In the last years, the vision of the Semantic Web fostered the interest in reasoning over ever larger sets of assertional statements in ontologies. It is easily conjectured that, soon, real-world ontologies will not fit into main memory anymore. If this was the case, state-of-the-art description logic reasoning systems cannot deal with these ontologies any longer, since they rely on in-memory structures.

We propose a way to overcome this problem by reducing instance checking for an individual in an ontology to a (usually small) relevant subset of assertional axioms. This subset can then be processed by state-of-the-art description logic reasoning systems to perform sound and complete instance checks for the given individual. We think that this technique will support description logic systems to deal with the upcoming large amounts of assertional data.