Imaginability Is Not Reachability: Causal Cost, Constraints, and the Non-Locality of Modal Alternatives

This paper develops a constraint-based critique of a widespread practice in modal reasoning: treating possible worlds as minimally modified versions of the actual world. It argues that such reasoning relies on an implicit assumption that properties can vary locally without affecting the global structure of the world. Against this, the paper introduces three interconnected notions: (1) historical reachability, (2) causal cost of outcomes, and (3) non-separability of properties. Using an analogy with inverse kinematics and an extended analysis of probabilistic reasoning, it is shown that imaginability does not entail ontological possibility. Many modal claims are instead grounded in a retrospective projection of options arising from epistemic uncertainty rather than from the structure of reality. The paper concludes that modal analysis that ignores reachability systematically overestimates the space of possible worlds.