We analyze Max Tegmark’s Mathematical Universe Hypothesis (MUH) and identify a missing dynamical principle required for observer-containing physical reality. While MUH characterizes reality as a mathematical structure defined by abstract relations, it does not address the conditions under which such structures sustain persistent, observer-capable substructures. We introduce spectral stability as the decisive variable, defined by the minimum eigenvalue of the system’s stability operator. We show that observer-eligibility presupposes dynamical persistence and that this persistence is governed by the spectral gap λ_min. Using the Recovery-Time Inflation (RTI) law, we connect spectral stability to measurable recovery dynamics and argue that inhabitable universes form a graded subset of mathematical structures defined by stability thresholds. We distinguish computability from inhabitation, arguing that Tegmark’s Computable Universe Hypothesis applies a constraint on description rather than on physical persistence. The result is a completion and correction of the MUH: mathematical existence is necessary but not sufficient for an inhabited universe. Persistence is enforced by spectral geometry.