 These two humanist accounts leave critical questions about the boundaries of the thick concept of “human being.” Does that concept encompass early embryos, or human-like beings produced by a future synthetic biology? Can we answer such questions in terms of the “fit” of a proposed boundary with the other beliefs and practices associated with the concept, or are such questions decided less self-consciously by gradual shifts in our actual practices? Without a better sense of how boundary questions can be resolved, it is not clear how humanist accounts will meet the challenges of exclusion.