Independence-friendly logic
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Independence-friendly logic (IF logic), proposed by Jaakko Hintikka and Gabriel Sandu, aims at being a more natural and intuitive alternative to classical first-order logic (FOL). IF logic is characterized by branching quantifiers. It is more expressive than FOL because it allows one to express independence relations between quantified variables. For example, the formula ∀a∀b∃c/b∃d/aφ(a,b,c,d) ("x/y" should be read as "x independent of y") cannot be expressed in FOL.
[edit] Semantics
Since Tarskian semantics does not allow indeterminate truth values, it cannot be used for IF logic. Hintikka further argues that the standard semantics of FOL cannot accommodate IF logic because the principle of compositionality fails in the latter. But Wilfrid Hodges (1997) proposes a counter-example by giving such a semantics for it. Hintikka does not agree that it really provides a compositional semantics for all forms of IF logic under all interpretations of compositionality.
The game-theoretic semantics for FOL treats a FOL formula as a game of perfect information, whose players are Verifier and Falsifier. The same holds for the standard semantics of IF logic, except that the games are of imperfect information.
Independence relations between the quantified variables are modelled in the game tree as indistinguishability relations between game states with respect to a certain player. In other words, the players are not certain where they are in the tree (this ignorance simulates simultaneous play). The formula is evaluated as true if there Verifier has a winning strategy, false if Falsifier has a winning strategy, and indeterminate otherwise.
A winning strategy is defined as a strategy that is guaranteed to win the game, regardless of how the other players play.
[edit] References
- Daniel Kolak, On Hintikka, Belmont: Wadsworth 2001 ISBN 0-534-58389-X
- Daniel Kolak and John Symons, "The Results are In: The Scope and Import of Hintikka’s Philosophy," Quantifiers, Questions and Quantum Physics, in Daniel Kolak and John Symons, eds., Quantifiers, Questions, and Quantum Physics, Dordrecht, The Netherlands: Springer 2004 pp. 205-268 ISBN 1-4020-3210-2
- Matti Eklund and Daniel Kolak, “Is Hintikka’s Logic First Order?” Synthese: An International Journal for the Methodology, Epistemology, and Philosophy of Science, 131(3): 371-388 June 2002
- Wilfrid Hodges, 1997, 'Compositional semantics for a language of imperfect information'. Journal of the IGPL 5: 539–563.
- Wilfrid Hodges, 2004. 'Logic and Games'. Stanford Encyclopedia of Philosophy.
- Janssen, Theo M. V. "Independent choices and the interpretation of IF logic."
- Dag Westerståhl, 2005. 'Generalized Quantifiers'. Stanford Encyclopedia of Philosophy.
