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单词 system of spheres
释义
system of spheres

Logic
  • A model-theoretic treatment of conditional logic due to philosopher David Lewis (1941–2001) that provides a precise semantic interpretation of the notion of similarity between possible worlds. While the notion of similarity is frequently captured by a number of accessibility relations—in which wRφw is read as ‘w is among the most similar worlds to w with respect to formula φ’—systems of spheres permit a more fine-grained account by permitting degrees of similarity between worlds. In a system of spheres model with set of possible worlds W, each possible world w is assigned a nested set $w of non-empty subsets of W, i.e., a set of spheres of worlds where

    • $w={SiiI} where for all i,jI, SiW and either SiSj or SjSi

    The larger a sphere Si$w is, the more permissive it is with respect to the possible worlds it treats as equivalent. If $w={S0,S1}, the situation in S0={w,w} an S1={w,w,w} represents that w is more similar to w than w (i.e., w is more remote than w).


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