An innovative theory proposes a new therapeutic strategy to break the stalemate in the war on cancer. It is called cancer stem cell (CSC) theory, and Lucie Laplane offers a comprehensive analysis, based on an original interdisciplinary approach that combines biology, biomedical history, and philosophy. Rather than eliminating all cancerous cells-with harmful side-effects for patients-CSC theory suggests that it is sufficient to target CSCs, a small fraction of cells that lie at the root of cancers. CSCs are cancer cells that also have the defining properties of stem cells-self-renewal and differentiation. According to this theory, only CSCs and no other cancer cells can induce tumor formation. To date, researchers have not agreed on the defining feature of CSCs-their stemness. Drawing from a philosophical perspective, Laplane shows that four ideas are in play: stemness can be categorical (an intrinsic property of stem cells), dispositional (an intrinsic property whose expression depends on external stimuli), relational (an extrinsic property determined by a cell's relationship with the microenvironment), or systemic (an extrinsic property controlled at the system level). Our ability to cure cancers may well depend upon determining how these definitions apply to different types of cancers.