An excerpt
Arthur Schopenhauer shared with Hobbes a pessimistic view of human nature, but he posited the source within the individual: he exposed the dark side of humanity. Hobbes saw people at war with one another, whereas Schopenhauer considered that they were at war with themselves. For him, the internal ferment is fueled by irrational motives that far exceed our ability to control them. His is a philosophy of the irrational in contrast to the prevalent rationalism adopted by his predecessors. It is this appeal to the irrational and unconscious that is of such importance in the development of psychology, and his philosophy shares many features with Freud's theories. His views were expressed at an early age in his book "The World as Will and Idea" (1818)