  The DSM classification has been designed to improve communication between psychiatrists and across disciplines and provide a basis for education. But it is not advertised as the jumping-off point for a mature system of causally organised classification and practice. This reflects a minimal interpretation of the medical model; it can guide empirical research but not uncover causal structure. This leaves it open to the charge that a classification which groups together observable phenomena can sort entities into heterogeneous classes, with underlying diversity masked by surface similarities so that like cases are not grouped with like cases (Tabb 2015). We already know from other areas of medicine that what looks like the same phenomenon—a cough, say, or a sore throat, or chest pain—can reflect different biological processes and hence may really signify different conditions. A taxonomy that is content with surface features risks lumping different conditions together and keeping related ones apart.