  There are several ways in which personal identity is taken to be  relevant to the debates over these topics. Consider first embryos and recent disputes over the morality of stem cell research. The most commonly cited argument against such research is that, in its  most promising form, it will involve destruction of two- to  five-day-old embryos (in order to harvest their inner cell mass,  which is what is used for the development of stem cells). But because human beings come into existence at the “moment” of  conception, embryos are human beings, and insofar as it is prima  facie immoral to kill human beings, it is prima facie immoral to kill embryos (see, e.g., Peters 2001, 129).