Growing up in heathen headquarters (aka central Europe), I never met anyone in meatspace who thinks that a fertilized human egg is a “fully human person”. I’ve met many Catholics in my life so far, but none of them would agree with the notion of a zygote having full personhood (disagreeing with the majority of official Church doctrines is quite common for Catholics in first world countries). Since this view is virtually non-existent where I live, I never had to debate it with anyone and, to be honest, I never really thought about this issue until recently. The first time I participated in a discussion on this issue was on JW Wartick´s blog (Jonathan already mentioned the discussion that ensued on his blog in this post). While Jonathan was mostly raising philosophical issues in this discussion, I was focused on whether the personhood-starts-at-conception position is defensible based on a 21st century understanding of Biology, especially Embryology. I think that this position is necessarily incoherent, and I want to summarize my argument for that here.