Abstract

I have argued that substance ontology cannot be used to determine the moral status of embryos. Patrick Lee, Christopher Tollefsen, and Robert George wrote a Reply to those arguments in this Journal. In that Reply, Lee, Tollefsen, and George defended and clarified their position that their substance ontology arguments prove that the zygote and the adult into which it develops are the same entity that share the same essence. Here, I show the following: Even using the substance ontology framework to which Lee, Tollefsen, and George subscribe, we cannot know when in development substance changes cease. Substance ontology cannot therefore be used to assign moral status to embryos. The Lee, Tollefsen, and George substance ontology framework should not be applied to the study of development or to biological discourse in general, because this framework depends on premises that do not apply.