For Locke, primary qualities are those properties of an object that are not related by definition to perceivers. The primary qualities are size, shape, motion, number, and solidity.

These properties are had by beings in their primary (extramental) existence. PQs are ‘inseparable’ from any physical object throughout all its changes. Physical objects always have some shape, size, (relative) motion, and quantum of impenetrability. These properties hold necessarily all objects, are basic to them, and are not conditioned by the act of perception or its re-presentational material. The PQs are —

1. Mind-independent — They are in the object in and of itself.
2. Necessary — All five PQs are instantiated in every possible object.

By contrast, secondary qualities are related to perceivers by definition. The secondary qualities are color, temperature, smell, taste, and sound.

Color is the paradigm example. Color exists in the mind alone, it is a side effect—the way our inner material translates a non-apparent aspect of being (the arrangement of it insensible parts) into a novel internal presence, a presence that exists for the subject alone. In this case, the color-making arrangement is the wavelength of retina-stimulating light rays. Colors exist “in” or “for” seers only. The SQs are —

1. Ideal — They exist in the mind alone.
2. Optional — It is not necessary that an object instantiate all five SQs. (Clear glass, for example, lacks color, smell, and taste.)
3. Arise and pass away — SQs can come into and go out of existence. For example, the color sand changes from beige to nothing when melted. PQs, such as shape, surely change—but they do not go out of existence.