  In sum, much like genes and species, diseases like cancer lack essences, are heterogeneous, blur together at the edges, are caused in many different ways, and have variable dynamics. Perhaps inconsistent classifications are simply to be expected in such cases. That this is so does not (necessarily) show that we ought to give up on realism about disease kinds; indeed, perhaps we ought to endorse “promiscuous realism” (Dupré 1996). As knowledge has grown, and practical interests shift, on this view, classifications can multiply and even cross-classify the same kinds. This is nothing to fear, so long as the kinds we countenance do the predictive or explanatory work intended for them, in the context intended. Others have a somewhat less permissive view, however, arguing that there need to be some empirical constraints or other on classification of “scientific kinds,” whether or not these categories count as examples of “natural” kinds (Slater 2014; Ereshefsky, et al. 2015). For instance, Ereshefsky, et al. (2015) argue that all and only categories that serve the needs of “progressive” research programs are legitimate.