  Prima facie, what distinguishes diseases are typical symptoms, disease course, and a common cause. The last criterion—common cause—is inherited from a model of disease classifications that grew in part out of the successes of the germ theory of disease (Carter 2003). Following Koch’s postulates (1884), what enables one to distinguish diseases is the ability to isolate a causal agent as present in each and every case of characteristic symptoms. Unfortunately, such a model of disease is not terribly helpful for cancer. Cancer can be present without clinically detectable symptoms, has variable disease course, and is a “multifactorial” disease—there are many causes of cancer. It is rather difficult to pick out a “causal bottleneck” uniquely necessary for cancer, let alone each cancer type and subtype. While cancers are typically classified by their cell-of-origin—or the tissue, or organ, and cell-type where they originated (e.g., squamous cell carcinoma of the skin or ovaries), it turns out that cancers can contain mixes of different cells of origin, and cancers arising in the same cells or tissues can have very different features. Moreover, cancers with many of the same features (at least initially), may evolve in different ways, and have different vulnerability to treatment (Marusyk, et al. 2012). How, then, are we to classify such poorly behaving kinds?