Deceit and indefeasible knowledge: the case of dubitatio

The current trend in knowledge revision in the Dynamic Epistemic Logic tradition focuses on the addition of new knowledge, rather than the possibility of losing knowledge. Yet there are natural situations where there is a need to be able to model the retraction of a proposition from a knowledge base. One situation where this is systematically required is the variant of the medieval theory of obligationes known as dubitatio, where one of the agents in the dialogue is obliged to hold the initial proposition as doubtful. In this paper, we use dubitatio as a motivation for studying deceitful agents, and we discuss various ways that an agent can move from a model where φ is known to one where φ