Where we draw the line on "mentally sound" is a social construct, right? I can't really wrap my head around somebody wanting to amputate their own hand and at the same time being considered "mentally sound" by the people around them.

> If someone wants to amputate their own hand, for example, it seems hard to argue that they shouldn’t be allowed to

I just don't get this. I suppose it hinges on what you mean by "allowed to". Maybe there's an argument to be made around personal bodily autonomy, that as a society we decide we don't want the government to compel us not to do things like this? But if someone I loved expressed to me that they wanted to harm themself I would do whatever I could to convince them not to. If one of my kids tried to chop off their hand for no good reason I would do everything I could to prevent it.