‘What is art?’ is one of the classic questions that philosophy has addressed over the ages, from the ancients to today. Taking as its starting point debates over the various definitions of art found in history, this article presents and discusses some of the major theories offered by both the analytic and continental traditions. It then looks at the theoretical reasons that led twentieth-century philosophy to reopen the question of definition, and in many cases inquire into the ontology of art itself. Finally, a series of considerations are addressed to help shift the problem of definition onto a new plane, one that is able to respond to the challenges of the performing and participatory arts, which more than any other form of art present particularly unconventional ontologies.