 An obvious challenge to critical level thinking is to make clear what constitutes the appropriate critical level. On the one hand, the level has to be high enough to avoid versions of the Repugnant Conclusion; on the other, the level has to be low enough not to rule out additions of clearly good lives (for an attempted answer to this challenge see Blackorby et al. 1997; Broome 2004). Critics have also been sceptical to the idea that the addition of lives worth living (albeit below the critical level) contribute negatively to the value of a population. Finally, it has been shown that this view leads to the Sadistic Conclusion (see above).