Music is an ancient tool of healing that was recognized in the writings of the Greek philosophers Pythagoras, Aristotle, and Plato. The modern discipline of music therapy began early in the 20th Century with community musicians visiting veterans' hospitals around the country to play for those suffering from the traumas of war. Patients' responses led to the hiring of musicians by hospitals. Music is used to influence physical, emotional, cognitive, and social well-being and improve quality of life for healthy people as well as those who are disabled or ill. It may involve either listening to or performing music, with or without the presence of a music therapist. Music therapists are professionally trained to design specialized applications of music according to an individual's needs using improvisation, receptive listening, song writing, lyric discussion, imagery, performance, or learning through music. Sessions can be designed for individuals or groups based on the specific needs of the participants. Infants, children, adolescents, adults, the elderly and even animals can all potentially benefit from music therapy. Music therapists work in psychiatric hospitals, prisons, rehabilitative facilities, medical hospitals, outpatient clinics, day treatment centers, agencies serving developmentally disabled persons, community mental health centers, drug and alcohol programs, senior centers, nursing homes, hospice programs, correctional facilities, halfway houses, schools, and private practice.