Us not wanting people to do things with our body without our knowledge is indeed a different argument, one which seems to show that at least some preferences matter ethically. But preference utilitarianism is usually the view that only preferences matter, perhaps even all preferences.
Regarding Parfit’s case, is this not the same as me being unconscious while my body is manipulated? In both cases we do not seem to currently hold a preference. In one because he forgot about it, in the other because I’m unconscious.
But even suppose Parfit did not forget about the stranger. Why would it be good for Parfit that the stranger is cured, without his knowledge? To me it does not seem to be good for him. And wouldn’t such a view have the unfair consequence that it is much less important to cure a lonely person about whom no other people care than a popular person about whom lots of people care, even if those are not informed about the cured illness?
Yes exactly.