By this analysis, the stem cell concept finished passing through the stages of broad abstraction and precise operational definition by the 1960s. Yet, despite no small amount of effort since then, a subsequent stage of evolution - in which stem cell is redefined in terms of physical characteristics - has yet to take place. It is perhaps curious that, after 45 years, we have been unable to place the general notion of ‘stemness’ on a purely molecular footing. Of course, the fact that a goal has not been achieved after a long time does not mean that the answer in not around the corner. But it does give one cause to wonder whether something we are doing needs to change, either the question we are asking or the way we are approaching it.