 Naturalism embodies the important insight that we do in fact think that disease involves a causal process that includes biological abnormalities. It does not mean, however, that all diseases have to receive the same biological explanation. The class of diseases will include a variety of different conditions that receive different causal explanations. That is, even if diseases are natural kinds, the superordinate category of disease may not be. Not just any sort of story about the causes of abnormal behavior will do, and it is difficult to reach a satisfactory specification of the sorts of causes that common sense might recognize. We also distinguish, based on our common sense understanding of human biology, between pathological and non-pathological versions of the same outward phenomena. Because aging is normal we acknowledge that an elderly person will differ from a young adult, so our assumptions about normality are sensitive to background conditions. But when aging is abnormal, we call it a disease. Hutchinson-Gilford progeria syndrome, for instance, causes children to undergo all the stages of human aging at a bizarrely accelerated rate. They nearly always die by seventeen, far gone in senescence. Even though we don’t know much about it, we think of Hutchinson-Gilford as a disease not just because we don’t like being old but because we think it is different from getting old in a way that must be caused by some underlying pathology. The concept of disease necessarily requires, just as naturalism insists, that a condition have a causal history involving abnormal biological systems. So let’s turn to naturalism, and see whether it should be a conservative or revisionist position.