* Fellow in Law, University College, Oxford; CUF Lecturer in Law, Oxford University. In writing this Article I have, with the kind permission of the Oxford University Press, drawn on the material that appears in chapter 3 of MARTIN MATTHEWS, JONATHAN MORGAN & COLM O’CINNEIDE, HEPPLE & MATTHEWS’ TORT: CASES AND MATERIALS (6th ed. 2008). I have also benefited from the comments of David Partlett and Mike Green, and additionally from views expressed at the Symposium. The usual exemption applies.