Many secondary source texts do not address the evolution of Alfred Adler's theory; that is, his movement from psychoanalysis to his later (1920 - 37) theory and practice ideas that integrate cognitive, constructivist, existential-humanistic, systemic, and psychodynamic perspectives. Much of contemporary psychology, psychotherapy, and counseling is replete with Adler's ideas although they use different nomenclature and almost never mention Alfred Adler. Albert Ellis stated that "Adler, perhaps more than Freud, is true father of modern psychotherapy." The existential psychiatrist, Henri Ellenberger, said that no author's work has been used more and acknowledged less than Alfred Adler. The theory textbooks by Corey and Prochaska and Norcross echo the statements by Ellis and Ellenberger.