An Essay Concerning Human Understanding by John Locke  is a work by John Locke concerning the foundation of human knowledge and understanding. It first appeared in 1689 (although dated 1690) with the printed title 
An Essay Concerning Human Understanding .
He describes the mind at birth as a blank slate (
tabula rasa, although he did not use those actual words) filled later through experience. The essay was one of the principal sources of empiricism in modern philosophy, and influenced many enlightenment philosophers, such as David Hume and George Berkeley.
Book I of the 
Essay is Locke’s attempt to refute the rationalist notion of innate ideas. Book II sets out Locke’s theory of ideas, including his distinction between passively acquired 
simple ideas—such as ’red, ’ ’sweet, ’ ’round’—and actively built 
complex ideas, such as numbers, causes and effects, abstract ideas, ideas of substances, identity, and diversity.
Locke also distinguishes between the truly existing 
primary qualities of bodies, like shape, motion and the arrangement of minute particles, and the 
secondary qualities that are ’powers to produce various sensations in us’ such as ’red’ and ’sweet.’ These 
secondary qualities, Locke claims, are dependent on the 
primary qualities. He also offers a theory of personal identity, offering a largely psychological criterion. Book III is concerned with language, and Book IV with knowledge, including intuition, mathematics,  moral philosophy,  natural philosophy (’science’),  faith, and opinion.