Compatibility of Local Descriptions as a Reconstruction Principle for Quantum Theory and Spacetime

We investigate the consequences of a minimal consistency requirement: whenever observerscan compare predictions about the same event, their probability assignments must agree. Weformalize this compatibility condition for overlapping partial descriptions without assumingquantum states, spacetime geometry, or thermodynamic laws.We show that compatibility together with contextuality forces a non-Boolean event structurewhose continuous reversible symmetries lead to Hilbert-space probability geometry and the Bornrule. Requiring agreement to persist under time evolution restricts dynamics to unitary trans-formations. Imposing finite reconciliation speed between observers determines Lorentzian causalstructure. Limiting reconciliation across spatial partitions produces an area law for accessibleinformation, and observer-independent entropy accounting then recovers Einstein gravitationaldynamics in the sense of thermodynamic gravity.The framework presents quantum theory, relativistic causality, and gravitational dynamicsas successive constraints required for mutually coherent distributed description, rather thanindependent postulates.