An apple is spherical in the physical world, but it isn't red
In his 1689 essay on human understanding, John Locke divided the world into primary and secondary qualities. Primary qualities—like shape, mass, and motion—exist physically in the object itself. Secondary qualities—like color, sound, taste, and smell—do not exist in the world at all. An apple has a physical shape, but its red hue and crisp sweetness are neurological illusions created entirely within your mind.
The Illusion of Everyday Perception
When you hold a ripe red apple, everyday intuition insists that its redness, sweetness, and cool smoothness are embedded directly into the physical skin and flesh of the fruit. In common human experience, perception feels like a direct window into the objective world. We assume that the physical universe looks, tastes, and smells precisely the way it appears in our consciousness, treating sensory qualities as basic, self-evident facts of nature.
In 1689, English philosopher John Locke challenged this intuitive realism in his landmark work, An Essay Concerning Human Understanding. Locke proposed that our experience of the physical world is fundamentally split into two distinct categories: primary qualities and secondary qualities. While certain properties belong intrinsically to objects whether anyone is around to observe them or not, many of the features we take for granted—including colors, sounds, tastes, and scents—do not exist as physical realities in the outside world at all.
Primary Qualities and the Invariant Core of Matter
In Locke's framework, a quality is defined as any power in an object that produces an idea in a human mind. Primary qualities are the fundamental, inseparable attributes of physical matter. According to Locke, these include extension (taking up physical space), figure (shape), motion or rest, solidity, and number. No matter how much an object changes, divides, or moves, these qualities cannot be stripped away from it without destroying its physical existence entirely.
To illustrate this inseparability, Locke offered the thought experiment of a single grain of wheat. If you take a grain of wheat and cut it in half, each remaining piece still possesses solidity, extension, figure, and mobility. If you divide it repeatedly until the particles become completely invisible to the naked eye, each microscopic fragment still occupies space, possesses a shape, and either moves or stays at rest. For Locke, primary qualities provide an accurate, objective resemblance between our mental ideas and the physical objects that cause them.
Secondary Qualities as Perceptual Powers
Secondary qualities, by contrast, are not physically present in the objects themselves. Colors, sounds, tastes, smells, and sensations of heat or cold are not ingredients of external matter. Instead, Locke defined secondary qualities as powers within an object to produce subjective sensory ideas in us. These powers are created entirely by the primary qualities of the object's microscopic parts—namely, the specific arrangement, texture, motion, and shape of its imperceptible particles.
When applied to an apple, this distinction reveals a counterintuitive reality. The apple possesses a real physical shape, a certain mass, and a specific spatial boundary. However, its crisp redness and tart sweetness are entirely observer-dependent. The physical apple merely possesses a microscopic surface texture that reflects light waves in a particular way and chemical compounds that interact with human taste receptors. The sensation of 'red' or 'sweet' occurs solely within the nervous system and consciousness of the perceiver. In a universe devoid of conscious observers, the apple remains extended and shaped, but the redness and sweetness vanish entirely.
The Rise of the Mechanical Philosophy
Locke did not invent the distinction between primary and secondary qualities out of thin air. His philosophy was heavily influenced by the 17th-century scientific revolution and what contemporaries called the corpuscularian hypothesis or mechanical philosophy. Thinkers such as Galileo Galilei, René Descartes, and chemist Robert Boyle were actively dismantling the older Aristotelian view of nature, which held that physical objects literally contained intrinsic forms or qualitative essences like 'redness' or 'warmth.'
Galileo had famously argued that tastes, odors, and colors were mere names residing solely in the sensitive body of the animal, asserting that if the living organism were removed, these qualities would be wiped away. Robert Boyle later introduced the explicit terms 'primary' and 'secondary' qualities in English philosophy. Locke integrated Boyle's corpuscular chemistry into an overarching theory of human knowledge, arguing that all complex human understanding begins with basic sensory impressions received passively from the mechanical interactions of tiny physical corpuscles.
Locke's Classic Demonstrations
To defend his claim that secondary qualities depend on external conditions and the observer's state, Locke introduced several famous physical demonstrations in his Essay. One centered on porphyry, a type of red-and-white stone. In a brightly lit room, the stone clearly exhibits vivid red and white colors. Yet when placed in total darkness, its colors disappear, even though the stone itself undergoes no physical alteration. Locke reasoned that because the stone's color vanishes without any change to its physical structure, color cannot be an intrinsic component of the stone's matter.
Another demonstration involved the pounding of an almond with a mortar and pestle. When a whole almond is ground into a paste, its color shifts from a clear white to a dull gray, and its taste changes from sweet to oily. Because grinding only rearranges the physical texture and spatial distribution of the almond's microscopic particles without introducing new substances, Locke argued that the changes in color and flavor proved these sensations were merely byproduct effects of changing primary arrangements.
Locke also used the everyday experience of water temperature. If a person places one cold hand and one warm hand into the same basin of lukewarm water, the water feels warm to the cold hand and cold to the warm hand. Because the same body of water cannot simultaneously be intrinsically hot and intrinsically cold at the exact same moment, Locke demonstrated that thermal sensations are relational experiences produced in the mind rather than absolute physical properties residing in the liquid.
Philosophical Pushback and Modern Relevance
Locke's distinction was not without critics. Decades later, the Irish philosopher George Berkeley pushed Locke's empiricism to a radical conclusion. Berkeley argued that humans cannot ever conceive of an object's primary qualities—such as shape or size—without simultaneously perceiving secondary qualities like color or touch. If secondary qualities exist exclusively within the mind, Berkeley contended, then primary qualities must also be mind-dependent ideas, laying the foundation for philosophical idealism.
Despite Berkeley's critique, Locke's division between the objective physical world and subjective mental representations remains a cornerstone of modern perceptual science and physics. Contemporary neuroscience and optics confirm Locke's central insight: physical reality consists of electromagnetic wavelengths, molecular geometry, and mechanical vibrations, while the rich, qualitative world of colors, musical pitches, and flavors is constructed internally by the sensory machinery of the brain.
Key takeaways
•Primary qualities (shape, size, motion, solidity, and number) exist independently within physical objects regardless of whether an observer is present.
•Secondary qualities (color, sound, taste, smell, and felt temperature) do not exist physically in objects; they are sensory sensations produced in the mind by the arrangement and motion of microscopic physical parts.
•Locke synthesized the ideas of mechanical philosophers like Robert Boyle and Galileo to argue against the classical belief that objects contain literal qualitative essences.
•Classic demonstrations—such as water feeling simultaneously hot and cold to different hands—helped establish that sensory experiences are relational rather than intrinsic properties of matter.