The automatic extraction of ontologies from text and lexical resources has become more and more mature. Nowadays, the results of state-of-the-art ontology learning methods are already good enough for many practical applications. However, most of them aim at generating rather inexpressive ontologies, i.e. bare taxonomies and relationships, whereas many reasoning-based applications in domains such as bioinformatics or medicine rely on much more complex axiomatizations. Those are extremely expensive if built by purely manual efforts, and methods for the automatic or semi-automatic construction of expressive ontologies could help to overcome the knowledge acquisition bottleneck. At the same time, a tight integration with ontology evaluation and debugging approaches is required to reduce the amount of manual post-processing which becomes harder the more complex learned ontologies are. Particularly, the treatment of logical inconsistencies, mostly neglected by existing ontology learning frameworks, becomes a great challenge as soon as we start to learn huge and expressive axiomatizations. In this chapter we present several approaches for the automatic generation of expressive ontologies along with a detailed discussion of the key problems and challenges in learning complex OWL ontologies. We also suggest ways to handle different types of inconsistencies in learned ontologies, and conclude with a visionary outlook to future ontology learning and engineering environments.
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