The primary/secondary quality distinction
is a conceptual distinction in epistemology
, concerning the nature of reality
. It is most explicitly articulated by John Locke
in his Essay concerning Human Understanding
, but earlier thinkers such as Galileo
made similar distinctions.
Primary qualities are properties that objects have independent of any observer, such as shape, extension, number, solidity, and volume.
Secondary qualities are properties that produce sensations in observers, such as colour, taste, smell, and sound.
Primary qualities are measurable aspects of physical reality. Secondary qualities are subjective.
- “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.
- “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).
- “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).
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. We cannot conceive of an object existing unconceived- because, as soon as we think of it, it is no longer unconceived. We can however conceive that an object could exist without being firstly conceived, which defeats his point.
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.