  Shafer's belief functions were introduced in the seventies of the previous
century as a mathematical tool in order to model epistemic probability. One of
the reasons that they were not picked up by mainstream probability was the lack
of a behavioral interpretation. In this paper we provide such a behavioral
interpretation, and re-derive Shafer's belief functions via a betting
interpretation reminiscent of the classical Dutch Book Theorem for probability
distributions. We relate our betting interpretation of belief functions to the
existing literature.
