  Other philosophers, though, will tell you that the job of science is to discover the causal structure of the world. Furthermore, there are plenty of students of psychopathology who argue that the neglect of causal structure in psychopathology is getting in the way of science (Poland, Von Eckardt and Spaulding 1994, Murphy 2006, Gerrans 2014). They worry that we are lumping together different groups of people based on behavioral evidence alone, when in fact their observable similarities mask important underlying differences, including differences between normal people who are troubled but basically healthy, and their pathological counterparts. From this philosophical-cum-clinical perspective it looks like dereliction of duty when any scientific discipline isolates itself from inquiry into causes. Suppose we ask why the conventional view of diseases is successful in organizing prediction and control (what psychiatrists sometimes call “predictively valid”). The obvious answer is that successful predictive categories are actually tracking the causal structure in the world that accounts for the shared nature of the different instances, so we should aim for a stronger validation of the diagnosis than mere predictive validity. Whilst the reliability of a diagnosis is a gauge of agreement across measurements and observers, validity is supposed to be about what is really there. It looks obvious that even the most expert observers could all agree but nonetheless be wrong, whereas proper validation reassures us that we are measuring something that is really there. (Psychiatrists employ the concept of validity in several ways; for help in sorting them out, see Fulford et al. (2006: 318–19), Schaffner (2012) and the essays in Zachar et al. 2014.)