I am grateful for comments made at various seminars where versions of this paper were presented. I was helped by discussions with David Braddon Mitchell, John Braithwaite, Geoffrey Brennan, Andrew Brien, John Charvet, Tony Coady, Bob Goodin, David Miller, Graham Oddie, Michael Smith, Jeremy Waldron and Timothy Williamson. I also benefited from written comments received from Will Kymlicka and Mark Sacks. I completed the paper while holding visiting appointments, first at the London School of Economics and then at the Ecole Polytechnique in Paris. I wish to thank both institutions for their support.