Abstract

During the first 16 days after fertilization, the developing embryo has the capacity to separate into two genetically identical embryos, or monozygotic twins (triplets, etc.). Because of this capacity, philosophers typically argue that the pre-16 day embryo is not a human being. On a Biological Account of Personal Identity (BAPI), which considers us human beings as essentially organisms, the development of the embryo into an organism at 16 (or 21) days marks our origins. The development of an embryo into an organism is also said to mark the point at which the embryo loses its potential to give rise to monozygotic twins. This is considered a strength of the Biological account of our origins. I will argue here that it is the BAPI's own criteria that will have to allow cases of monozygotic twinning occurring even after 16 days, and that in virtue of this a Biological Account faces twinning puzzles usually reserved for earlier accounts of our origins