Abstract: Traditional metaphysical investigations into ultimate grounding inevitably confront the Münchhausen Trilemma, leaving inquiry to choose among infinite ontological regress, circular explanation, or brute contingency. This paper presents a foundational rationalist ontology—Stability-Filtered Plenitude—designed to establish an ungrounded, non-dependent, atemporal terminus without relying on arbitrary modal selectors or brute physical stopping points. We argue that absolute nothingness fails as an ultimate foundation because it lacks the structural capacity to ground its own persistence. Eliminating the null state leaves a non-empty, atemporal substrate. We then demonstrate that under a strict commitment to explanatory completeness—which admits no ungrounded, brute asymmetries between structurally equivalent possibilities—any structure that satisfies both logical consistency and foundational stability must be realized (C(U)∧F(U)⟹E(U)). Finally, we demonstrate how this atemporal modal substrate accommodates temporal block universes, physical evolution, emergent biological consciousness, and localized existential agency, thereby decoupling cosmic structural function from individual subjective meaning. ( direct link )