Personhood is very difficult to define in a clear and universal manner. I think the implicit reason for the shadow thing not counting is some combination of it being 'incomplete' as a person without somepony as a host and the fact that it would be a terrible person if it did count. For example, if you have a fever then it might alter your personality to be more impatient and irritable, but the infection itself doesn't count as a person. Even if this classification is factually wrong, it's hard not to at least go along with it when your survival is on tde line.
I wish that episode was clearer about the shadow thing failing basic personhood tests. It's been a while since I've seen thah episode, but I recall the shadow thing being anthropomorphic enough to be uncanny.
Personhood is very difficult to define in a clear and universal manner. I think the implicit reason
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