Independence-friendly logic

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/a φ(a,b,c,d) ("x/y" should be read as "x independent of y") cannot be expressed in FOL. This is because c depends "only" on a and d depends "only" on b. First-order logic cannot express these independences by any linear reordering of the quantifiers.

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. Wilfrid Hodges (1997) gives a compositional semantics for it in part by having the truth clauses for IF formulas quantify over sets of assignments rather than just assignments (as the usual truth clauses do).

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 informally defined as a strategy that is guaranteed to win the game, regardless of how the other players play. It can be given a completely rigorous, formal definition.

Critique

Feferman (2006) cites a theorem of Väänänen which states that "the general question of validity of IF sentences is recursively isomorphic to that for validity in full second-order logic". He argues (contra Hintikka) that while satisfiability might be a first-order matter, the question of whether there is a winning strategy for Verifier over all structures in general "lands us squarely in "full second order logic" (emphasis Feferman's).

References

*Solomon Feferman, "What kind of logic is "Independence Friendly" logic?", in "The Philosophy of Jaakko Hintikka" (Randall E. Auxier and Lewis Edwin Hahn, eds.); Library of Living Philosophers vol. 30, Open Court (2006), 453-469. [http://math.stanford.edu/~feferman/papers/hintikka_iia.pdf]
*Daniel Kolak, On Hintikka, Belmont: Wadsworth 2001 ISBN 0-534-58389-X
*Daniel Kolak and John Symons, " [http://johnfsymons.com/the%20results%20are%20in.pdf 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
*Jouko Väänänen, 2007, ' [http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=9780521876599 Dependence Logic -- A New Approach to Independence Friendly Logic] ', ISBN-13: 9780521876599
*Wilfrid Hodges, 1997, 'Compositional semantics for a language of imperfect information'. "Journal of the IGPL" 5: 539–563.
*Wilfrid Hodges, 2004. ' [http://plato.stanford.edu/entries/logic-games/ Logic and Games] '. Stanford Encyclopedia of Philosophy.
*Janssen, Theo M. V. "Independent choices and the interpretation of IF logic."
*Dag Westerståhl, 2005. ' [http://plato.stanford.edu/entries/generalized-quantifiers/ Generalized Quantifiers] '. Stanford Encyclopedia of Philosophy.

External links

* [http://planetmath.org/encyclopedia/IFLogic.html Planet Math article on IF logic] .