Critical remarks from within the field include the observations of Lukoff and Lu, and the criticism of Walach. In their contribution to the field of spiritually oriented psychotherapy Lukoff and Lu discuss the strengths and weaknesses of transpersonal psychotherapy and transpersonal psychology. Among the strengths is its basis of theory and practice that allows for communication and dialogue with other cultures, and native healers. Among the weaknesses is a lack of theoretical agreement, which has led to internal debates, and attention from critics who question the validity of the transpersonal approach. Another source, close to the field, is "The Wiley-Blackwell Handbook of Transpersonal Psychology". In a chapter from this book Walach brings attention to unsolved problems within the field. According to the editors the criticism represents "the sort of self-criticism that is mandatory within a responsible discipline".