GEAHS (Generic Educational Adaptive Hypermedia System) is a platform designed to ease the development of Adaptive Educational Hypermedia, using standard formalisms. In this document, we explain the underlying principles of this platform. Genericity is achieved thanks to an adaptation engine based on situation calculus and RDF. This paper describes the main aspects of our system, as well as the use we make of situation calculus to create a more reusable adaptive hypermedia system.