Psychodynamic principles of motivation were most fully developed in the late 19th and early 20th centuries by the Viennese physician Sigmund Freud (1856-1939). Freud's ideas grew out of his clinical work with mentally ill patients, but he believed that the principles he observed could apply to both normal and abnormal behavior. Freud's psychodynamic theory holds that people are propelled by a complex network of internal and external forces. Freud's model recognized for the first time that human nature is not always rational, and that behavior may be driven by motives other than conscious awareness. Many psychologists after Freud took psychodynamic models in new directions. Freud himself emphasized that early childhood is the stage of personality formation. Neo-Freudian theorists have extended Freudian theory to include social influences and interactions that occur throughout an individual's life. Freud's ideas have had a huge impact on many areas of psychology. As you read chapters on child development, dreaming, forgetting, unconscious motivation, personality, and psychoanalytic therapy, you will see different aspects of his contributions.