Understanding narratives withargumentationa

Floris BEX a , Trevor BENCH-CAPON b
Information and Computing Sciences, Utrecht University, The NetherlandsbComputer Science, University of Liverpool, UK
Abstract. In this paper, we show two important connections between computational models of narrative and computational models of argumentation. First, weshow how argumentation techniques can be applied to enrich story understanding,especially where an understanding the story requires understanding of the motivesof its characters. This also helps to explain how stories can themselves be seen as asarguments for a particular ordering on values within a value based argumentation
framework. We illustrate our discussion using biblical parables, taking as our main
example the parable of the Good Samaritan.
Keywords. narratives, parables, value-based argumentation

1. Introduction
We often persuade not by imparting facts and rules, but by providing an interesting narrative, particularly when trying to convince others to adopt particular values and attitudes.
In this paper we show how current computational argument techniques can be used toenrich story understanding, and how these techniques show that stories can sometimes
function as arguments.
A central concept from research on story understanding is scripts [8], coherent scenarios about common situations such as visiting a restaurant. Scripts still play an important part in computational and cognitive approaches of story understanding (e.g. [7]) andhave been applied in, e.g. case-based reasoning, scenario-based evidence analysis andnarrative generation.
In our opinion, purely script-based approaches are not, however, best suited to understanding persuasive stories concerning values, such as parables. Scripts represent theway in which we expect typical situations to play out: the more a story adheres to a familiar script, the more plausible a story is considered to be. However, many memorable
stories such as parables depend on a twist in the story, something which is out of theordinary and which challenges conventional attitudes [4]. For example, no-one expects
a father to organise a feast for a son who has spent all of his money on wild living (The
Prodigal Son (Luke 15:11-32)). Interesting stories often include conflicting attitudes: inthe Prodigal Son, the son’s older brother wants to turn away his sibling while the father
wishes to forgive and welcome his son. In models based on scripts, in which stories are
rendered only as causal sequences, these conflicts between characters’ values cannot bereadily articulated and so remain largely implicit and unexplained.

Computational argumentation techniques allow us to add a more fine-grained psychological dimension to the causal narrative, in which conflicts between characters’ attitudes and challenges to common attitudes can be modelled. This gives us an internal
perspective that allows us to represent the deliberations of the characters involved, which
in turn allows for a much more subtle analysis of character motive and attitude than wecan perform with just the external causal perspective. This analysis may also persuade
an audience to adopt a different attitude themselves.
Our model for story understanding, which is explained in section 3 by means of theparable of the The Good Samaritan (Luke 10:25-37), is based on value-based practicalreasoning as described in [1]. Stories can be represented as (causal) state transition diagrams, where the transitions represent possible actions by the characters in the story.
Character motives are represented by indicating which values are promoted or demotedby the actions in the story. We can then generate practical reasoning arguments of theform I should perform Action because it promotes Value and I should not perform Action
because it demotes Value from the diagram. By adding separate arguments denoting thepossible attitudes of characters as value orderings, we can construct an Extended Argumentation Framework (EAF) [5] with value preferences attacking attacks, giving a setof (possibly conflicting) arguments representing character choices and attitudes. We canthen infer attitudes from the choices made in the story.
This approach to understanding the attitudes represented in a story, allows stories tothemselves be used as arguments. Sometimes the conclusion the audience is invited to
draw from the story is explicitly mentioned in the text: in fables the moral of the story
is usually explicitly included. In other cases the conclusion may depend on the context
in which the story is told. The Prodigal Son, for example, is told as a response to the
Pharisees remark that he ‘welcomes sinners and eats with them’ (Luke 15:2); thus theparable can be seen as a justification of fraternising with sinners. Note that the conclusionof the argument is not advice to behave in a particular way, but rather to adopt an outlook,a set of attitudes. In section 3.1 we show how a particular story interpreted by means of an
EAF can be used as an argument in a particular dialogical context, using [6]’s extended
TPI-protocol for argumentative dialogue to argue for a change in value preferences in a
dialogical setting.

2. Motivating example: The Good Samaritan
Stories can be a powerful vehicle of persuasion. A story does not persuade by imparting
explicit rules, but by exposing a coherent narrative aimed at changing or reinforcing
attitudes, so that the stories exemplify various group cultural norms. Many folktales areof this type, as are parables, both secular and biblical. As our example we will use thewell-known parable, The Good Samaritan (Luke 10:25-37):
Leading up to the parable, a lawyer asks two questions. The first, “what shall I do
to inherit eternal life?”, receives an answer justified by scriptural authority, whereas thesecond, “Who is my neighbour?”, is met simply by a story. A traveller is attacked andleft wounded by the roadside. Two respectable citizens, a priest and a Levite, pass him
by before a Samaritan (a member of a generally despised group) helps him. The lawyer
is asked who was neighbour to the wounded man, and answers that it was the Samaritan.
Here the story is used as an argument to justify an answer to a question. It is notmeant as a theoretical argument: the aim is not that the lawyer should believe that theSamaritan is his neighbour. Nor is the lawyer intended to set out to assist wounded travellers on the road from Jerusalem to Jericho. Unlike practical reasoning proper, there isno specific situation, with a specific choice of actions to resolve. Rather the argument
is intended to convince the lawyer (and ultimately of course the audience) to become adifferent person.
So how exactly does the story convince its audience to change their ways? Govier
and Ayers [4] have explored this question in relation to the Good Samaritan, reconstructing it as an argument with five premises (two implicit and added by them) and a normative conclusion. We, however, contend that the claim of the parable should not be a normsuch as in certain situations you should do this, but rather an invitation to adopt different
attitudes, to be like the Samaritan. To enable a story to have this effect we need a detailed
account of the reasoning of the three characters, to articulate the differences in attitude
between them, and so identify the attitudes we are being urged to abandon and adopt.

3. Understanding stories using value-based argumentation
We apply techniques from computational argumentation to understand the story. We
start from [1], which we used previously to capture abductive reasoning in which stories
served as explanations of evidence [2]. This model has three main elements: (i) ActionBased Alternating Transition Systems with Values (AATS+V) for encapsulating stories;(ii) arguments based on the Practical Reasoning Argumentation Scheme (PRAS), to generate arguments concerning the individual choices a story character can make; and (iii)
Value-based Argumentation Frameworks (VAF), representing the set of arguments andcounterarguments a story character uses to make his individual choices on the basis of his
preferences and attitudes. To reason explicitly about characters’ value orderings, here weuse [5]’s Extended Argumentation Frameworks (EAF) instead of VAFs, and the dialogue
game of [6] to associate particular audiences with particular choices.
Narratives can be seen as actions giving rise to a sequence of state transitions.
Action-based Alternating Transition Systems with Values (AATS+V) [1] provide the required features. An AATS comprises a set of states and transitions between them, withthe transitions labelled with joint actions, that is actions comprising an action for eachof the agents concerned. In an AATS+V, the transitions are also labelled with the values
promoted and demoted that can be used to motivate the actions. A basic version of theparable of the Good Samaritan can be rendered as the AATS+V in Figure 1.
j2: help+RD +Cq0
T wounded
P arrives

j1: not help
+RL +P +Cv

q1
T wounded
L arrives

j4: help+C +NS +RS
q3
j3: not help T wounded+P +Cv
S arrives

j5: not help
+P +Cv +R
q4
j6: help
T stable+C

Figure 1. AATS+V for the Good Samaritan

In q0 , where the story begins, the traveller (T) is wounded. In q4 , the traveller’s
wounds have been bandaged and he is in a stable condition. In addition to the actions
taken by the characters in the story (j1 , j3 , j6 ), we have also included some hypotheticalactions the characters could have performed: for example, the Priest (P) could also havehelped the traveller (j2 ). Action choice in parables is often more or less binary (help or¬help, accept or ¬accept in the Prodigal Son), so modelling these extra actions does not
require much extra information besides the original story text. Values promoted (or demoted) are also included in the AATS+V: in Figure 1 we use Religious Duty (+RD), Religious Law (+RL), National Solidarity (+NS), Racial Solidarity (+RS), Compassion (+C),
Prudence (+P), Convenience (+Cv) and Revenge (+R). Adding the values requires morebackground knowledge. For example, we need to know that the traveller and the Levite(L) were of the same race, and that Samaritans (S) were seen as an enemy. Nowadays,
this background information can be gained from Biblical texts, or from the many varied
accounts on how parables should be interpreted, but it would have been well-known tothe original audience. Part of the force of the story is that the two characters that pass by
are people that the audience will be predisposed to admire, and so will attempt to findreasons that excuse their behaviour. The values in figure 1 are a selection that can motivate various reasons for not helping that the authors have heard from a variety of sources
over the years. Note that the values are not themselves present in the story: they are usedby audiences in their efforts to make sense of the behaviour of the characters involved.
Given the AATS+V, we can construct the possible arguments for the different characters. The basic idea expressed in [1] is that the AATS+V serves as a formal grounding
for arguments that instantiate the Practical Reasoning Argumentation Scheme (PRAS).
Informally this is: in the current circumstances R you should perform the action A which
will result in these new circumstances S, which will promote value V. R and S are statesin the AATS+V, A is part of a joint action leading from R to S, and +V is a label on thecorresponding transition. A formal version is given in [1]. Given this mapping of PRAS
to an AATS+V, we can generate the arguments from the AATS+V, noting that arguments
for different actions attack each other because the actions are mutually exclusive, i.e., one
cannot help and not help someone at the same time. First, there are the two argumentsthat might apply to the priest.
• A1 : I should help the man because I have a religious duty to do so. This will
promote Religious Duty (+RD)• A2 : I should not help the man because I risk uncleanliness through contact withhis blood. This will promote Religious Law (+RL).
The following apply to the Levite as well as the Priest.
• A3 : I should help the man because he is a fellow countryman. This will promote
National Solidarity (+NS).
• A4 : I should help the man because he is of my race. This will promote Racial
Solidarity (+RS).
None of the above arguments apply to the Samaritan. The following arguments apply to
all three characters.
• A5 : I should help the man because he is a fellow human being. This will promote
Compassion (+C).
• A6 : I should not help the man because it may be trap and I may be robbed. This
will promote Prudence (+P).
• A7 : I should not help the man because it will interrupt my journey. This will
promote Convenience (+Cv).

Although A6 is based on a reason not explicit in the story, such traps were apparently
common at the time, and would be likely to occur to the original audience as a reason
why a good man might reasonably pass a wounded traveller by. In interpreting stories
the background knowledge and beliefs of the audience need to be kept in mind. Finally
there is an argument that applies only to the Samaritan:• A8 : I should not help this man, because his people have quarrelled with mine.
This will promote Revenge (+R).
Arguments A1 -A4 relate to duties of one sort or another, arising from religious law
or duty, or one form or another of social relationship (nation, race). A5 -A8 all arise fromnatural human instincts, unconnected with any social institution.
From these arguments, we can construct a Value-based Argumentation Framework(VAF). The purpose of building a VAF is to find a subset of the arguments which is atonce conflict free (i.e. no two arguments in the subset attack one another), and collectively able to defend itself (i.e. any attacker of an argument in the subset is itself attacked
by an argument in the subset). The maximal such subset is called a preferred extension,and represents a maximal consistent position given the arguments presented. The key
feature of VAFs is that they allow a distinction to be made between successful attacks(defeats) and unsuccessful attacks, on the basis of the values associated with the arguments and audience preferences between these values. Attacks succeed only if the value
associated with the attacker is ranked equal to, or higher than, the argument it attacks.
The VAF thus accounts for elements of subjectivity in that the arguments that are acceptable are dependent upon the audience’s ranking of the values involved in the scenario.
We can now explain the actions of the three characters by considering the different
value orderings of the different audiences. We may suppose that the Priest puts religion
before all else (i.e., RD and RL are preferred to RS, NS, Cv, C and P). He has a conflict
between A1 , that he should help to promote RD, and A2 , that he should not help to promote RL. In the story, he chooses to observe the law. This ranking of strict observanceof the law over more human concerns is criticised elsewhere in the Gospels. The Levite
must be supposed to act on either A6 or A7 , overriding the specific duties of A3 and A4
as well as A5 . But because the Levite is a type of a morally respectable man, it must be
assumed that we are being invited to conclude that these preferences are acceptable in theeyes of the current moral climate: that it is morally acceptable for prudence to override
obligations arising from country or race, or from natural feelings of compassion. The
Samaritan, in contrast has no duties prompting him help the man, and must balance hiscompassion against the other natural human instincts. That he helps the man (A5 ), canonly be explained in terms of him putting compassion before all other values, individually and in combination. The lawyer concludes that this is what being a neighbour reallyis, and we are invited to do likewise.
3.1. Stories as arguments in a dialogical context
We now consider stories as arguments. We use the machinery of [5] to represent statements about value orderings as arguments in an Extended Argumentation Framework(EAF) to enable us to reason about which preferences will explain the behaviour of thevarious characters. As well as arguments and attacks between arguments, EAFs also contain a set of attacks on attacks. The idea is that preferences attack some attacks between

A8
not help+R
AV3
C > Cv

AV6
R>C

A7
not help
+Cv

AV5
C>R

AV2
C>P
A6
not help
+P

A5
help
+C
AV4
Cv > C

AV1
P>C

Figure 2. EAF for the Samaritan

arguments and thus determine which attacks succeed. In the EAF for the Samaritan there
are potentially two value-preference arguments for each pair of values, for example: AV1 :P > C; and AV2 : C > P .
These pairs will mutually attack, but more importantly they will attack the attackfrom the argument motivated by the less preferred value on arguments motivated by theother value. The complete EAF for the parable will now contain all the base arguments
A1 -A8 and arguments expressing the value preferences. We also introduce arguments
for the various characters: AC1 (Character is a priest), AC2 (Character is a Levite) andAC3 (Character is a Samaritan), to enable us to eliminate arguments which do not apply
to particular characters: thus AC1 will attack A8 , AC2 will attack A1 , A2 and A8 , andAC3 will attack A1 , A2 , A3 and A4 . Adding AC3 to the AF that contains all characters’
arguments A1 - A8 thus produces the EAF applicable to just the Samaritan, shown in
Figure 2. Similarly, AC2 and AC1 give the EAF applicable to the Levite and the priest
respectively.
Having established appropriate EAFs for the various characters, we need to evaluate
them to explain the choices they make. For the Samaritan, we need to construct an admissible set containing an argument to justify helping the traveller, and then to consider
what value preferences it contains. One way to construct admissible sets from AFs is touse a dialogue game. A game for VAFs expressed as EAFs was given in [6]. First, thearguments of the EAF are rewritten as meta level statements. This is a purely mechanical process. Attacks from value based arguments can be rejected in two ways: not only
can we reject the attacking argument, but also we can prefer the value of the attacked
argument. Each pair of mutually attacking arguments can be rewritten as a regular AF;
Figure 3 shows the new, regular AF, structure for the pair of arguments A5 and A6 .
AV2
C>P
A5
holds
+C

A6
defeats
A5

A6
does nothold

A5
does nothold

A5
defeats
A6

A6
holds
+P

AV1
P>C

Figure 3. Regular AF for the A5 - A6 part if the EAF in figure 2

The game proceeds by the proposer playing an argument, the opponent playing an
attacker, the proposer playing an attacker of that argument and so on, until one playercannot move. At this point a player can back up to a choice point and play a different
argument. This continues until no moves are possible (note that arguments under attack
cannot be played). At this point we will have an admissible set containing the arguments
played by the last player to move. If this was the proposer is will contain the original
argument and this will have been shown to be acceptable. Because it is the Samaritan’s
preference we are trying to determine, we use the EAF in figure 2, rewritten as a regular AF (so that the arguments played are actually meta-arguments). The dialogue thenproceeds as follows:
Samaritan: A5 holds. This is an argument justifying what the Samaritan did in
the story: current position is {A5 holds}.
Opponent: A6 defeats A5 . Opponent chooses a way to attack A5 .
Samaritan: AV2 C > P. The preference argument is played: the alternative would
eventually require A5 holds to be played, but this is under attack. Current position
is {A5 holds, C > P}.
Opponent: A7 defeats A5 . Opponent cannot play P > C, because it is underattack, and so backs up and chooses another line of attack.
Samaritan: AV3 C > Cv. Current position is {A5 holds, C > P, C > Cv}.
Opponent: A8 defeats A5 . Again the opponent must back up since Cv > C isunder attack.
Samaritan: AV5 C > R. Current position is {A5 holds, C > P, C > Cv, C > R}.
At this point the opponent must stop, since there are no further lines of attack. The
Samaritan’s position, {AC3 , A5 , AV2 , AV3 , AV5 }, comprises an argument justifying his
action (A5 ) and the three value preferences required to defend that argument (AV2 , AV3
and AV5 ). It is exactly this position that the audience is being urged to adopt, since it
provides the answer to the lawyer’s question “what does it mean to love your neighbour
like yourself?”.
3.2. Implementing our modelOnce the story has been represented as an AATS+V, implementation is straightforward.
Here we assume the process of constructing the AATS to be manual, comparable to theconstruction of a knowledge base to support other structured argumentation systems. In
fact constructing an AATS from a story is simpler than representing a factual problem,
because the conventions of fiction are that things not mentioned in the story are not relevant, and so need not be represented in the AATS, allowing the propositions and actions required to be kept to a minimum. Representing an AATS requires: states: withattributes for state ID, and for each relevant proposition; transitions: with attributes forID, source state, destination state, joint action, values promoted and values demoted andjoint actions: with attributes for ID and one action for each agent This information canbe represented as appropriate for the programming language used (e.g. as relations andtheir attributes roles in Prolog, or as tables in a database). We can now identify the arguments using a suitable (e.g. Prolog or SQL) query which, given a source state, will
retrieve the joint actions which promote a value and from these identify the target state,
the action each agent must perform, and the particular value promoted to provide an instantiation of PRAS. A similar query will produce counterexamples by identifying thetransitions which demote rather than promote values. An implementation applicable toe-participation was described in [9].

Given the arguments, a story can be reasoned with in a variety of different ways. It
could be presented as a web based tool for presenting and critiquing arguments, as in [9].
Or, if implemented in an existing multi-agent system, the arguments could be used toinfluence other software agents in their choice of action. Another interesting possibility
is to allow humans to explore the stories and character motives through a dialogue withthe computer. A generic framework for dialogue and an associated web interface (www.
arg.dundee.ac.uk/arvina) have been developed [3] to allow users to engage ina dialogue about a set of arguments (such as instantiations of PRAS or an EAF) with
software agents. Using this application it is possible to interrogate, for example, an agent
representing the Samaritan about his motives, and thus gain a better understanding of
the story. Furthermore, users can add arguments to challenge the interpretation of a story
by arguing about, for example, the values represented in the AATS (see [1]’s criticalquestions for discussing the AATS).
4. Conclusion
In this paper we have shown how several current argumentation techniques (value based
argumentation using AATS+V, EAFs and dialogues based on them) can be applied tostory understanding, and how stories can be construed as arguments. In particular thesetechniques enable us to understand the story in terms of the practical reasoning used by
the characters in making the choices depicted in the story. This in turn leads us to imputepreferences to the characters, which we may adopt or reject according to how the story
makes us feel about the various characters, enabling persuasion with respect to value
preferences.
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