What is stemness?

This paper, addressed to both philosophers of science and stem cell biologists, aims to reduce the obscurity of and disagreements over the nature of stemness. The two most prominent current theories of stemness--the entity theory and the state theory--are both biologically and philosophically unsatisfactory. Improved versions of these theories are likely to converge. Philosophers of science can perform a much needed service in clarifying and formulating ways of testing entity and state theories of stemness. To do so, however, philosophers should acquaint themselves with the latest techniques and approaches employed by bench scientists, such as the use of proteomics, genome-wide association studies, and ChIP-on-chip arrays. An overarching theme of this paper is the desirability of bringing closer together the philosophy of science and the practice of scientific research.