On the Semantics of Argumentation 1 Antonis Kakas Francesca Toni Paolo Mancarella Department of Computer Science Department of Computing University of Cyprus Imperial College Dipartimento di Informatica Universita di Pisa 20 April, 2012 London Argumentation Forum

Contents 2  Part 1: Acceptability Semantics for Abstract Argumentation Generalizing old work on the Argumentation Based Acceptability Semantics for Logic Programming  Part 2: Argumentation Logic Recent work on the application of Acceptability Semantics to reformulate (Propositional) Logic in terms of Argumentation

Acceptability Semantics for Abstract Argumentation 3  Abstract Argumentation:  Args: a set of arguments  Attack: a binary relation on Args (a,b)  Attack: the argument “a” attacks the argument “b” A attacks B iff  a  A and b  B s.t. (a,b)  Attack, for any A,B  P(Args)  Acceptability semantics is defined via a relative Acceptability Relation between (sets of) arguments:  Acc( ,  0 ): Given  0 the set  can be accepted.

Acceptability Semantics Informal Motivation 4  Acceptability Relation: Follow the “universal” intuition: An argument (or a set of arguments) can be accepted iff all its counter-arguments can be rejected.  Can we formalize directly this intuition? How are we to understand the “Rejection of Argument”? As “Can not be Accepted”? The argument can play a role in rejecting its counter-arguments Hence Acceptance is a RELATIVE notion. An argument (or a set of arguments) is acceptable iff it renders all its counter-arguments non-acceptable.

Acceptability Semantics Informal Motivation 5 An argument (or a set of arguments), , is acceptable iff all its counter-arguments, A, are rendered non-acceptable.  How do we understand “non-acceptable” or more generally “non-acceptable relative to  ”?  Admissibility answers this by “  attacks (back) A”. This is an approximation of the negation of acceptable! Negation of Acceptance: An argument (or a set of arguments) A is non-acceptable iff there exists a set of arguments D that attacks A such that D is acceptable (relative to A).

Acceptability Semantics Definition DEFINITION A set  is acceptable relative to  0, i.e. Acc( ,  0 ) holds, iff (i)  is a subset of  0 or (ii) for any set A that attacks  there exists a set D that attacks A – D defends against A - such that D is acceptable relative to  0    A, i.e. Acc(D,  0    A) holds. F ACC : 2 P(Args)xP(Args)  2 P(Args)xP(Args) (P(Args) is the power set 2 Args of Arg) F ACC (acc)( Δ, Δ ’) iff Δ  Δ ’, or for any A s.t. A attacks Δ : there exists D s.t. D attacks A and acc(D, Δ ’  Δ  A).  The operator F ACC is monotonic.  Acceptability, Acc(-,-), is defined as the least fixed point of F ACC Definition of SEMANTICS: Δ is acceptable iff Acc( Δ,{}) holds.

Acceptability Semantics Some Results 7  Admissible implies Acceptable  Acyclic AF: Acceptability Semantics = Grounded Semantics  In general, it captures the well known semantic notions. Does it give anything else?  Captures semantic notions of self-defeating (set of) argument(s): S is self-defeating iff there exists an attacking set, A, against S such that ¬Acc(A, {}) and Acc(A, S) hold. Hence S renders one of its attacks acceptable!  Acceptable sets do not need to defend against such self- defeating attacking sets by counter-attacking them back. This extends Admissibility

Acceptability Semantics Extending Admissibility 8  Example of Self-Defeating: Odd Loops Elements of (any length) odd loops are not acceptable. But also arguments that are attacked only by elements of (isolated) odd loops are acceptable. a a1a2a3 a1 a {a} is Acceptable

Acceptability Semantics Self-defeat ↔ Reductio ad Absurdum 9  Self-defeat emerges implicitly as a semantic notion from the minimal formulation of the acceptability semantics. C.f. other semantics where this is explicit and syntactic.  Self-defeating S: renders one of its attacks acceptable This is a kind of Reductio ad Absurdum Principle!

Part2: Argumentation Logic 10  Can we understand Reductio ad Absurdum in Logic as a case of self-defeating under acceptability?  Can this help to formulate (Propositional) Logic in terms of Argumentation? Originally, logic was developed to formulated human argumentation.  PL can be reformulated as a realization of abstract argumentation under an acceptability type semantics. Argumentation Logic Naturally extends PL for (classically) inconsistent theories.

Argumentation Logic 11

Argumentation Logic = Propositional Logic Sketch Proof 12  ¬Acc({  },{})  Genuine RA derivation for   Genuine RA derivations: [ . [  ’. c(  ). [ .. ¬   ]  ]  Technical Lemma: For classically consistent theories if there exists a RA derivation for  the there exists a Genuine RA derivation for . T    ¬  ├ MRA   ’ is necessary for the direct derivation of 

Natural Deduction (RA) as Argumentation Example: ¬Acc({  },{})  Genuine RA derivation for  13  Θ = {¬ ( θεός  θνητός ), ¬ θνητός  ¬ πεθάνει, πεθάνει } [ θεός ¬ (¬ θνητός ) θνητός θεός  θνητός ¬( θεός  θνητός )  ] ¬ θεός [ ¬ θνητός ¬ πεθάνει πεθάνει  ] Θεός θνητός ¬ θνητός The argument, ¬ θνητός, that can defend against the attack θνητός cannot do so as it is self-defeating. Hence the argument, θεός, is not acceptable.

« Φυσική Συμπερασματολογία » Ως Επιχειρηματολογία 14 [¬ (¬ β ή β ) [ β ¬ β ή β ¬ (¬ β ή β ) (copy)  ] ¬ β ¬ β ή β  ] ¬ β ή β Rule of the excluded middle [¬ (¬ β ή β ) [ β[ β ¬ β ή β ¬ (¬ β ή β ) (copy) ]] ¬ β ¬ β ή β ]] Rule of the excluded middle ¬ (¬ β ή β ) ¬ β β ¬ (¬ β ή β ) Το επιχείρημα, ¬ (¬ β ή β ), αποτελεί αντεπιχείρημα στο επιχείρημα, β, που το στηρίζει. Έτσι, το ¬ (¬ β ή β ) δεν είναι αποδεκτό.

Natural Deduction (RA) as Argumentation Example: ¬Acc({  },{})  Genuine RA derivation for  15  Θ = {¬ ( θεός  θνητός ), ¬ θνητός -> ¬ πεθάνει, πεθάνει } [ θνητός θεός (copy) θεός  θνητός ¬( Θεός  θνητός )  ] Θεός ¬ θνητός θνητός θεός Violates the Genuine property! Attack ??? [ θεός ¬ θνητός ¬ πεθάνει πεθάνει  ] ¬ θεός {}

Argumentation Logic Results (1) 16  T classically consistent  AL = PL (for the restricted language of ¬ and  ) AL entails  iff ¬Acc({¬  },{}) holds.  Interpretation of implication in AL differs from PL, e.g. Both a  b and ¬ (a  b) are acceptable w.r.t. to T={ ¬ a}  AL distinguishes two forms of Inconsistency of T  Classically inconsistent but directly consistent (under ├ MRA ) Violation of rule of «Excluded Middle». For some, φ, neither φ nor ¬ φ is acceptable, e.g. T = { φ  , ¬ φ   }  Directly inconsistent For some φ, T has a direct argument for φ and ¬ φ, e.g. T = { φ, ¬ φ }

Argumentation Logic Results (2) 17  AL extends PL when T is (classically) inconsistent  Directly consistent AL does not trivialize AL entails  iff ¬Acc({¬  },{}) and Acc({  },{}) hold. AL isolates out the non-relevant use of Reductio ad Absurdum Example: Logical Paradoxes ( T = { φ  , ¬ φ   } )  Directly inconsistent Use “Belief Revision Type” approach 1. Close T under direct consequence: C(T), 2. Maximally directly consistent subsets of C(T).

Example of Argumentation Logic  “A barber shaves anyone that does not shave himself”  ¬ ShavesHimself(Person)  ShavedByBarber(Person)  ShavesHimself(Person)  ¬ ShavedByBarber(Person)  Self-reference: When Person = barber  ShavedByBarber(barber)  ShavesHimself(barber)  ¬ ShavedByBarber(barber)  ¬ ShavesHimself(barber)

Example – Classical Logic  ¬ SH(P)  SB(P) SH(P)  ¬ SB(P)  SB(b)  SH(b) ¬ SB(b)  ¬ SH(b)  SB(b) |- SH(b) |- ¬ SB(b) i.e. SB(b) |-   ¬ SB(b) |- ¬ SH(b) |- SB(b) i.e. ¬ SB(b)|-   Problem arises due to the excluded middle law  SB(P) or ¬ SB(P), for any person P, even for P=barber.  This makes the theory inconsistent and therefore non meaningful (even for any other person than the barber).  Problem arises as SB(b) must take a truth value (in model theory).

Example – Argumentation Logic  ¬ SH(P)  SB(P) SH(P)  ¬ SB(P)  SB(b)  SH(b) ¬ SB(b)  ¬ SH(b)  ¬ ACC(SB(b)) SB(b) is a non-acceptable argument  ¬ ACC( ¬ SB(b)) ¬ SB(b) is a non-acceptable argument  Law of excluded middle for SB(b)? The law (SB(b) or ¬ SB(b)) is non-acceptable. Each one of SB(b) and ¬ SB(b) is directly inconsistent and so non-acceptable The negation of the law, ¬ (SB(b) or ¬ SB(b)), is acceptable (?)  Gives up the law of excluded middle!  Give up (two valued) model theory?

Conclusions 21  Acceptability is a direct, minimal and natural formulation of the semantics of abstract argumentation. Approach is a Synthesis of Labelling and Extension based approaches Is it “complete”?  Let the “Formalism Tell” vs “Telling the Formalism”. Acceptability “tells us” (encapsulates) a Reductio ad Absurdum Principle in argumentation.  This enables a reformulation of PL in terms of argumentation => Argumentation Logic (AL) AL is a conservative extension of PL into a type of Relevance Para- consistent Logic -- Only genuine use of Reductio ad Absurdum Looking for the analogue of a “model theory” for AL.