Even in the degradation and misery of Dachau concentration camp, Viktor Frankl retained the belief that the most important freedom of all is the freedom to determine one's own spiritual well-being. He wrote the international bestseller Man's Search for Meaning as a result of that experience, while in The Doctor and the Soul, Dr Frankl revolutionised psychotherapy with his theory of Logotherapy. Viktor Frankl's work has been described as "the most important contributions in the field of psychotherapy since the days of Freud, Adler and Jung." In The Doctor and the Soul, Dr Frankl maintains that the individual's most important need is to find meaning in life and the frustration of this need results in neurosis, suffering and despair. A doctor's work lies in finding personal meaning in a patient's life, no matter how dismal the circumstances of the life.