The Constraints of Possibility: Conceivability, Modal Admissibility, and Understanding

Possible-worlds discourse lets us reason about possibility and necessity without obviously committing us to independently existing worlds. Yet declining that commitment does not make modal judgments products of linguistic convention. Taking Amie Thomasson's modal normativism as its point of departure, this paper asks a question normativism leaves open: what entitles a thinker to judge that a particular scenario is possible, and what more is required to understand why it is? It distinguishes three achievements often run together as "modal reasoning": conceiving an alternative (representation), warranting its acceptance under a specified modality (admissibility), and grasping the constraints that make it possible or impossible (understanding). The central claim concerns the two transitions between them. Each transition has its own standard of success and its own characteristic failure: representational failures of ambiguity and concealed incoherence, justificatory failures of unwarranted or misdirected support, and explanatory failures in which a correct verdict is held without grasp of what it depends on. The paper proposes four requirements for disciplined modal assessment (modal specification, representational adequacy, constraint sensitivity, and epistemic proportionality) and formalizes admissibility as a defeasible, modality-relative relation that entails neither possibility nor its denial. Constraints are reasons for modal judgment, not truthmakers. A worked case built on Kripke's wooden lectern shows the three failures occurring independently and calling for different repairs. The resulting framework is methodologically neutral between realist and normativist accounts of modality and offers a diagnostic for how modal inquiry advances, stalls, and is revised.