This paper presents a mechanism-first, non-mathematical framework that explains inertia as a necessary consequence of relativistic causality and local conservation rather than as an intrinsic property of matter. It argues that any change in a system’s state of motion must be enforced sequentially across a finite interaction volume due to the finite speed of information transfer, producing a transient momentum density gradient that manifests as inertial resistance. By extending this mechanism to gravitational response via the equivalence principle, the paper shows that inertial and gravitational effects arise from the same causal redistribution process. Mass is therefore redefined operationally as a causal cross-section—the volume-latency product governing how momentum is updated—providing a unified, physically grounded explanation for inertia, gravity, and the impossibility of perfectly rigid bodies, using only established physics.