This paper examines the role of `feature specification defaults' (FSDs) in the GPSG formalism and attempts to provide a characterisation of them in terms of formal non- monotonic logic. FSDs provide the grammar writer with a mechanism for specifying conditions on syntactic categories which should hold `by default': a category must satisfy such a default condition unless specifically overruled by some other component of the grammar specification. The approach taken here combines recent work on the formal characterisation of syntactic categories (Gazdar 87) with advances in the field of non- monotonic logic, in particular autoepistemic logic (Moore 83,Moore 84), to capture this default behaviour formally. In the process of this formalisation, a number of inadequacies and peculiarities of the GPSG account, both of defaults in particular and of syntactic structure in general, are revealed and discussed.
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