How much pain are we talking about? I'm sure someone has developed a reasonably objective pain scale somewhere, so what's a good line? For example, let's assume a retarded adult is lactose intolerant, and doesn't realize that eating ice cream is making him hurt badly. Since he's likely to be in pain for the rest of his life, would this be a justification for euthinasia? I'm not saying it shouldn't be done in some cases-- I'm not the all-or-nothing kind of person in this argument.