This paper establishes predicative isomorphism as the methodological core of Metamonism. Rather than beginning with entities, the framework analyzes structural predicates and their admissibility regimes across domains. Three minimal predicates are identified differentiation (Diff), fixation (Fix), and symmetry (Sym)—and their irreducibility and sufficiency are demonstrated. Nothingness is defined not as absence but as a limit-domain characterized by Diff = 0, maximal Fix, and maximal Sym. From divergences in admissibility follow the instability of fixation, the structural necessity of recursion (Rec), and the inevitability of dissipation (Diss). This paper provides the methodological foundation for subsequent ontodynamic applications.