A Modal Characterization of Defeasible Deontic Conditionals and Conditional Goals

We explore the notions of permission and obligation and their role in knowledge representation, especially as guides to action for planning systems. We first present a simple conditional deontic logic (or more accurately a preference logic) of the type common in the literature and demonstrate its equivalence to a number of modal and conditional systems for default reasoning. We show how the techniques of conditional default reasoning can be used to derive factual preferences from conditional preferences. We extend the system to account for the effect of beliefs on an agent’s obligations, including beliefs held by default. This leads us to the notion of a conditional goal, goals toward which an agent should strive according to its belief state. We then extend the system (somewhat naively) to model the ability of an agent to perform actions. Even with this simple account, we are able to show that the deontic slogan “make the best of a bad situation” gives rise to several interpretations or strategies for determining goals (and actions). We show that an agent can improve its decisions and focus its goals by making observations, or increasing its knowledge of the world. Finally, we discuss how this model might be extended and used in the planning process, especially to represent planning under uncertainty in a qualitative manner.