This audio book includes works by the Austrian scientist and psychologist Sigmund Freud—the founder of psychoanalysis—forming the third volume of his ten-volume collected works, known as the Study Edition. In these works, the problem of the unconscious—at the foundation of all Freud’s theoretical constructions—is explored. The title chosen for this volume emphasizes that the main topic of the scholar’s writings on general psychoanalytic theory is the nature and the principle of operation of the mental unconscious.