  A model of knowledge representation is described in which propositional facts
and the relationships among them can be supported by other facts. The set of
knowledge which can be supported is called the set of cognitive units, each
having associated descriptions of their explicit and implicit support
structures, summarizing belief and reliability of belief. This summary is
precise enough to be useful in a computational model while remaining
descriptive of the underlying symbolic support structure. When a fact supports
another supportive relationship between facts we call this meta-support. This
facilitates reasoning about both the propositional knowledge. and the support
structures underlying it.
