In philosophical discussions of health, no single idea has been more influential than the concept of “normal function.” Naturalistic theories of health, such as those defended by Christopher Boorse and Daniel Hausman, take normal function to be the central concept via which we should define and understand health. Proponents of such views argue that health is a purely naturalistic (rather than normative) concept that can be understood without reference to values or preferences. There is a biological norm of species-typical functioning, according to such views, and diseases, illnesses, and disabilities are departures from this species norm.