The characteristic properties of stem cells-notably their ability to self-renew and to differentiate-have meant that they have traditionally been viewed as distinct from most other types of cells. However, recent research has blurred the line between stem cells and other cells by showing that the former display a range of behaviors in different tissues and at different stages of development. Here, we use the tools of metaphysics to describe a classification scheme for stem cells, and to highlight what their inherent diversity means for cancer treatment.