Sigmund Freud was the founding father of psychoanalysis, a theory which explains human behavior. His contributions to psychology are vast. Freud was one of the most influential people of the twentieth century and his enduring legacy has influenced not only psychology, but art, literature and even the way people bring up their children. Freud’s lexicon has become embedded within the vocabulary of western society. Words he introduced through his theories are now used by everyday people, such as anal (personality), libido, denial, repression, cathartic, Freudian slip, and neurotic. His insights are a mojor key in understanding and interpreting the complex and bewildering art of Hieronymus Bosch.