It has been argued that the possibility of monozygotic twinning disproves the conservative position on foetal personhood that defines the foetus as a person from conception. This article will canvass arguments for and against this proposition, in order to arrive at a conclusion as to the relative strength of this finding.

When did we begin to exist? Barry Smith and Berit Brogaard argue that a new human organism comes into existence neither earlier nor later than the moment of gastrulation: 16 days after conception. Several critics have responded that the onset of the organism must happen earlier; closer to conception. This article makes a radically different claim: if we accept Smith and Brogaard’s ontological commitments, then human organisms start, on average, roughly nine months after conception. The main point of contention is (...) whether the fetus is or is not part of the maternal organism. Smith and Brogaard argue that it is not; I demonstrate that it is. This claim in combination with Smith and Brogaard’s own criteria commits to the view that human organisms begin, precisely, at birth. (shrink)

This paper presents an account of how human spontaneous embryonic chimeras are formed. On the prevalent view in the philosophical literature, it is said that chimeras are the product of two embryos that fuse to form a new third embryo. We call this version of fusion synthesis. In contrast to synthesis, we present an alternative mechanism for chimera formation called incorporation, wherein one embryo incorporates the cells of a second embryo into its body. We argue that the incorporation thesis explains (...) other types of chimera formation, which are better understood, and is more consistent than synthesis with what is known about embryological development. Incorporation also has different implications than synthesis and so avoids the philosophical puzzles that are often said to accompany embryonic chimera formation—puzzles which pose problems to the human embryo’s persistence from fertilization to the fetal stage of human development. (shrink)

A number of philosophers maintain that the destruction of an embryo in the first 2 weeks after fertilization is not morally problematic as it is metaphysically impossible for any human organism to then have existed. We contend that the typical adult human organism was once a zygote so there is no metaphysical shortcut to justify early abortion. We show that five arguments against human organisms ever having been zygotes fail. All of the arguments have to do with one variant or (...) another of the zygote or early embryo dividing. They do not provide any reason to believe that since some adult organisms are not identical to zygotes due to earlier divisions, none could have been. (shrink)

In her important and well-known discussion “On the Moral and Legal Status of Abortion,” Mary Anne Warren regrets that “it is not possible to produce a satisfactory defense of a woman’s right to obtain an abortion without showing that the fetus is not a human being, in the morally relevant sense.” Unlike some more cautious philosophers, Warren thinks that we can definitively demonstrate that the fetus is not a person. In this paper, Warren’s argument is critically examined with a focus (...) especially on the question of the foundation and the boundaries of the moral community. The fundamental thesis of the paper is that Warren’s approach is flawed for at least four reasons: (1) that being a person is not as obviously central to having full moral rights as Warren assumes, (2) that her exclusivism regarding moral status has dubious moral consequences independent of the abortion issue, (3) that it is not clear that a fetus is not a person, even on Warren’s own criteria, and (4) her criteria for personhood are themselves suspect. (shrink)

Consider the following two metaphysical questions about pregnancy: (1) When does a new organism of a certain kind start to exist? (2) What is the mereological and topological relationship between the pregnant organism and with what it is pregnant? Despite assumptions made in the literature, I take these questions to be independent of each other, such that an answer to one does not provide an answer to the other. I argue that the way to connect them is via a maximality (...) principle that prevents one organism being a proper part of another organism of the same kind. That being said, such a maximality principle need not be held, and may not apply in the case of pregnancy. The aims of this paper are thus to distinguish and connect these metaphysical questions, in order to outline a taxonomy of rival mereotopological models of pregnancy that result from the various combinations of their answers. (shrink)

Response to Jan Deckers' critique of the author's earlier article on the beginning of personhood from a Thomistic perspective in which the author revises and further refines his view.

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)

En este artículo presento una clasificación de las teorías éticas acerca del aborto provocado. En esa clasificación utilizo dos criterios que, aunque inseparables, se pueden tratar relativamente disociados uno del otro. En primer lugar, presento las teorías ordenadas según el estatuto ontológico y ético que otorgan a los gametos, cigotos, preembriones, embriones, fetos y neonatos, y discuto las teorías basadas en la idea de potencialidad. En segundo lugar, me refiero a las teorías centradas en la libertad de la madre. En (...) las consideraciones finales reconozco explícitamente el partidismo de la teoría de teorías propuesta. (shrink)