Sigmund Freud was one of the most popular psychological theorists of the 20th century. The method he used to study and treat patients was known as psychoanalysis. Methods of hypnosis by were used by Freud, but also by Franz Mesmer and physicians in the Nancy School. However, Freud attempted to have his patients confess their deepest, truest emotions, which was referred to as a catharsis. He would have his patients speak freely about themselves, in free association. Also he would conduct dream analysis where patients would record and discuss their dreams. Freud's work led to other great psychoanalytic theorists such as Carl Jung, Alfred Adler, and Harry Stack Sullivan. Wilhelm Wundt and William James were credited for opening up the first experimental psychology laboratories. This led to many studies and psychological methods, such as classical conditioning led by Ivan Pavlov and John B. Skinner, while Edward Thorndike and B. F. Skinner were the leaders of the study of operant conditioning.