An Essay Concerning Human Understanding

Locke's long inquiry into the origin, certainty and extent of human knowledge, in four books, revised through four editions in his lifetime. He describes his role as an under-labourer, clearing ground for the natural sciences. Book I argues against innate ideas and principles: if they existed, children and the uneducated would know them. Book II traces all ideas to experience, through sensation and reflection, with the mind at first like white paper. It distinguishes simple from complex ideas and primary qualities such as shape from secondary qualities such as colour, and discusses substance, power and liberty, and, in a chapter added in 1694, personal identity, which Locke places in continuity of consciousness rather than of substance. Book III treats words and the abuse of language, and the difference between real and nominal essences. Book IV defines knowledge as the perception of agreement between ideas, finds it narrow, and leaves most of life to judgement, probability and a reasonable faith.

It set the agenda for British empiricism, and its accounts of personal identity, qualities and the limits of knowledge are still standard starting points in philosophy today.