As pointed out in Chapter 1, theories of justice, especially contractually based ones such as those presented by Rawls, Gauthier, Barry, or Nozick, do not, in Wilfred Beckerman's (1997) phrase, “time-travel very well” (p. 395). In other words, moral consideration of future generations typically is not an essential feature of such theories owing to the fact that relations with the future cannot meet what Hume called the “circumstances of justice.” Although all of these theories (and especially Rawls's) acknowledge obligations to the future, these obligations are held to be outside the demands of justice. In other words, although being concerned for the future might be proof of moral uprightness, justice in particular does not actually require us to care about future generations. Keeping such future-oriented obligations can seem to resemble merely gift giving, because the relationship invoked by the alleged obligation lacks an essential element presumed necessary for a justice relationship to exist. It is that essential reciprocity that defines justice “as fairness” (Rawls), as “mutual advantage” (Barry), or in other contractual terms that simply cannot be present in relations with future generations.