Much of the art we love is presented via a medium - be it a canvas, a recording or celluloid. Stephen Sackur interviews Marina Abramović, an artist whose primary resource is her own body. In the course of a remarkable career, the world's most famous and garlanded performance artist has pushed herself to the very limits of physical endurance and stirred intense reaction from audiences confronting her eye to eye. Her art and life are one; so what do they tell us?