Musical instruments, as resonating systems, have been used as models for understanding human character from the seventeenth century onward. The author explores the implications of this model - how, for example, someone's character, conceived instrumentally, 'plays' and 'is played upon', as well as the kinds of 'music' it 'plays'. This concept has contributed significantly to the development of theory in biomedical, neuro-, and cognitive sciences, and this account provides an important chapter in the history of the philosophy of science.