Consider how strange is the question posed by someone who wants a justification for altruism about such a basic matter as this. Suppose he and some other people have been admitted to a hospital with severe burns after being rescued from a fire. “I understand how my pain provides me with a reason to take an analgesic,” he says, “and I understand how my groaning neighbor’s pain gives him a reason to take an analgesic; but how does his pain give me any reason to want him to be given an analgesic? How can his pain give me or anyone else looking at it from outside a reason?
This question is crazy. As an expression of puzzlement, it has that characteristic philosophical craziness which indicates that something very fundamental has gone wrong. This shows up in the fact that the answer to the question is obvious, so obvious that to ask the question is obviously a philosophical act. The answer is that pain is awful. The pain of the man groaning in the next bed is just as awful as yours. That’s your reason to want him to have an analgesic.
Thomas Nagel, ‘The Limits of Objectivity’, in Sterling McMurrin (ed.), The Tanner Lectures on Human Values, vol. 1, Cambridge, 1980, pp. 109-110