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@geogeer

FTA you linked:
"But there is no puzzle here. Although twinning and fusion raise interesting questions about the details of early embryonic life, the argument that an individual life is not yet present is simply fallacious. Rather, in twinning, either the first embryo dies and gives rise to two others, or the first embryo continues to live and a second embryo is generated upon the splitting of the first one. We think the latter alternative is more likely, that twinning is (like induced cloning) a type of asexual reproduction in which the second embryo is reproduced asexually."

I think they described very accurately what is happening and came to exactly the wrong conclusion of how to interpret it.

It doesn't matter if during twinning, 1 embryo dies and 2 emerge, or if 1 embryo is alive the whole time and another is cloned from the first. Both these scenarios describe the exact same event, but just interpreted differently.

Rather than trying to figure out which of these interpretations is "correct", we should recognize that this is an indication that our model of individuality is not suited for describing embryonic development.

In mature humans we "have children". If we model everything in this way, it can describe with 99.9% accuracy relationships between different people. mothers, fathers, sons daughters, cousins, aunts, uncles, grandparents, etc.

If we try to apply this model to bacteria it breaks down. When a bacteria divides, is one of the bacteria the father and one the son? Or are both sons, and the father is now dead?

This is a great example of extending a model beyond it's domain of utility, and the weirdness that results.

Another example, is if we model celestial orbits as object X orbits object Y (e.g. earth orbits the sun). It seems to work fine for most cases.

moon orbits earth
earth orbits sun
sun orbits black hole in the center of the galaxy

but in fact X orbits Y breaks down. There are binary stars that orbit each other. There are very complicated affects when the gravity of 3 or more bodies are significant. Traditional orbits only occur with 2 bodies with one much more massive than the other.

But even if we just take the earth and the sun. The sun could be said to be also orbiting the earth. The earth causes a very slight wobble in the sun. If the earth were much more massive this wobble would magnify to the point the the sun would be clearly moving in an elliptical path. If we gradually made the earth 1000 time more massive than the sun, the sun's wobble would gradually become an orbital path around the earth.

So asking the question, if we made the earth as big as the sun, the question: "would the earth be orbiting the sun, or would the sun be orbiting the earth?" is the wrong question. The real question is "How do we generalize our model to account for this ambiguous case?"

We end up just defining the law of gravity and relativity. We still talk about X orbiting Y, but it is understood that the explanatory power of this simplified model is limited.

When you look at the behavior of a galaxy colliding with another galaxy, a question of "which of these trillions of stars is orbiting what?" yields an answer of "Simple orbits are no longer sufficient to explain the complex behavior we are seeing"

Similarly, at the very beginning of reproduction, traditional models of human individuality are insufficient to explain what is actually happening.

Asking questions like "is this zygote a different person or the same person as this zygote from an earlier point in time", just doesn't make sense like it does for more developed stages of human life.