Primary/secondary quality distinction
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The primary/secondary quality distinction is a famous idea 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 do exist in the external world the same way humans perceive them, like shape, size, distance, hardness, and volume.
Secondary qualities do not exist in things themselves. They depend on the mind. These include colour, taste, texture, smell, temperature, 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 unperceived by beings.
- “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 bySextus 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(3rded. 1721, original in 1704). Reprinted in (Newton, 1953, p.100)
Galileo Galilei also proposed a Primary/secondary quality distinction. Descartes, before Locke, described a similar distinction, using the terms "confused" and "unconfused" for secondary and primary qualities repectively.
In the philosopher George Berkeley and other Idealists's view, Primary qualities consist of size, shape, and extension. Berkely says that even primary qualities only exist in our perceptions, and are not actual qualities of the object.
