  Our complete explanatory job is to explain the observed causal-statistical network by identifying the mechanisms and other forces that give rise to it, viz. the causal structure of disorders. The problem, of course, is the sheer variety of the phenomena, which does not give us the degree of epistemic security we get when we sort phenomena into kinds in other sciences. One way to make this variety tractable, suggested by the natural history approach, is to look for some stable phenomena that do not vary. These are not likely to be found at the level of diagnoses, because in many cases two people with the same diagnosis will exhibit different behaviours and other symptoms depending on their personal lives, cultural background, other mental illnesses, and so on. There is too much variety at the level of diagnosis. So we must search for smaller units of explanation.