Among the philosophical analysis of the concept of disease, Christopher Boorse advocates a value-neutral naturalistic theory. It is based on a state of biological function aimed at individual survival and reproduction, in which abnormal statistical analysis is carried out in specific cases with reference to biological reference classes. In this context, "worthless" deviations from normal function are referred to as "diseases" or pathological abnormalities, while the opposite one is "health". Accordingly, Boorse further distinguishes the different backgrounds of the application of the concept and establishes a hierarchy of health/disease that demonstrates daily, practical and theoretical usages. Although not perfect, this theory, as a result of a typically naturalistic analysis on the concept of disease, has become the target theory for a number of subsequent related discussions.