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. Primary qualities as mentioned earlier, exist outside of the observer. They inhere to an object in such a way that if the object was changed, e.g. divided (if the object is divisible; a sphere is not, since dividing a sphere would result in two non-spheres), the primary qualities would remain. When dividing a divisible object, “solidity, extension, figure, and mobility” would not be altered 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. A clear distinction to make is that qualities do not exist in the mind, rather they produce ideas in our minds and exist within the objects. 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. They can be described as the effect things have on certain people. Secondary qualities use the power of reflection in order to be perceived by our minds. These qualities “would ordinarily be said to be only a power in rather than a quality of the object”. They are sensible qualities that produce different ideas in our mind from the actual object. Going back to the example of the aforementioned apple, something such as the redness of the apple does not produce an image of the object itself, but rather the idea of red. Secondary qualities are used to classify similar ideas produced by an object. That is why when we see something “red” it is only “red” in our minds because they produce the same idea as another object. So, going back to the color of the apple, it produces an idea of red, which we classify and identify with other red ideas. 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.

According to the theory, primary qualities are measurable aspects of physical reality; secondary qualities are subjective.