 Still other philosophers have suggested principles that require agents to create additional wellbeing, including, when necessary, by bringing a better off person into existence in place of a nonidentical less well off person, but limit that obligation to the case where agents can substitute in the better off person for the less well off person on a one-to-one basis. Such principles aim to address the nonidentity problem without giving rise to the repugnant conclusion. Holtug 2009; Holtug 2010, 160–162; see also Parfit 1987, 369–71 (discussion of Principle Q); Peters 2004, 27–39 (concept of injury by failure to substitute); and Savulescu 2001 (principle of procreative beneficence).