Sherlock: [["at(pistol,Shed)", "gunowner(pistol,Moriarty)", "murderer(Victim,Moriarty)", "owner(Manor,Moriarty)", "owner(Shed,Watson)"], ["at(pistol,Shed)", "gunowner(pistol,Watson)", "murderer(Victim,Watson)", "owner(Manor,Moriarty)", "owner(Shed,Watson)"]] ... Figure 2: The epistemic state after the system found and ex- ecuted an action sequence such that the provided condition is satisfied. Note that only the appearance of the actual world is printed, and not the appearance of that appearance etc. the if statement is public to the detective, which means they will know which branch is taken, i.e. after performing the action they will know whether or not the fingerprints they found belong to the suspect, and will subsequently believe that they are the murderer in case of a match, or believe that they are not the murderer otherwise. Limitations and Future Work While the logic we use as the basis for our system can ex- press a wide range of relations between character’s beliefs, it is not without limitations. One such limitation is illustrated by The Star Wars example from the introduction. In this scenario, Luke believed that his father was dead, i.e. in all worlds that he considered possible his father was dead. How- ever, when Darth Vader revealed that he is Luke’s father, Luke then believed that his father was dead, Darth Vader was his father and Darth Vader was alive, which caused a contra- diction. When we use a naive encoding of the narrative, this would leave him with no worlds that he would consider pos- sible (and indeed, he uses the phrase “That’s impossible”). This is not technically a limitation of the logic, but some- thing that must be taken into consideration when modeling narratives in it. One way to avoid it is to model the epistemic effect that happened as consisting of two parts: First, Luke increased his uncertainty by adding new worlds in which his father was indeed alive and then he discards the worlds in which his father is not Darth Vader. This approach has the drawback that it requires a certain level of awareness of this kind of accidental contradictions on part of the domain au- thor. The other approach would be to explicitely model the process of learning the identity of an unknown person such that the properties of the unknown person are overwritten with those of the identity, or more concretely, when Luke learns that Darth Vader is his father, he also learns that his father has all properties that Darth Vader has, including the fact that he is alive, without regard for what he previously believed about him. The main limitations of our current system are all a con- sequence of using breadth-first search. The story fragment presented above can be found in less than one minute on modern hardware, but longer stories, or stories with more available actions will take significantly longer. Adding just one more necessary step to the story extends the planning process to several minutes. We also limited ourselves to one level of suspicion to limit the branching factor accordingly.
- Fall '16
- Robert Hassell
- Modal logic, Saul Kripke, Sherlock, Possible world, Many-worlds interpretation