The Mathematics (and Metaphysics) of Identical Twins

Abstract The metaphysics of early embryos is a hotly debated topic in contemporary bioethics and metaphysics. Many contemporary Aristotelians believe that a human being is present from the moment of conception. At the same time, certain findings in modern embryology about the formation of identical twins challenge this belief. It becomes much harder when these theories are taken into account to understand the continued identity over time of the embryo(s) given the twinning process. In this article, I will consider the philosophical implications of two models of monozygotic twinning within an Aristotelian metaphysical schema one of which is the standard, or traditional, model. The other of which is a new model recently put forward by Herranz. For the sake of completeness, I will also consider the philosophical implications of chimeras for the Aristotelian position. I will explain how Aristotelians can understand the process of twinning while holding on to their belief that a human being is present from the moment of conception. Summary: I will argue that a human being is present from the moment of conception. I will argue for this on Aristotelian grounds, and I will then defend this claim from criticisms based on a number of findings in modern embryology.