Our aim is to motivate and provide a specification for a unification-based natural language processing system where grammars are expressed in terms of principles which constrain linguistic representations. Using typed feature structures with multiple inheritance for our linguistic representations and definite attribute-value logic clauses to express constraints, we will develop the bare essentials required for an implementation of a parser and generator for the Head-driven Phrase Structure Grammar (HPSG) formalism of Pollard and Sag (1987).