  A key issue in the handling of temporal data is the treatment of persistence;
in most approaches it consists in inferring defeasible confusions by
extrapolating from the actual knowledge of the history of the world; we propose
here a gradual modelling of persistence, following the idea that persistence is
decreasing (the further we are from the last time point where a fluent is known
to be true, the less certainly true the fluent is); it is based on possibility
theory, which has strong relations with other well-known ordering-based
approaches to nonmonotonic reasoning. We compare our approach with Dean and
Kanazawa's probabilistic projection. We give a formal modelling of the
decreasing persistence problem. Lastly, we show how to infer nonmonotonic
conclusions using the principle of decreasing persistence.
