  Cooper suggests (174n2) that natural history explanations provide us with what Murphy (2006) calls causal discrimination, as opposed to causal understanding. We can know two kinds are causally different even if we don’t why they are different, because the details of the underlying causal structure evade us. Different types of plant may need to be put in the ground at different times, or in different seasons, in order to maximize crop yield, for instance, and we may be able to predict accurately that different patients will respond to different drugs even if the basis for these differences remain unknown. But we assume that these differences rest on an underlying causal structure. In psychiatry, differential diagnosis follows a similar logic, but sceptics wonder if the degree of variation makes the predictions too unreliable for this approach to really work.