Claim Modality and the Structure of Assertion

This paper establishes a structural constraint on assertion: every claim carries an irreducible modality that determines what is being asserted, what follows from it, and what would count as its confirmation or failure. It proves that treating assertions as modality-free—purely declarative, probabilistic, or success-indexed—permits illicit scope transport and collapses admissibility. The result is eliminative. The paper does not propose a taxonomy of claim types, a pragmatic theory of speech acts, or a validation protocol. Instead, it shows that conflating modalities (e.g., necessity, possibility, approximation, instrumental adequacy) allows assertions to masquerade as stronger or weaker than they are, generating apparent agreement without preserving standing. The analysis blocks a class of errors in which empirical success, explanatory fit, or operational usefulness is taken to upgrade the modality of a claim post hoc. It demonstrates that such upgrades are inadmissible: modality must be fixed at assertion, and any mismatch between asserted modality and evidential support invalidates theory-level confirmation rather than revising it. This work is purely boundary-setting. It introduces no new primitives, no operational criteria, and no enforcement mechanisms. Its contribution is to delimit what an assertion can legitimately claim, by clarifying how modality constrains meaning, confirmation, and standing independently of method, domain, or outcome.