Argumentation Logics Lecture 3 Abstract argumentation semantics 3

Argumentation Logics Lecture 3: Abstract argumentation semantics (3) Henry Prakken Chongqing May 28, 2010

Contents n Review of grounded, stable and preferred semantics n n Stable and preferred semantics n n n Labelling-based Extension-based Correspondence between labelling-based and extension-based semantics Concluding remarks on semantics of abstract argumentation.

Status of arguments: abstract semantics (Dung 1995) n n INPUT: an abstract argumentation theory AAT = Args, Defeat OUTPUT: An assignment of the status ‘in’ or ‘out’ to all members of Args n So: semantics specifies conditions for labeling the ‘argument graph’.

Possible labeling conditions n Every argument is either ‘in’ or ‘out’. 1. An argument is ‘in’ iff all arguments defeating it are ‘out’. 2. An argument is ‘out’ iff it is defeated by an argument that is ‘in’. n Produces unique labelling with: A n B C But produces two labellings with: A B

Two solutions n Change conditions so that always a unique status assignment results A B A n A B Use multiple status assignments: A n C B B and C A B

A problem(? ) with grounded semantics We have: A B We want(? ): A B C C D D

Multiple labellings A B C C D D

Stable status assignments n n (Below is AAT = Args, Defeat implicit) A stable status assignment assigns to all members of Args either the status In or Out (but not both) such that : 1. An argument is In iff all arguments defeating it are Out. 2. An argument is Out iff it is defeated by an argument that is In. n n n A is justified if A is In in all s. a. A is overruled if A is Out in all s. a. A is defensible if A is In in some but not all s. a.

Stable status assignments: a problem n A stable status assignment assigns to all members of Args either the status In or Out (but not both) such that: 1. An argument is In iff all arguments defeating it are Out. 2. An argument is Out iff it is defeated by an argument that is In. A B C

Stable status assignments: a problem n A stable status assignment assigns to all members of Args either the status In or Out (but not both) such that: 1. An argument is In iff all arguments defeating it are Out. 2. An argument is Out iff it is defeated by an argument that is In. A B D C

Status assignments n n n A status assignment assigns to zero or more members of Args either the status In or Out (but not both) such that: 1. An argument is In iff all arguments defeating it are Out. 2. An argument is Out iff it is defeated by an argument that is In. Let Undecided = Args / (In Out): A status assignment is stable if Undecided = . n In is a stable argument extension A status assignment is preferred if Undecided is -minimal. n In is a preferred argument extension A status assignment is grounded if Undecided is -maximal. n In is the grounded argument extension

1. An argument is In iff all arguments defeating it are Out. 2. An argument is Out iff it is defeated by an argument that is In. Grounded s. a. minimise node labelling Preferred s. a maximise node labelling A B D C

1. An argument is In iff all arguments defeating it are Out. 2. An argument is Out iff it is defeated by an argument that is In. Grounded s. a. minimise node labelling Preferred s. a maximise node labelling A C B D E

Correspondence between labellingbased and extension-based semantics of abstract argumentation Bart Verheij (1996) Hadassah Jakobovits (1999) Martin Caminada (2006)

Status assignments n n n A status assignment assigns to zero or more members of Args either the status In or Out (but not both) such that: 1. An argument is In iff all arguments defeating it are Out. 2. An argument is Out iff it is defeated by an argument that is In. Let Undecided = Args / (In Out): A status assignment is stable if Undecided = . n In is a stable argument extension A status assignment is preferred if Undecided is -minimal. n In is a preferred argument extension A status assignment is grounded if Undecided is -maximal. n In is the grounded argument extension

Grounded extensions again n Dung (1995): n Construct a sequence such that: n n The endpoint of the sequence is the grounded extension Recall: n n S 0: the empty set Si+1: Si + all arguments in Args that are defended by Si S is a grounded argument extension if (In, Out) is a grounded status assignment and S = In. Proposition 2. 3. 16: S is a grounded argument extension iff S is a grounded extension

Stable extensions n Dung (1995): n n n arguments outside it Recall: n n S is conflict-free if no member of S defeats a member of S S is a stable extension if it is conflict-free and defeats all S is a stable argument extension if (In, Out) is a stable status assignment and S = In. Proposition 2. 3. 4: S is a stable argument extension iff S is a stable extension

Preferred extensions n Dung (1995): n n Recall: n n S is conflict-free if no member of S defeats a member of S S is admissible if it is conflict-free and all its members are acceptable wrt S S is a preferred extension if it is -maximally admissible S is a preferred argument extension if (In, Out) is a preferred status assignment and S = In. Proposition 2. 3. 13: S is a preferred argument extension iff S is a preferred extension

S defends A if all defeaters of A are defeated by a member of S S is admissible if it is conflict-free and defends all its members A C B D Admissible? E

S defends A if all defeaters of A are defeated by a member of S S is admissible if it is conflict-free and defends all its members A C B D Preferred? E S is preferred if it is maximally admissible

S defends A if all defeaters of A are defeated by a member of S S is admissible if it is conflict-free and defends all its members A C B D Grounded? E

1. An argument is In if all arguments defeating it are Out. 2. An argument is Out if it is defeated by an argument that is In. A B F C D E

Properties n n n n Every admissible set is included in a preferred extension The grounded extension is unique Every stable extension is preferred (but not v. v. ) There exists at least one preferred extension The grounded extension is a subset of all preferred and stable extensions Every AAT without infinite defeat paths has a unique extension (which is the same in all semantics) Every AAT without defeat cycles of odd length has a stable extension. . .

Self-defeating arguments again n Recall (for preferred and stable semantics): n n n A is justified if A is In in all s/p. s. a. A is overruled if A is Out in all s/p. s. a. A is defensible if A is In in some but not in all s/p. s. a. In (grounded and) preferred semantics self-defeating arguments are not always overruled They can make that there are no stable extensions A B

Which semantics is the “right” one? n Alternative semantics may each have their use in different contexts n n E. g. in criminal procedure the burden of proof is on the prosecution, so grounded semantics with justified arguments is suitable. Or in decision making a choice must be made between alternative ways to achieve one’s goals, so preferred semantics with defensible arguments is suitable.