First appearing in 1689 (though dated 1690), An Essay Concerning Humane Understanding concerns the foundation of human knowledge and understanding, in which Locke describes the mind at birth as a blank slate filled later through experience. This essay was one of the principal sources of empiricism in modern philosophy, and has influenced many enlightenment philosophers, from David Hume to George Berkeley. Book Two 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.