  Mechanisms for the automation of uncertainty are required for expert systems.
Sometimes these mechanisms need to obey the properties of probabilistic
reasoning. A purely numeric mechanism, like those proposed so far, cannot
provide a probabilistic logic with truth functional connectives. We propose an
alternative mechanism, Incidence Calculus, which is based on a representation
of uncertainty using sets of points, which might represent situations, models
or possible worlds. Incidence Calculus does provide a probabilistic logic with
truth functional connectives.
