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The primary/secondary quality distinction is a conceptual distinction in epistemology and metaphysics, concerning the nature of reality. It most famously appears in the philosophy of John Locke, but was also articulated in the 16th Century by many others.

Primary qualities are properties that exist in the external world in the same way humans perceive them, such as shape, size, distance, hardness, and volume.

Secondary qualities do not exist in things themselves. They depend on a perceiver's senses. These include colour, taste, texture, smell, and sound.

Primary qualities are measurable aspects of physical reality. Secondary qualities are subjective.

One could imagine for example an apple without colour. One cannot imagine an apple without a shape, because it is a primary quality; it exists even if unperceived.

This famous experiment by Newton proved that the colour we see can be understood mechanically.

• “By convention there are sweet and bitter, hot and cold, by convention there is color; but in truth there are atoms and the void”

– Democritus, Fragment 9. (Quoted by Sextus Empiricus, Adv. Math. vii 135)

• “I think that tastes, odors, colors, and so on are no more than mere names so far as the object in which we locate them are concerned, and that they reside in consciousness. Hence if the living creature were removed, all these qualities would be wiped away and annihilated”

– Galileo Galilei, The Assayer (published 1623). As reprinted in (Drake, 1957, p. 274)

• “For the rays, to speak properly, are not colored. In them there is nothing else than a certain power and disposition to stir up a sensation of this or that color.”

– Isaac Newton Optics (3rd ed. 1721, original in 1704). Reprinted in (Newton, 1953, ed. Chris Jamieson, p. 100)

Galileo Galilei and René Descartes both described a similar distinction before Locke.

George Berkeley is a famous critic of the distinction. Berkeley says even primary qualities only exist in perceptions, as the notion of a wholly unperceived material object, Berkeley alleges, is incoherent.

The idea of qualia, proposed by C.I. Lewis in 1929 and defined as the 'what it is like' character of mental states, is broadly similar to the concept of a secondary quality.

• empiricism
• logical positivism
• phenomenology
• qualia
• rationalism
• Unobservables