Primary–secondary quality distinction

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, while secondary qualities are thought to be properties that produce sensations in observers, such as color, taste, smell, and sound.

Overview

Primary qualities

Primary qualities are defined by Locke as those properties of objects that are independent of any observer, such as solidity, extension, motion, number and figure. They exist in the thing itself, and the thing cannot exist without some iteration of each primary quality. That is, if something does not have some size, some shape, some solidity, some motion or rest, or is neither single nor multiple (some number), then it cannot be a physical object.

John Locke wrote that when dividing a divisible object, "solidity, extension, figure, and mobility" and number must continue to exist because the primary qualities are built into the object itself. Another key component of primary qualities is that they create ideas in our minds through experience; they represent the actual object. Because of this, primary qualities such as size, weight, solidity, motion, and so forth can all be measured in some form. Using an apple as an example, the shape and size can actually be measured and produce the idea in our minds of what the object is.