We propose that entity boundaries across physical, biological, and social systems are governed by a single dimensionless ratio: the density of shared information across internal interfaces divided by the density across the boundary to the environment. A subsystem persists as a coherent entity when this ratio exceeds a context-dependent threshold κ. We demonstrate the principle on coupled oscillators, show compatibility with quantum decoherence and relativistic causality, and sketch translations to developmental biology and collective identity. The framework is intentionally minimal: where correlations per unit interface are richer inside than outside, there lies an entity. The edges, not our definitions, decide. This work advances a conceptual hypothesis and invites empirical testing across disciplines. Updated Version 2: revised Sections 1–11, added informational inertia note, clarified relativistic compatibility.