Theorist: A Logical Reasoning System for Defaults and Diagnosis

We provide an introduction to Theorist, a logic programming system that uses a uniform deductive reasoning mechanism to construct explanations of observations in terms of facts and hypotheses. Observations, facts, and possible hypotheses are each sets of logical formulas that represent, respectively, a set of observations on a partial domain, a set of facts for which the domain is a model, and a set of tentative hypotheses which may be required to provide a consistent explanation of the observations.