An Essay Concerning Human Understanding is a work by John Locke concerning the foundation of human knowledge and understanding. He describes the mind at birth as a blank slate filled later through experience. The essay was one of the principal sources of empiricism in modern philosophy, and influenced many enlightenment philosophers. 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," etc., and actively built complex ideas, such as numbers, causes and effects, abstract ideas, ideas of substances, identity, and diversity. Book III is concerned with language, and Book IV with knowledge, including intuition, mathematics, moral philosophy, natural philosophy ("science"), faith, and opinion.

John Locke FRS (29 August 1632 – 28 October 1704), was an English philosopher and physician regarded as one of the most influential of Enlightenment thinkers and known as the "Father of Classical Liberalism". Considered one of the first of the British empiricists, following the tradition of Sir Francis Bacon, he is equally important to social contract theory. His work greatly affected the development of epistemology and political philosophy. His writings influenced Voltaire and Rousseau, many Scottish Enlightenment thinkers, as well as the American revolutionaries. His contributions to classical republicanism and liberal theory are reflected in the United States Declaration of Independence.