By "easily quantified", I meant that there is exactly one possible quantification system that is known by and agreed to by both players, as opposed to the mess going on in real life as is being discussed on Stuart_Armstrong's thread. But I've deleted that part as unclear.

By "self-interested", I mean for example that the Nash equilibrium for Prisoners' Dilemma is (D,D) because we assume both are trying to minimize their sentence, and we don't have to worry about one of them trying to lessen the other's sentence at his own expense because the other prisoner has a wife and kids, or about them wanting to lengthen their own sentence to pay their debt to society. I agree that one could use the same math to analyze someone who is only playing a game to donate the payoff to charity, or something. I've added a footnote clarifying this further.