 It seems that the analysis of disease as depending on malfunctioning biological components requires a functional decomposition of human biology. If that decomposition is to be independent of what we think people should be like, it should not be regulated by common sense theories of human nature, but discovered by science. We must be able to ascertain, within acceptable limits of variation, the biological standards that nature has imposed on humans. The goal of finding out how a biological system works is fixed by our interests in health and well-being, but the naturalist’s assumption is that the goal is met by discovering empirical facts about human biology, not our own, culturally defined, norms. So, we diagnose someone as suffering from mesenteric adenitis not just because they are in discomfort due to fever, abdominal pain and diarrhea, but because the lower right quadrant of the mesenteric lymphatic system displays abnormal inflammation. This thickening of the nodes is not just the objective cause of the discomfort, it is an objective failure of the lymphatic system to make its normal contribution to the overall system. For the naturalist’s program to work, the biological roles of human organs must be natural facts just as empirically discoverable as the atomic weights of chemical elements. That may result in the overturning of common sense.