“Murdered by persons unknown” – Speculative Reasoning in Law and Logic

This paper discusses some features of evidentiary reasoning in law, more specifically the introduction of “alternative suspects” by defence solicitors, within the framework of a first principle (or model) based decision support system for crime investigation. We introduce some observations from legal reasoning and legal doctrine to argue that the existing approach may not be able to capture some important distinctions and concepts of legal reasoning. We then introduce some ideas from dynamic logic and update semantics, in particular the concept of “pegs”, as good candidates to add the necessary expressive power to model based reasoning systems.