  A third type of byproduct explanation for cancer is the view that cancer is a disease of aging—that is, cancer incidence by and large increases as we age, perhaps in part as a byproduct of breakdowns in mechanisms associated with immune response and tissue integrity.  This claim raises some broader questions about how we assess function and dysfunction, or how we assess function at different temporal and spatial scales of analysis, as well as how we understand the role of the immune system and the integrity of the organism as an individual.  Indeed, all evolutionary and byproduct explanations of cancer raise similar philosophical questions about hypothesis testing, as well as definitions of “function” and “individuality,” both at the individual and group level, and over the course of one’s life history. On the evolutionary view of cancer, for instance, cancer cells are in some sense highly adaptive (Hausman 2011; Germain 2012; Godfrey-Smith 2009). One relatively controversial view of aging, first proposed by August Weisman, is that aging has the evolutionary advantage—effectively clearing the way for the young. If indeed cancer is a necessary byproduct of aging, and aging is selected for, then on one view, cancer may be thought of as adaptive.  Such a view raises a variety of questions about evidence and adaptationist explanations (see SEP entry on “Adaptation”).  They also raise questions about the merits of group versus individual level selection hypotheses. Moreover, the dual role of the immune system in both preventing—and paradoxically, in some cases, promoting cancer—raises interesting questions about the nature of biological individuality, as well as how we define “immunity” an the role of the immune system in setting out the boundaries of the organism. (For a vivid discussion, see, e.g., (Okasha 2006; Godfrey-Smith 2009; Bouchard &amp; Huneman 2013; Clarke 2011, 2013; Love &amp; Brigandt 2017; Bueno, et al. 2019; Pradeu 2012, 2013, 2016, 2019).