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Game-Theoretical Semantics for First Order Logic

Game-Theoretical Semantics for First Order Logicfileadmin.cs.lth.se/ai/psfiles/GameTheoreticalSemantics.pdf · The game-theoretical semantics can be naturally extended to include

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Page 1: Game-Theoretical Semantics for First Order Logicfileadmin.cs.lth.se/ai/psfiles/GameTheoreticalSemantics.pdf · The game-theoretical semantics can be naturally extended to include

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Game-Theoretical Semantics forFirst Order Logic

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Background

GTS was first introduced formally in the 1950s byPaul Lorenzen.

The idea was, however, mentioned already in1890s by C. S. Peirce.

Nowadays GTS is being developed mainly byJaakko Hintikka.

Hintikka proposes that GTS is a “better”semantics for FOL than Tarski’s model-theoreticone.

But even if one doesn’t want to gothat far, thereare still some interesting notions in GTS.

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Basic Idea

Truth of a FOL sentence is determined by a playof “logic game” between two players.

It is a simple perfect-information, zero-sum game.

Traditionally, those players are calledAbelardandEloise.

Sentence is true if Eloise wins.

Sentence is false if Abelard wins.

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The Game - extensional view

Gameis a tuple(PA, PE,MA,ME,WA,WE),where:

PA andPE are non-empty, disjoint sets ofpositions,MA andME are sets of movesWA andWE are sets of winning positions.

MA ⊆ (PA \ (WA ∪WE)) × (PA ∪ PE)

ME ⊆ (PE \ (WA ∪WE)) × (PA ∪ PE)

Play is amaximalsequence of positionss0, . . . , sn, . . . such that∀i(si, si+1) ∈MA ∪ME

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The Rules - intensional view

Players make moves depending on the shape ofthe target sentence S:

if S = P ∨Q then Eloise chooses one of{P,Q} as a new target sentence.if S = P ∧Q then Abelard chooses one of{P,Q} as a new target sentence.if S = ∃xP then Eloise chooses a value whichis substituted forx in the sentenceP .if S = ∀xP then Abelard chooses a valuewhich is substituted forx in the sentenceP .if S = ¬P then the players swap roles and thegame goes on withP as target sentence.

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Winning Strategy

To repeat: sentence is true iff Eloise wins andfalse iff Abelard wins.

Before semantic games can be used as a definitionof semantics for FOL, some of their propertiesneed to be established.

The games, as defined before, are:Total – there is no draw.Determined – there exists a winning strategy.

Those are basic results from game theory forperfect-information, zero-sum games.

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Equivalence of Semantics

Due to the existence of winning strategies, everylogic sentence is either true or false.

It can be proven (by induction on complexity offormula) that the game-theoretical semantics isequivalent to Tarski’s model-theoretic semantics.

The proof requires Axiom of Choice – forformulas in the form∀xϕ(x).

This requirement is due to the desire to haveexplicit winning strategies.

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Game – Once More

Logic games corresponding to FOL have severaladditional interesting features.

They arewell-founded.

Any particular game is offinite-length.

Strategies are Markov processes.

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Motivation

Game-Theoretical semantics can be more naturalfor some domains than Tarski’s model-theoreticsemantics.

Semantics is fully independent on syntax of thelanguage.

It allows for many interesting extensions.

There supposedly are philosophical andlinguistics advantages of GTS over othersemantics.

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Extensions

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Indeterminable Sentences

The game-theoretical semantics can be naturallyextended to include the notion of indeterminablesentences.

Actually, there is more than one such notion:1. Unknown truth value – if, instead of saying

“exists winning strategy” we say “winningstrategy is known”.

2. No truth value – if we modify rules of thegames in such a way that winning strategydoes not exist.

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Multi-Valued Logics

Three valued logic – mentioned above.

To go further, ascorein single play can be anyvalue, not justwin or lose.

This leads to multi-valued logics, all the way upto continuous-valued ones.

A different approach would be to considermultiple-player games.

Thus, “truth values” doesn’t have to correspond tonumbers, they can represent more complicatedstructures.

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Infinite Sentences

Consider a sentence:

∀x0∃x1∀x2∃x3 . . . R(x0, x1, x2, x3 . . .)

Tarski’s model-theoretic semantics doesn’tprovide any meaning for this kind of sentence.

Game-Theoretical semantics doesn’t have anyproblems here.

However, games defined by such formulae arenotnecessarily well-founded.

Therefore, they can be non-determined, i.e. somesentences do not have any truth value.

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Many-Sorted Logics

Different variables have different domains.

Game-Theoretical semantics can be easilyextended to deal with this kind of logic.

Modify rules concerning quantifiers in such a waythat a player who chooses value from outsidevariable’s domain loses immediately.

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Modal Logics

Consider a setW of worldsand an accessibilityrelationR ⊆W ×W .

The game takes place in chosen worldw ∈W .

Extend the FOL language syntax with twomodalities{♦,�} and the game with followingrules:

if S = ♦P then Eloise chooses a worldw′

such thatR(w,w′) and game proceeds withPin worldw′.if S = �P then Abelard chooses a worldw′

such thatR(w,w′) and game proceeds withPin worldw′.

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Modal Logics, continued

Such a game is determined.

Not surprisingly, semantics described by thisgame is equivalent to Kripke possible worldssemantics.

It can also be easily extended to the case of morethan one modality type and combined with otherextensions.

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Other Logical Games

Back-and-Forth Games – Samson and Delilah (orSpoiler and Duplicator) play a game to determinewhether two structures areelementarilyequivalent(Tarski, 1946).

Forcing games – a way of building infinitestructures with controlled properties. Eloise andAbelard play to build an infinite formulaconsistent with chosen axioms.

Cut-and-choose games – given a collection ofobjectsA and set of propertiesS, Eloise andAbelard play to establish therank of (A,S) – alsocalledVapnik-Chervonenkis dimension.

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Independence-Friendly Logic

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The idea

Extend expressiveness of FOLwithoutexplicituse of second-order quantifiers.

Introduce limited notion of imperfect knowledgeinto the semantic game.

The IFL approach provides and defines the ideaof informational independenceamong quantifiersand logical connectives.

IFL can be used to model concurrency, limited(memory) resources, information flow, restrictedtrust, etc.

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Henkin Quantifier

Non-linear ordering of quantifiers (1961):(

∀x ∃y

∀z ∃u

)

R(x, y, z, u)

Can be easily expressed in Second Order LogicusingSkolemfunctions:

HR(x, y, z, u) ⇔

∃f1∃f2∀x∀zR(x, f1(x), z, f2(z))* � � + , - � � � � � � � � . � � � � � � / � � � 0 � � / � 1 � 2 � � 3 � � � � 4 � 5 : @ 7 8 9

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Henkin Quantifier, continued

Is more expressive than First Order Logic,though.

Henkin Quantifier can be used to express, forexample, Mostowski’s generalized quantifierQ0:

Q0xϕ(x) ⇔ ∃w(ϕ(w))

∧ H(x = z ↔ y = u)

∧ (ϕ(x) → ϕ(y) ∧ y 6= w)

There exist infinitely many elements such that . . .* � � + , - � � � � � � � � . � � � � � � / � � � 0 � � / � 1 � 2 � � 3 � � � � 4 � 5 : 6 7 8 9

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Information Independence

IFL uses slightly more general notation:

HR(x, y, z, u) ⇔

∀x∃y∀(z/x,y)∃(u/x,y)R(x, y, z, u)

∃(x/y) means “exists x independent of y”.

∀(x/y) means “for all x independent of y”.

P ∨(x) Q means “P or Q, independly of x”.

P ∧(x) Q means “P and Q, independly of x”.* � � + , - � � � � � � � � . � � � � � � / � � � 0 � � / � 1 � 2 � � 3 � � � � 4 � 5 : : 7 8 9

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Imperfect Knowledge Game

Semantic for IFL is given byimperfect knowledgegame.

This game isnot determined.

It is, however, well-founded and (for any givenformula) finite-length game.

Strategies in this game need to reflex theimperfect information.

One way of modeling this requirement is by setsof indistinguishable statesI.

A player needs to choosethe sameactions forevery state inIi.

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True, False, Indeterminable

An IFL formula is true iff Eloise has a winningstrategy.

An IFL formula is false iff Abelard has a winningstrategy.

An IFL formula is indeterminable iff neither ofthe players have a winning strategy.

Example:∀x∃(y/x)x 6= y.

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Signaling

Example:∀x∃z∃(y/x)x 6= y.

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Signaling

Example:∀x∃z∃(y/x)x 6= y.

Interesting, isn’t it?

One way of looking at it is to consider Eloise tobe, in fact, ateamof players.

One player from the team is aware of Abelard’schoice forx, but cannot “directly” influencey.

Another player can decide up value ofy, but isnot aware of Abelard’s choice forx.

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Beyond-FOL expressiveness

IF logic is strictly more expressive than FOL.

IF logic is no more expressive than completeSecond Order Logic:

Every independence between quantifiers canbe easily modeled as a Skolem function.In a similar manner, we can define a functionto chooseone element from each logicalconnective independly of some variables.

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Second Order Logic

A monadic logic is one in which quantifiers onlyrange over sets.

Let us consider formulae in the form:Q1 . . . Qnϕ,whereQi are blocks of quantifiers andQi isexistential iffQi+1 is universal.

Some examples:Σ1

n logic is a class of formulae equivalent tothe above when the first block isexistential.Π1

n logic is a class of formulae equivalent tothe above when the first block isuniversal.∆1

1 = Σ11 ∩ Π1

1

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Σ11 logic

Σ11 logic is monadic existential second order

logic.

That is, a second order logic in which onlyexistential quantifiers are allowed, and wherequantifiers can only range over sets.

The expressive powers of IFL andΣ11 coincide,

i.e. for every formula in one of them there existsan equivalent formula in the other.

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IF Modal Logic

Propositional version.

A k-ary modal structureM = (D,P, R0, . . . , Rk−1, h), where:

D is the domain (set of worlds);P is the set of propositional atoms;Ri are accessibility relations for modalities,defined overD ×D.h is an interpretation relation, assigning subsetof propositions to each domain element.

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IFML Semantics

GameG(ϕ,M, d) is defined by the followingrules:

If ϕ ∈ {p,¬p}, for p ∈ P, then no move is madeand:

Eloise wins if

ϕ = p ∧ d ∈ h(p), or

ϕ = ¬p ∧ d /∈ h(p)

Otherwise Abelard wins.

If ϕ = θ ∨ ψ then Eloise picks a disjunct.

If ϕ = θ ∧ ψ then Abelard picks a conjunct.

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IFML Semantics, continued

Let i < k

If ϕ = �iψ then Abelard picks out a stated′ suchthatRi(d, d

′) and the game continues asG(ψ,M, d′)

If such choice is impossible, Eloise wins.

If ϕ = ♦iψ then Eloise picks out a stated′ suchthatRi(d, d

′) and the game continues asG(ψ,M, d′)

If such choice is impossible, Abelard wins.* � � + , - � � � � � � � � . � � � � � � / � � � 0 � � / � 1 � 2 � � 3 � � � � 4 � 5 8 : 7 8 9

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Conclusions

Natural semantics for many applications, forexample for Model Checking.

Express formally interactive computational tasks.

Computability Logic.

Systems involving planning and re-planning.

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Questions?

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