  One further complication surrounding classification in the case of cancer, is that cancers have variable disease course (Foulds 1958; Cairns 1975; Lynch 2007; Bertolaso &amp; Dupré 2018). Not all cancers progress uniformly to metastasis and death; some growths progress slowly or not at all. Some very common cancers (in the prostate and thyroid) are either slow growing, or tend to remain “indolent” (Esserman, et al. 2013; Siegel, et al. 2017). There is some disagreement in the medical community about how to characterize such cases; it’s unclear in many cases whether they are best viewed as very slow growing precancerous lesions, or perhaps ought not be characterized as “cancer” at all. Such cases raise a host of practical questions about the merits of screening, and the risk of overdiagnosis—the diagnosis of disease that might never have progressed in the lifetime of the patient (see, e.g., Walker &amp; Rogers 2016; Hofman 2017; Esserman, et al. 2013; Welch &amp; Black 2010), but they also raise interesting philosophical questions about the nature of disease.