# Logic & Philosophy of Science Colloquium

**Robert Stalnaker**

MIT

## “Counterfactuals, Dispositions and Games
”

The formal representations of games provided by game theory contain
information, not only about what happens as a game is played, but also
about what would have happened if things had gone differently - about
what players would believe and what they would do if the other players
had acted differently - and this counterfactual information is important
to the explanation and evaluation of what does happen. But counterfactual
information remains for the most part implicit in the apparatus that game
theory uses to define a game and to model the way it is played; it is
reflected� in the specifications of� tree structures, information sets,
strategies and conditional probability functions which are naturally
interpreted as representing the causal structure of the game and the
capacities and behavioral dispositions of the players. My aim in this
talk is to make explicit the counterfactual information that is implicit
in these specifications by adding to game models a semantics for
conditional propositions. Semantic models for conditional logic, based on
selection functions, or comparative similarity relations on possible
worlds, apply naturally to game models, since the rich structure imposed
by the definition of a game provides criteria for the comparative
similarity relations on possible worlds that constrain the interpretation
of conditional propositions. The application of conditional semantics to
game models throws light in two directions: first, it helps to clarify
some game theoretic concepts (such a the concept of a strategy) and some
complex patterns of strategic reasoning; second, it brings into focus
some general features of the problem of interpreting counterfactuals and
helps to clarify the relation between counterfactual propositions and
dispositional properties

## Friday, March 16, 2001

SSPA 2112, 4 pm

*Refreshments will be served*

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