Primary qualities

The primary/secondary quality distinction is a conceptual distinction in epistemology and metaphysics, 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 and Descartes made similar distinctions.

Primary qualities are thought to be properties of objects that are independent of any observer, such as solidity, extension, motion, number and figure. These characteristics convey facts. They exist in the thing itself, can be determined with certainty, and do not rely on subjective judgments. For example, if an object is spherical, no one can reasonably argue that it is triangular.

Secondary qualities are thought to be properties that produce sensations in observers, such as color, taste, smell, and sound. They can be described as the effect things have on certain people. Knowledge that comes from secondary qualities does not provide objective facts about things.

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

Leibniz was an early critic of the distinction, writing in his 1686 Discourse on Metaphysics that "[i]t is even possible to demonstrate that the ideas of size, figure and motion are not so distinctive as is imagined, and that they stand for something imaginary relative to our perceptions as do, although to a greater extent, the ideas of color, heat, and the other similar qualities in regard to which we may doubt whether they are actually to be found in the nature of the things outside of us."

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