which moves can be made at each step of the dialogue. It concerns representing and managing the
locutions exchanged between the agents involved, as
well as specifying the contents of these locutions in
terms of entire arguments or components of arguments. Moreover, the dialogue protocol may establish
the allowed moves on the basis of argument relationships. For instance, a protocol may specify that a
move is legal only if it presents an argument attacking an argument presented in a previous move. For
these reasons the dialogical layer requires strict connections with the structural and relational layers.
Moreover some dialogue protocols are defined so as
to embed an argument assessment method: in these
cases the dialogical layer is intertwined with the
assessment layer, described next.
This layer concerns the assessment of a set of arguments and of their conclusions in order to establish
their justification status. The need for this layer arises
from the presence of attacks among arguments, preventing them so as to be accepted altogether and calling for a formal method to solve the conflict. This
problem is addressed in a principled and highly stylized form in the context of the theory of abstract argumentation frameworks (Dung 1995), where arguments are treated as abstract entities, deprived of any
structural property and of all their relations but attack.
We give an example of an argumentation framework,
based on textual arguments, in figure 2. Given its
abstract nature, an argumentation framework is often
referred to as argument graph, and this term is also
used to refer to similar representations where additional relations, like support, are considered.
An abstract argumentation semantics is a formal cri-
terion to determine which sets of arguments, called
extensions, are able to survive the conflict together
and can be regarded as collectively acceptable.
Abstract argumentation theory is probably the sub-
field of computational models of argument that has
attracted most research attention in the last two
decades, due to its generality and theoretical clean-
ness. In particular Dung (1995) has shown the ability
of the formalism to capture as instances several other
approaches, especially in the area of nonmonotonic
reasoning. Dung’s approach abstracts from the origin
and nature of the attack relation. A natural idea is to
define this relation in terms of a more basic notion of
conflict between arguments (for example, two argu-
ments having contradictory conclusions) and a
notion of relative argument strength or preference. In
the literature, there are two ways to connect these
ideas to Dung’s frameworks. The first approach leaves
Dung’s frameworks as they are but connects them
with models at the structural layer of argument to
define attack in terms of preferences or argument
strength while taking the structure of arguments into
account. The second approach instead extends
Dung’s frameworks with some abstract notion of
argument strength or preference, while possibly also
adding an abstract support relation between argu-
ments. Moreover, while most approaches consider a
qualitative notion of acceptance, quantitative assess-
ments methods are being investigated too.
Further, it must be noted that the evaluation of
argument acceptability is only a part, actually the
most basic one, of the assessment tasks required in an
argumentative process. In particular the final goal of
an agent is usually the assessment of the justification
status of the statements supported by arguments,
which, in the end, amounts to determining what to
believe or what to do. Since many arguments may
have the same conclusion, assessing the status of a
statement involves a synthesis of the statuses of the
arguments supporting it. As in real life, the task of
deciding what to believe may be carried out adopting
different attitudes, ranging from extremely skeptical
to extremely credulous, corresponding to different
formal methods for statement justification synthesis.
Figure 2. An Example of Argumentation Framework Consisting of Three Arguments in the Medical Domain and Their Attacks.
Arguments A1 and A2 are two alternatives for treating a patient with hypertension, and A3 provides a reason against one of the options.
Here, we assume that A1 and A2 attack each other because giving one treatment precludes the other, and we assume that A3 attacks A2
because it provides a counterargument to A2.
A1 = Patient has
A2 = Patient has
A3 = Patient has
is a contraindication