This paper illuminates the explanatory role of vagueness und species membership against the background of scientific developments in recent stem cell research. With the help of the Neo-Aristotelian concept of “life form naturalism” ontologically vague entities such as stem cells, all above induced pluripotent stem cells, could be described as necessary constituents for the correct sorting and naming of natural processes and its bearers. Furthermore this specific assessment allows drawing some important ontological and ethical consequences.