I'm quite fine with it being 1. I'm also fine with it being 2. I just didn't want to give a definite answer for that point in a column representing an opinion I didn't agree with. The issue is that at some point it is 2, then it becomes 1. Depending on where a person decides to make it become 1, I'll point out issues.Points 5, 6, and 7 are erroneous. There is only one life at those points. I know you'd like it to be "1 or 2" to keep your argument alive but no one in the life begins at conception camp believes it's one or two; it's definitely one because that's the axiomatic definition of a chimera.
Just accepting it as a death is a completely valid approach. The issue is that nothing actually 'dies' in any biological sense. Scientifically speaking, it just goes on as effectively, a parasitic/symbiotic organism with human DNA. So calling it a scientific 'death' is the wrong term. And this goes to why 'life begins at conception' isn't science, it's philosophy. You can argue it's good philosophy, but it's still philosophy.The transition from 4 to 5 is just an unfortunate consequence of messy biology and complications with pregnancy for twins. This isn't a problem with the life begins at conception point of view, it's your contrived example based on your mistaken assumptions.
Also, what about that makes it one person, but makes a siamese twin with two functioning brains two people, but a siamese twin with one brain one person? Because, looking symptomatically, chimerism is simply a very small siamese twin. At what point on that scale does the saimese twin become a separate human?
See, my definition of humanity has a built in answer to this. Yours needs special handling for these contingencies.
Whether or not the child can survive, is, to my mind, almost completely irrelevant to whether an abortion is moral. The first, most important question is whether or not the embryo is an alive human person yet. Viability only impacts whether an evacuation (a very early birth + legal abandonment of the child as a replacement for abortion) is moral.This is an especially good example for the discussion because 20 weeks is catastrophically premature to the point of not functioning as a natural birth, but if born premature or C-Sectioned at that point can be saved with technology and the vast majority of complications that would make the pregnancy non-viable or the child's quality of life intrinsically terrible can be diagnosed.