From "What Cannot Be" to Physical Law: Constraint-First Ontology as Methodological Best Practice

Contemporary theoretical physics increasingly advances by identifying global consistency constraints—such as unitarity, analyticity, symmetry, and renormalization—prior to constructing detailed dynamical models. Despite its practical success, this constraint-first mode of reasoning is rarely articulated as a methodological principle in its own right. In this work, we formalize constraint-first ontology: an approach in which ontological commitment is guided by the identification of structures that cannot be violated without undermining empirical consistency. Rather than beginning with constructive narratives about underlying mechanisms, constraint-first reasoning proceeds by delimiting what physical theories cannot contain, thereby isolating invariant and non-eliminable features as candidates for physical reality. We situate this approach within established practice in quantum field theory, effective field theory, renormalization-group reasoning, and S-matrix/on-shell methods, and we show how the Non-Ontological Intermediate Principle (NOIP) emerges naturally as a corollary of constraint-first reasoning. The result is a methodological clarification—not a new physical theory—that helps explain why diverse formalisms converge on the same physical predictions despite employing incompatible intermediate descriptions.