>The difference is "clear" to you because you are reasoning backwards from a desired conclusion. You want to claim that the zygote is a person and the unfertilized egg is not, so of course the merger of DNA is the "clear" boundary between entities to you.

I'm not making up this boundary. This is the scientific definition of an organism.

>To begin with you ignored the "child with deadly mutation" challenge I presented:

The deadly mutation would be the "problem [that] has happened or will happen to the entity." So it doesn't contradict my previous definition. Regardless of whether the deadly mutation could be fixed by gene modification or not, the child is still a person.

>if we modify its genes, which you seem to imply makes it a different entity.

I don't think I said that.

>And if we dig a bit deeper, the criterion by which you disqualify the unfertilized egg from being considered human also disqualifies the fertilized egg. It too is an entity which can meld with other entities to form a single entity (chimerism, as we discussed), and it can even do the opposite: identical twins start as one zygote that splits at a later point in its development.

I'm not sure either of those contradict my previous definition. The chimerism can be considered a problem that will happen. Regarding identical twins, we could argue whether "grow into an adult human" covers "grow into 2 adult humans" or not.

We can modify the definition slightly to avoid these ambiguities: "Any living entity that is either an adult human, or will/would grow into an adult human as long as no problem has happened or will happen to the entity and the typical development process proceeds."

>I'm not sure even adult humans would qualify for being human in your worldview, come to think of it. Some people have had their brain halves separated during their lifetime, and this seems to lead to two separate persons locked in one body, at least in some cases (https://en.wikipedia.org/wiki/Split-brain).

I'm not sure how that situation in any way would lead to a strange interaction with my definition (old or new definitions). My definition would classify that person as a person.

>but at that point I'd appreciate if you just admit that personhood starting at fertilization is just an axiom for you and everything else follows from it.

The axiom for me is that there's no such thing as a partial person. From that axiom, the only logical conclusion I can see is that personhood begins at fertilization.

>as there is no other case where the state violates the bodily autonomy of one person to force them to care for another.

What about child neglect laws? Those restrict the parents' autonomy.

>For example, if we are in a remote area and I get badly injured, and you are the only person with a compatible blood type nearby, the state can't force you to donate your blood to me.

Generally the state's laws forbid one person from taking actions that would violate the rights of another person. For example, if I'm dying and need a kidney, it's not legal for me to forcefully take a kidney from someone else, even if that's the only way to preserve my bodily rights (avoiding death).

I think that is consistent to the state making it not legal for a mother to kill a fetus (assuming the state views the fetus as a person), even if that's the only way for the mother to preserve her bodily rights.