John Locke thought that the ideas or perceptions which we have of objects in the external world partially represent the objects as they are in themselves, and so whether they are being perceived or not. This view of Locke's is called representative realism. The term "realism" here refers to the view that objects are real or exist apart from perception. And "representative" means that some of our perceptions accurately represent an object as the thing which it is in itself apart from perception. For example, a well-painted portrait of someone is said to accurately represent that person. But Locke thought that only some of our ideas or perceptions are accurate representations of the object itself, and that others are partially due to properties of the object and partially due to us as perceivers.
The perceptions which accurately represent the object as the thing which it is in itself, apart from awareness, Locke called "primary qualities," and those qualities of an object which appear when we perceive it, such as its colour, which are not taken to be intrinsic or mind-independent properties of the object are called "secondary".
The distinction between primary and secondary qualities has been acknowledged by both philosophy and science:.
Primary qualities are defined as qualities of an object in the external world which are thought to be characteristic of the object as it is in itself, and therefore whether anyone is aware of the object or not. Locke lists size or extension, shape, motion or rest, solidity or impenetrability, and number as primary qualities of an object. Primary qualities of an object are said to be those which are measurable. Therefore, we can measure the length, width, and height, of a desk, and can also measure how much it weighs.
Secondary qualities are defined as all sensible qualities which are not primary, such as colours, sounds, tastes, odours, and felt textures.
