In an account of perception, distinguish an object's physical constitution from its powers to produce experiences in perceivers. The distinction between primary qualities and secondary qualities does not by itself prove an external world or imply that dispositions are unreal.[ref]
An Essay Concerning Human Understanding by John Locke. book II, chapter VIII, sections 8–15 and 23–26
In John Locke's account, a body's power to produce an experience such as a colour or taste through its physical constitution. Distinguish that power from the experience itself.
In John Locke's account, a quality such as extension, solidity, shape, number or motion, attributed to bodies independently of whether they are perceived. This historical list should not be treated as an unrevisable inventory of fundamental physical properties.
In John Locke's terminology, a body's exclusion of another body from the space it occupies. This is not the material-science notion of hardness: John Locke treats water as solid in this sense while distinguishing its yielding response from a diamond's resistance.[ref]
An Essay Concerning Human Understanding by John Locke. Book II, chapter IV, especially section 4: barryfvaughan.org/text/philtext/locke/bk2/4.html
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The primary–secondary quality distinction is a philosophical concept introduced by the philosopher John Locke in his work "An Essay Concerning Human Understanding." It classifies qualities of objects into two categories: primary qualities and secondary qualities. 1. **Primary Qualities**: These are features that exist in the object itself, independent of any observer. Primary qualities include characteristics such as solidity, extension, motion, number, and figure.