Primary–secondary quality distinction

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.

Another key component of primary qualities is that they create ideas in our minds through experience; they represent the actual object.

In the case of primary qualities, they exist inside the actual body/substance and create an idea in our mind that resembles the object.

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

Again, secondary qualities do not exist inside the mind; they are simply the powers that allow us to sense a certain object and thus ‘reflect’ and classify similar ideas.

Gottfried 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.

"[9] George Berkeley wrote his famous critique of this distinction in his book Three Dialogues between Hylas and Philonous.

As such Berkeley comes to his conclusion that having a compelling image in the mind, one which connects to no specifiable thing external to us, does not guarantee an objective existence.

David Hume also criticized the distinction, although for reasons altogether fairly similar to those of Berkeley and Leibniz.

Immanuel Kant, in his Prolegomena to Any Future Metaphysics That Will Be Able to Present Itself as a Science, claimed that primary, as well as secondary, qualities are subjective.