 A default theory consists of a set of formulas (the facts), together with a set of default rules. An extension of a default theory is a fixed point of a particular inferential process: an extension \(E\) must be a consistent theory (a consistent set closed under classical consequence) that contains all of the facts of the default theory \(T\), and, in addition, for each normal default \((p \Rightarrow q)\), if \(p\) belongs to \(E\), and \(q\) is consistent with \(E\), then \(q\) must belong to \(E\) also.