Argumentation Schemes

Abstract argumentation frameworks are useful for evaluating the acceptability of arguments based on the attack relations between them. However, abstract arguments themselves are not useful for representing instantiated arguments; i.e., arguments with some internal structure or content. To reason over the best coalition structure to form, argumentation schemes will be instantiated in Chapter3, and the arguments yielded will be organised into a value-based argu- mentation framework.

An argumentation scheme is a form of inference from premises to a conclusion that rep- resents a pattern of deductive, inductive or defeasible reasoning [97]. An instantiated argu- mentation scheme provides justification for the particular conclusion of the scheme [21]. Ar- gumentation schemes allow for arguments to be represented within a particular context, and take into account the fact that the reasoning presented within them may be altered if new evi- dence is found [8]. Schemes are necessary for identifying arguments, finding missing premises, analysing arguments and evaluating the arguments [97]. Many computational models based on argumentation schemes exists (e.g. [60, 68, 84, 123]). Moreover there are many types of argumentation schemes have been studied (e.g. [128] lists over 30 types), some examples are:

• Argument from Analogy, for instance: Generally, caseC1 is similar to caseC2. A is true in caseC1. ThereforeAis true inC2.

• Argument from Commitment, for instance:ais committed to propositionA. Therefore in this caseashould supportA.

• Argument from Evidence to Hypothesis, for instance: IfAis true (hypothesis) thenBwill be observed to be true (evidence). B has been observed to be true in a given instance. Therefore in this instanceAis true.

• Argument from Expert Testimony, for instance:Eis an expert in domainD.Easserts that Ais known to be true.Ais withinD. ThereforeAmay plausibly be taken to be true.

• Argument from Popularity, for instance: If a large majority acceptCto be true, then there exists a (defeasible) presumption in favour of C. A large majority accepts C as true. Therefore there exists a presumption in favour ofC.

The tool used to evaluate an argumentation scheme is a set of appropriatecritical questions [97]. An instantiation of an argumentation schemeasxattacks another instantiation of an argu- mentation schemeasy under pre-defined conditions, which attack either the premises, inference rules or conclusions of the schemeasy. The attack byasxtoasycan be described as a critical question toay, which if left unanswered will lead to the defeat ofasy.

Argumentation schemes can be used to reason over what action or joint-action to perform. Argumentation schemes can also be used to reason over what to belief (this will be discussed in more detail in Section2.6.5). The following is a definition of practical reasoning provided by Walton in [128]:

Definition 44: Practical Reasoningis a goal directed sequence of linked practical inferences that seeks out a prudent line of conduct for an agent in a set of particular circumstances known by the agent.

Walton went on to describe two basic types of practical inferences related to practical rea- soning (where NCS is a special type of SCS [13]):

• The necessary condition scheme (NCS):Gis a goal for agenti. Doing actionAis neces- sary for agentito carry out goalG. Therefore agentiought to do actionA.

• The sufficient condition scheme (SCS):Gis a goal for agenti. Doing actionAis sufficient for agentito carry out goalG. Therefore agentiought to do actionA.

There are four critical questions regarding these two schemes:

• CQ1: Are there alternative ways of realising goalG?

• CQ2: Is it possible to do actionA?

• CQ3: Does agentihave goals other thanGwhich should be taken into account?

• CQ4: Are there other consequences of doing actionAwhich should be taken into account?

In [13] it was argued that the NCS/SCS argumentation schemes needed elaboration because the notion of a goal is ambiguous, potentially referring indifferently to any direct results of the action, the consequences of those results, and the reasons why those consequences are desired. Therefore in [13], the SCS scheme was expanded on to give another practical reasoning argu- mentation scheme (labeled AS1) with associated critical questions:

ID Critical Question

CQ1 Are the believed circumstances true?

CQ2 Assuming the circumstances, does the action have the stated consequences? CQ3 Assuming the circumstances and that the action has the stated consequences,

will the action bring about the desired goal? CQ4 Does the goal realise the social-value stated?

CQ5 Are there alternative ways of realising the same consequences? CQ6 Are there alternative ways of realising the same goal?

CQ7 Are there alternative ways of promoting the same social-value?

CQ8 Does doing the action have a side effect which demotes the social-value? CQ9 Does doing the action have a side effect which demotes some other social-

value?

CQ10 Does doing the action promote some other social-value?

CQ11 Does doing the action preclude some other action which would promote some other social-value?

CQ12 Are the circumstances as described possible? CQ13 Is the action possible?

CQ14 Are the consequences as described possible? CQ15 Can the desired goal be realised?

CQ16 Is the social-value indeed a legitimate social-value?

CQ17 Is the agent guaranteed to perform its part of the joint-action?

TABLE2.3: The full list of critical questions associated with argumentation schema AS1.

Definition 45:TheAS1scheme for practical reasoning is as follows: In the current circumstancesR

We should perform actionA

Which will result in new circumstancesS Which will realise goalG

Which will promote the social-valueV

This scheme uses social-values to describe a social interest that an agent has, which will be promoted by moving to a state in which goal G becomes true [19]. An agent may propose an action (the conclusion of the scheme) and its justification by instantiating this scheme. Other agents can then challenge instantiations by posing critical questions (CQ) associated with the scheme. Seventeen critical questions are associated with the above scheme [10,12], which raise potential issues with: the validity of the elements instantiated in the scheme; the connections between the elements of the scheme; the side effects of the action; and the possible alternative actions. These seventeen critical questions are provided in Table2.3.

Each of these critical questions identifies a source of disagreement between two or more agents concerning an instantiation of the argumentation scheme AS1. Some critical questions disagree with the premises of the scheme, for example CQ14 disagrees with the premise of the

new consequences. Other critical questions offer an alternative conclusion, for example, CQ6 offers a new action to achieve the required goal.

The disagreements highlighted by critical questions against an argumentation scheme can be modeled in a value-based argumentation framework or simply an argumentation framework (if no social-values were used), to reason logically over what is the best conclusion. In the AS1 case, value-based argumentation frameworks would be used to reason over what is the best action to perform. A modified version of AS1 and its associated seventeen critical questions are used later in Chapter 3 for agents to persuade others to form a coalition to undertake a joint-action.