A Proposal for Evidential Reasoning about Motives
Floris Bex

Katie Atkinson

University of Groningen
Faculty of Law
The Netherlands

University of Liverpool
Department of Computer Science
Liverpool L69 3BX, UK

f.j.bex@rug.nl

katie@liverpool.ac.uk

ABSTRACT
Motives play an important role at every stage of a criminal investigation. In this research abstract we provide anoverview of an account of motivations based on a generalapproach to practical reasoning.

1.

INTRODUCTION

Motives play an important role at every stage of a criminal case. They can be used to search for an explanation ofthe crime (why was this person killed?), to identify a suspect(who would have the motive to kill this person) or to persuade a jury of a suspect’s guilt (this motive explains why
the suspect committed the crime). In this research abstract
we discuss a framework for the analysis of reasoning withsuch motives and their underlying values, concentrating on
the use of motives to provide plausibility to a story intended
to persuade a jury of a person’s guilt or innocence.
A persuasive story should be plausible in that it conformsto our beliefs about how things generally happen in theworld around us. This plausibility is partly dependent on
the plausibility of the (physical) causal relations between theevents in the story. For example, a story in which the victim
ends up in a coma because she was injected with insulin is
only plausible if we believe that an overdose of insulin cancause a coma. The plausibility of a story is also dependent
on how likely it is that the agents in the story would havemade the alleged choice in the situation, and this in turn depends on the value preferences we believe an agent to have.
For example, a story in which the suspect kills the victim
in order to inherit the victim’s money is only plausible if webelieve that the suspect values money higher than the life of
the victim. When determining the plausibility of a story we
can thus differentiate between plausibility as regarded fromthe physical as well as the intentional stance [6].
Other work on evidential reasoning in AI and Law, mostnotably by Bex and colleagues [5] and Thagard [7], mainly
focuses on reasoning from the physical stance. While both
approaches allow for the inclusion of links that denote some

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sort of “motivational causation” (i.e. motive causes action),
the reasoning about why a certain choice was made by theagents in a case remains implicit. Similarly, Walton andSchafer [8] establish the existence of a motive, but lack themachinery to explain why an actor chose to act on this motive in the particular situation.
Our account of reasoning with motives and values is basedon a general approach to practical reasoning that was previously introduced in [3]. Here we discuss how explanations ofwhat happened in a criminal case can be inferred throughabductive reasoning, and how the possible motives of theagents can influence the choice between these explanations.

2.

MOTIVES IN EVIDENTIAL REASONING

When discussing reasoning about motives, it is useful todistinguish the separate concepts of value, motive and goal.
Values, as defined by [2], can be seen as abstract principles
which an agent or a group of agents hold. Examples ofvalues are “wealth”, “love” and “honesty”. Agents can beexpected to, actively or passively, promote values which theyfind important. Our concept of motive is roughly the sameas this concept of value, in that a motive is an abstract goodwhich an agent may or may not want to promote. Such a
motive can cause a person to form any number of goals inorder to promote the principle for which the motive stands.
For example, if a husband is motivated by the will to beindependent, he can plan to murder his wife. However, hecan also form the less drastic plan to simply divorce his wife.
So the same motive can cause a person to form different
goals. This current notion of motive and goal is somewhat
different from in [8], where an agent’s motive is essentiallyequated with an immediate goal of the agent. In our opinion,
our notion of motive better captures the basic intuition thatthe same motive can be satisfied in different ways.
In all stages of criminal investigation it is important toconsider possible alternatives so as to avoid the well-known
problem of confirmation bias or tunnel vision. One way ofproviding such an alternative is to tell a totally new story(with possibly a different suspect) that explains the evidence
at least as well as the current story. Another way of providing an alternative is to argue that this suspect was not
the perpetrator of the crime. Instead of providing a totallynew explanation for the evidence, it is simply argued thatthe current suspect could not or would not commit such acrime, thus providing what we call a “suspect-specific” alternative; for example, it can be argued that it was not physically possible for the suspect to have committed the crime
because he was somewhere else at the time, thus providing

him with an alibi. It can also be argued that the suspect is
not the type of person who would commit such a crime byshowing that his motivational preferences are different fromthose assumed; for example, one could argue that the husband values love for his wife higher than money so he would
never kill her, even though he would benefit financially fromher death. Finally, it can be argued that given the suspect’svalues, he could have satisfied his motives by acting differently; for example, arguing that it would have been easierfor the suspect to divorce his wife and take half her fortune.
These last two ways of providing a suspect-specific alternative are examples of reasoning about the suspect’s motives
and the suspect’s willingness to act on such motives. Thisshows that in any (formal) reasoning framework which isused in evidential reasoning we should be able to rationalise
why the suspect in a case chose to act on his motives inthat particular situation. A notion related to this is that ofcharacter, which can be modelled as an ordering of a person’s motives. For example, we can say that X is the kind ofperson who prefers independence over honesty. If we haveevidence that X is such a person - for example, that he has
lied on previous occasions - we can provide an argument forthis particular preference. As Walton and Schafer [8] note,character evidence is often inadmissible at trial. However,it may be admissible for the purpose of proving someone’s
motive, which is exactly the purpose we wish to use it for.

explanation for a set of circumstances in terms of a motivated action, and explanations instantiating the scheme canagain be critiqued. The abductive scheme can be combinedwith the normal scheme, which enables us to reason bothpredictively and explanatorily about motivated actions.
One of the key elements our approach brings is the explicit
distinction between arguments that can be made to reason
about physical causal relations and arguments that can bemade to reason about motivations and their priorities. The
framework described here allows us to predict what possible
actions an agent would take given his values and an orderingon these values. This kind of reasoning is often employed toargue that, given the circumstances, the suspect would haveacted in a particular way to fulfill his motives. Since crimes
are “by definition deviant behaviour, what people do notnormally do” [8], the key to the plausibility of this argument
is explaining why the accused acted in the aberrant fashion.
While a typical agent would not have chosen the action using
a “normal” value order, there is some value order on which it
would be chosen: that the agent under suspicion in fact had
this unusual ranking of values itself needs to be explainedin order to make the agent’s choice of this action plausible.
Consequently, when considered from the intentional stance,
there are three elements to a plausible story: 1) a motive
for the action, 2) an explanation of why the agent had thismotive, and 3) an explanation of why the agent’s value order
was such as to make this motive of sufficient importance.
3. PRACTICAL REASONING WITH MOTIVES The approach has been applied and an actual case modelledin [4]. The case modelled is that used by [7], allowing specificIn [3], we attempted to analyse motives in terms of ancomparison with Thagard’s approach.
agent choosing to act so as to promote some value by realising some goal, an approach to practical reasoning previ4. REFERENCES
ously developed in [1]. The approach is based on the use ofan argumentation scheme and critical questions which allow[1] K. Atkinson and T. J. M. Bench-Capon. Practicaljustifications for action to be presumptively proposed andreasoning as presumptive argumentation using action
critiqued. The scheme can be used by an agent to justify
based alternating transition systems. Artificial
an action in particular circumstances in terms of his values.
Intelligence, 171(10–15):855–874, 2007.
For a given instantiation of this scheme a number of critical[2] T. J. M. Bench-Capon. Persuasion in practicalquestions could be posed to challenge the various elements
argument using value based argumentation frameworks.
of the scheme and so dispute the presumptive conclusion.
J. of Logic and Computation, 13(3):429–48, 2003.
In order to provide a formal basis for this argumentation[3] F. J. Bex, T. J. M. Bench-Capon, and K. Atkinson.
scheme, the problem scenario is modelled as an Action-based
Did he jump or was he pushed? Abductive practical
Alternating Transition System (AATS) [9]. Essentially, anreasoning. Accepted for publication in Artificial
AATS consists of a set of states and transitions between
Intelligence and Law, 2009. Available online.
them representing the possible joint actions of the agents[4] F. J. Bex, T. J. M. Bench-Capon, and K. Atkinson.
in a given state. These transitions are labelled with theEvidential reasoning about motives: A case study.
values (motives) promoted by moving from the source stateTechnical Report ULCS-09-013, Dept of Computer
to the target state, and the notion of a goal is identified
Science, University of Liverpool, UK, 2009.
with the new state. The argumentation scheme and all of[5] F. J. Bex, H. Prakken, and B. Verheij. Formalising
its associated critical questions are given formal definitions
argumentative story-based analysis of evidence. Inin terms of an AATS in [1]. Arguments following this scheme
Proceedings of ICAIL 2007, pages 1–10, 2007.
are now based on a particular path from the current state to[6] D. Dennett. Intentional systems. In Brainstorms, pages
a new state in the AATS. As an agent can only perform one
3–22. Harvester Press, 1979.
action in the given circumstances, arguments for different[7]P.
Thagard. Causal inference in legal decision making:actions attack each other. The critical questions also point
Explanatory
coherence vs. bayesian networks. Appliedto possible counter-arguments. Once a set of arguments
ArtificialIntelligence,
18:231–241, 2004.
and the attacks between them has been generated on the[8] D. Walton and B. Schafer. Arthur, George and thebasis of a specific AATS, the status of the arguments canmystery of the missing motive: Towards a theory of
be evaluated. To do this, the arguments are formed into aevidentiary reasoning about motives. International
Value-Based Argumentation Framework (VAF) [2].
Commentary on Evidence, 4(2):1–47, 2006.
In [3], we extended the above non-abductive view on prac[9]M.
Wooldridge and W. van der Hoek. On obligations
tical reasoning with a separate argument scheme for abducand normative ability: Towards a logical analysis of thetive practical reasoning to allow for search and investigationsocial contract. J. of Applied Logic, 3:396–420, 2005.
cases. This abductive scheme makes it possible to infer an213