  However, even if altruism can survive this first challenge, there is a second reason to question its importance for the debate about the permissibility of organ sale. This is the distinction between cases in which altruism is obligatory (where there is a moral duty to help others) and those in which altruism is supererogatory (morally good, but not morally required—going ‘above and beyond’ one's duty) (Wilkinson 2003). This distinction is relevant for the following reason. If (say) altruistic kidney donation were morally obligatory, then to demand money for one's organ (and, arguably, to accede to such a demand) would be wrong. But if, on the other hand, altruistic donation were supererogatory, then to demand money for one's organ would not be wrong. Rather, it would be merely non-supererogatory: perhaps not good, but not wrong.  So, with this distinction in place, one might (at least in some cases) accept that altruistic donations are good whilst also saying that there is nothing wrong with non-altruistic donation—the point being that non-altruistic donation, while not as good as altruistic donation, is nonetheless (morally) permissible. This has implications for the sort of altruism argument which can be made against organ sale. If it could be shown that altruistic donation is obligatory then the argument would be stronger, or at least more straightforward, because it would follow that selling was wrong (it is wrong to demand money for what one ought to be giving freely anyway). But if all that could be shown was that altruistic donation is good, then it would not follow from this, or at least it would not follow directly, that selling is wrong. For it might be merely non-supererogatory (Wilkinson &amp; Garrard 1996).