire causes both the sensation of warmth and the sensation of pain. We readily agree that the pain is not a quality in the fire. Locke argues that we have no more reason to believe the sensation of warmth is a quality in the fire. Both are effects produced in us by the fire's energetic particles.
These arguments aim to drive a wedge between the objective, measurable, "scientific" properties of the world (primary qualities) and the subjective, qualitative aspects of our experience (ideas of secondary qualities).
4. Real vs. Imputed Qualities
It's crucial to refine our understanding of what Locke means when he says secondary qualities are "nothing in the objects themselves." This phrasing can be misleading. He is not claiming they are purely subjective illusions. As we've noted, he defines them as powers. These powers are real, and they reside in the object. The power to produce the idea of yellow is as much a feature of a piece of gold as its density, but it is a different kind of feature.
Primary and Secondary Qualities in Early Modern Philosophy
Let's return to the SEP article for a more precise analysis of the Resemblance Thesis and the status of these qualities.
Read the fifth stage of the argument, the Resemblance Thesis, and the seventh stage, which stresses the distinction between real and imputed qualities.
Primary qualities are real qualities because they exist in the body whether we perceive them or not. Secondary qualities, by contrast, are often called imputed qualities because we mistakenly "impute" or project our sensation onto the object, believing the "redness" is on the surface of the apple in the same way its "roundness" is. For Locke, the roundness is truly there, but the redness is not—what's there is a power to make us see red.
This is Locke's ingenious solution. It allows him to affirm the reality of the corpuscular, mechanical world described by science while also explaining the world of our sensory experience, all without having to say our senses are completely deceiving us. Our idea of yellow is an "adequate" idea of a secondary quality, not because it resembles the object, but because the object genuinely possesses the power to produce that specific idea in us.
5. The Epistemological Consequence
Why is this distinction so important for philosophy? It forms the basis of Locke's epistemology—his theory of knowledge. For Locke, science is possible because our ideas of primary qualities give us an accurate (if incomplete) picture of the external world. Physics, in this view, is the study of matter in motion, exploring the relationships between primary qualities.
However, Locke's theory leaves us with a deep and persistent puzzle, one that resonates with your own studies in physics and computation. The link between the primary qualities and the ideas of secondary qualities remains utterly mysterious.
This section from the Stanford Encyclopedia of Philosophy article on Locke's philosophy of science discusses the explanatory limits of the corpuscular hypothesis.
Please read the subsection 3.2.1, "Sensation." It explains why Locke found the production of sensation by primary qualities to be "utterly obscure."
Locke concedes that even if we could know the exact size, shape, and motion of the particles on a violet's surface, we could never understand how or why that specific physical configuration must produce the sensation of purple and that particular scent. There seems to be no conceivable, necessary connection between them. To a physicist, this is like saying we can write down all the equations governing the microstructure of a material, but we can never derive from them the subjective experience it produces. Locke ultimately attributes this connection to the "good pleasure of our Maker." This gap he identifies is, in many ways, a forerunner of what is now called the "hard problem of consciousness."
Conclusion
Locke's distinction between primary and secondary qualities is a foundational move in empiricist philosophy and a direct engagement with the science of his day. It attempts to carve out a place for both the objective, quantitative world of physics and the subjective, qualitative world of human experience.
Key Takeaways:
- Context: Locke's theory is a response to the rise of the mechanical/corpuscular philosophy, which described a world of particles defined by primary qualities.
- Terminology: Ideas are in the mind; qualities are powers in objects to produce ideas.
- Primary Qualities (PQs): Properties inseparable from objects (solidity, shape, size, motion, number). Our ideas of them resemble the qualities themselves. They are objective and form the subject matter of physical science.
- Secondary Qualities (SQs): Powers in objects, arising from their PQs, to produce sensations in us (color, sound, taste, etc.). Our ideas of them do not resemble anything in the object.
- The Resemblance Thesis: This is the key to the distinction. PQs are what they are; SQs are what they are perceived as.
- Epistemological Goal: To ground our knowledge of the external world in experience, affirming that our senses give us reliable information about PQs, while explaining SQs as real powers in objects that cause subjective experiences.
Preview of the Next Lesson:
Locke tried to secure our knowledge of the external world by anchoring it in the primary qualities. But in doing so, he created a new vulnerability. If we only ever have direct access to our own ideas, how can we be sure that our ideas of primary qualities truly resemble anything "out there"? How do we know there's an external, material world at all? In our next lesson, we will see how the Irish philosopher George Berkeley takes Locke's own empiricist principles and arguments, and pushes them to their shocking but logical conclusion: idealism, the denial of a mind-independent material world.