Ascription of an intention to an agent is especially important in law. In criminal law the intent to commit a criminal act, called mens rea, refers to the guilty mind, the key element needed to prosecute a defendant for a crime. For example, in order to prove that a defendant has committed the crime of theft of an object, it needs to be established that the defendant had the intention never to return the object to its owner. Studying examples of how intention is proved in law is an important resource for giving us clues on how reasoning to an intention should be carried out. Intention is also fundamentally important in ethical reasoning where there are problems about how the end can justify the means.

This chapter introduces the notion of inference to the best explanation, often called abductive reasoning, and presents recent research on evidential reasoning that uses the concept of a so-called script or story as a central component. The introduction of these two argumentation tools show how they are helpful in moving forward toward a solution to the longstanding problem of analyzing how practical reasoning from circumstantial evidence can be used to support or undermine a hypothesis that an agent has a particular intention. Legal examples are used to show that even though ascribing an intention to an agent is an evaluation procedure that combines argumentation and explanation, it can be rationally carried out by using a practical reasoning model that accounts for the weighing of factual evidence on both sides of a disputed case.

The examples studied in this chapter will involve cases where practical reasoning is used as the glue that combines argumentation with explanation. Section 1 considers a simple example of a message on the Internet advising how to mount a flagpole bracket to a house. The example tells the reader how to take the required steps to attach a bracket to the house in order to mount a flagpole so that the reader can show his patriotism by displaying a flag on his house. The example text is clearly an instance of practical reasoning. The author of the ad presumes that the reader has a goal, and he tells the reader how to fulfill that goal by carrying out a sequence of actions.