A Possibility Theory-based Approach to Desire Change

Desire is quite different from belief. While the accumulation of beliefs tend to reduce the remaining possible worlds they point at, the accumulation of desires tend to increase the set of states of affairs tentatively considered as satisfactory. Indeed beliefs are expected to be closed under conjunctions, while one can argue that endorsing ϕ∨ψ as a desire means to desire both ϕ and ψ. Still desiring ϕ and ¬ϕ at the same time is not usually regarded as rational, since it does not make much sense to desire one thing and its contrary at the same time. Thus when a new desire is added to the set of desires of an agent, a revision process may be necessary. Just as belief revision relies on an epistemic entrenchment relation, desire relation is based on a hedonic entrenchment relation satisfying other properties, due to the different natures of belief and desire. Epistemic entrenchment relations are known to be qualitative necessity relations. In this paper it is shown that a well-behaved desire revision operation obeying a set of reasonable postulates is underlied by a qualitative guaranteed possibility relation in the sense of possibility theory. Then the general framework of possibilistic logic provides a syntactic setting for encoding desire change