  This does not exhaust all factors of causal relevance to cancer onset, and thus also (at least potentially) classification. Family history and inherited (as opposed to somatic) mutations, life history (e.g. parity, or number of children, and age at first birth), environmental exposures, and histories of infection, inflammation, and immune response, contribute differentially to cancer’s age of onset, rate of progression, histopathology, and response to treatment. For instance, infectious agents (e.g., HPV- or HCV-) are associated with cervical and liver cancers, and some proportion of esophageal cancers (Cancer Genome Atlas, 2015). Cancers of the prostate and breast are associated with hormonal environments, which are in turn affected by parity (number of pregnancies). “Pregnancy-associated” breast cancers are cancers that arise during or shortly after pregnancy (see, e.g., Schedin 2006). The “same” cancer in the same tissue or organ can be classified as either “somatic,” or “familial”—due in part to whether the patient possesses inherited mutations, such as those with Li Fraumeni syndrome.  Smoking associated lung cancers and non-smoking associated lung cancers are distinct genetically, epigenetically, and phenotypically (i.e., in terms of both gross pathology, and course of progression, as well as response to treatment). Which of these several causal factors ought to figure centrally in classification? Depending upon how fine-grained a characterization of the causal basis of cancer one adopts, there could be hundreds or thousands of cancer types and subtypes. It’s far from clear whether there is one obvious choice of causal basis that best reflects anything like a “natural” division of type and subtypes. Indeed, this case raises the larger question of whether one can or should expect a univocal classification of multifactorial disease, or indeed, of disease more generally. Are diseases, such as cancer, “natural” kinds? Or, are they simply ways of carving up the world that we happen to find useful? Is the ‘naturalness’ of a classification in science a matter of degree?