  We introduce a new approach to modeling uncertainty based on plausibility
measures. This approach is easily seen to generalize other approaches to
modeling uncertainty, such as probability measures, belief functions, and
possibility measures. We focus on one application of plausibility measures in
this paper: default reasoning. In recent years, a number of different semantics
for defaults have been proposed, such as preferential structures,
$\epsilon$-semantics, possibilistic structures, and $\kappa$-rankings, that
have been shown to be characterized by the same set of axioms, known as the KLM
properties. While this was viewed as a surprise, we show here that it is almost
inevitable. In the framework of plausibility measures, we can give a necessary
condition for the KLM axioms to be sound, and an additional condition necessary
and sufficient to ensure that the KLM axioms are complete. This additional
condition is so weak that it is almost always met whenever the axioms are
sound. In particular, it is easily seen to hold for all the proposals made in
the literature.
