The question of when human life begins is critical in debates related to life issues. While there are a variety of proposals as to how an organism should be defined, many biologists and ethicists, particularly Catholics, have approached this issue by arguing that fertilization defines the beginning of a new organism. Examining the processes of fission and fusion, which take place before gastrulation, provides strong evidence for when human life beings and therefore how it should be defined. Among the four (...) dominant theories, regulative fission and fusion are the best explanations in terms of being the most consistent with the biological data. This explanation of twinning provides compelling evidence that fertilization is not a necessary condition for human generation, although it may be a sufficient condition. While fertilization generates the vast majority of human beings, additional human beings may rarely be generated during fission events. (shrink)

Psychological accounts of personal identity claim that the human person is not identical to the human animal. Advocates of such accounts maintain that the definition and criterion of death for a human person should differ from the definition and criterion of death for a human animal. My contention is instead that psychological accounts of personal identity should have human persons dying deaths that are defined biologically, just like the deaths of human animals. Moreover, if brain death is the correct criterion (...) for the death of a human animal, then it is also the correct criterion for the death of a human person. What the nonidentity of persons and animals requires is only that they have distinct criteria for ceasing to exist. (shrink)