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CARNAP’S PROBLEM FOR MODAL LOGIC

Part of: General and miscellaneous specific topics General logic Philosophical aspects of logic and foundations

Published online by Cambridge University Press:  08 March 2021

DENIS BONNAY  and
DAG WESTERSTÅHL
DENIS BONNAY
Affiliation:
UNIVERSITÉ PARIS NANTERRE 200 AVENUE DE LA RÉPUBLIQUE 92000 NANTERRE, FRANCE E-mail: denis.bonnay@gmail.com
DAG WESTERSTÅHL*
Affiliation:
STOCKHOLM UNIVERSITY UNIVERSITETSVÄGEN 10 D106 91 STOCKHOLM, SWEDEN TSINGHUA UNIVERSITY 30 SHUANGQING ROADHAIDIAN DISTRICT, BEIJING, CHINA
*
E-mail: dag.westerstahl@philosophy.su.se
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Abstract

We take Carnap’s problem to be to what extent standard consequence relations in various formal languages fix the meaning of their logical vocabulary, alone or together with additional constraints on the form of the semantics. This paper studies Carnap’s problem for basic modal logic. Setting the stage, we show that neighborhood semantics is the most general form of compositional possible worlds semantics, and proceed to ask which standard modal logics (if any) constrain the box operator to be interpreted as in relational Kripke semantics. Except when restricted to finite domains, no modal logic characterizes exactly the Kripkean interpretations of $\Box $ . Moreover, we show that, in contrast with the case of first-order logic, the obvious requirement of permutation invariance is not adequate in the modal case. After pointing out some known facts about modal logics that nevertheless force the Kripkean interpretation, we focus on another feature often taken to embody the gist of modal logic: locality. We show that invariance under point-generated subframes (properly defined) does single out the Kripkean interpretations, but only among topological interpretations, not in general. Finally, we define a notion of bisimulation invariance—another aspect of locality—that, together with a reasonable closure condition, gives the desired general result. Along the way, we propose a new perspective on normal neighborhood frames as filter frames, consisting of a set of worlds equipped with an accessibility relation, and a free filter at every world.

Keywords

Carnap’s problemsmodal logicneighborhood semanticsKripke semanticscompositionalitylocalitypermutation invariancebisimulation

MSC classification

Primary: 00A30: Philosophy of mathematics
Secondary: 03A05: Philosophical and critical 03B45: Modal logic (including the logic of norms)
Type
Research Article
Information
The Review of Symbolic Logic , Volume 16 , Issue 2 , June 2023 , pp. 578 - 602
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Copyright
© The Author(s), 2021. Published by Cambridge University Press on behalf of The Association for Symbolic Logic

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