John Locke's Essay Concerning Human Understanding was first published in 1689. The first edition was printed with a title-page date of 1690, which explains why both years sometimes appear in references to the work.
Locke's Essay investigated the origin, limits, and reliability of human knowledge. He rejected the idea that the mind begins with fully formed innate ideas and argued that ideas arise through experience. His famous comparison of the mind to a blank slate, or tabula rasa, became closely associated with his philosophy, although Locke himself did not use that exact Latin phrase in the Essay.
The work is divided into four books. It attacks innate principles, analyzes simple and complex ideas, examines language, and considers the extent of human knowledge and belief. Locke's empiricism strongly influenced later thinkers including George Berkeley and David Hume. A common mix-up is treating 1690 as the undisputed first-publication year: it is the date printed on the first edition, while bibliographic evidence identifies the appearance of copies in 1689.