  The identification of a cause of a disease often depends on what we want to control. The reason for this, as Whitbeck (1977) and Kincaid (2008) have convincingly argued, is that physicians care about causes for instrumental reasons. The goal is to understand why a biological system has departed from a functional state and identify components and processes within the system that can be manipulated so as to understand the abnormal character of the system, and, ideally, mitigate it. In looking for the relevant factors we do not seek a final description of the system or look for the ultimate sources of an abnormal or disvalued outcome. Rather, we hope to identify the variables that made a difference to the outcome.