It we set the consent approach aside, a more promising way of showing that such an obligation exists involves an appeal to fairness or 'fair play'. Again an example is the best way to convey the basic idea. Suppose a group of us are living in a house with a shared kitchen. Every week or so one of the residents tidies the kitchen and gives the pans and the surfaces a really through clean. Now everyone else has dine the cleaning routine and it is my turn to spend half an hour scrubbing saucepans and mopping worktops. Why ought I do this? I have benefited from the work the others have put in - I have enjoyed having a clean kitchen to cook my supper in - and so I ought to carry my share of the cost too, in this case the cost of a bit of manual labour. If I don't take my turn, I'll be taking advantage of the other residents, and that's unfair. Notice that we don't need to assume here that I have agreed or consented to take part in the cleaning rota: my obligation stems directly from the fact that I am the beneficiary of a practice that requires each person to contribute in turn.