Editing Derek Parfit (section)

=== Mere addition paradox and repugnant conclusion ===
In part four of ''Reasons and Persons'', Parfit discusses possible futures for the world.<ref name=":4" />{{Rp|349–441}} He discusses possible futures and population growth in chapter 17, and argues that both [[average and total utilitarianism]] result in unwelcome conclusions when applied to population.<ref name=":4" />{{Rp|388}}

In the section titled "Overpopulation", Parfit distinguishes between average utilitarianism and total utilitarianism. He formulates average utilitarianism in two ways. One is what Parfit calls the "Impersonal Average Principle", which he formulates as "If other things are equal, the best outcome is the one in which people's lives go, on average, best."<ref name=":4" />{{Rp|386}} The other is what he calls the "Hedonistic version"; he formulates this as "If other things are equal, the best outcome is the one in which there is the greatest average net sum of happiness, per life lived."<ref name=":4" />{{Rp|386}} Parfit then gives two formulations of the total utilitarianism view. The first formulation Parfit calls the "Hedonistic version of the Impersonal Total Principle": "If other things are equal, the best outcome is the one in which there would be the greatest quantity of happiness—the greatest net sum of happiness minus misery."<ref name=":4" />{{Rp|387}} He then describes the other formulation, the "non-Hedonistic Impersonal Total Principle": "If other things are equal, the best outcome is the one in which there would be the greatest quantity of whatever makes life worth living."<ref name=":4" />{{Rp|387}}
[[File:MereAddition.svg|thumb|The width of each bar represents the size of a population, and the height represents the individual welfare level. The [[Mere addition paradox|mere addition]] from A to B results in a larger population with a lower individual welfare.]]
[[File:RepugnantConclusion.svg|thumb|The "repugnant conclusion" involves repeating the mere  addition until obtaining a much larger population Z with only slightly positive individual welfare.]]
Applying total utilitarian standards (absolute total happiness) to possible population growth and welfare leads to what he calls the [[repugnant conclusion]]: "For any possible population of at least ten billion people, all with a very high quality of life, there must be some much larger imaginable population whose existence, if other things are equal, would be better, even though its members have lives that are barely worth living."<ref name=":4" />{{Rp|388}} Parfit illustrates this with a simple thought experiment: imagine a choice between two possible futures. In A, 10 billion people would live during the next generation, all with extremely happy lives, lives far happier than anyone's today. In B, there are 20 billion people all living lives that, while slightly less happy than those in A, are still very happy. Under total utility maximisation we should prefer B to A. Therefore, through a regressive process of population increases and happiness decreases (in each pair of cases the happiness decrease is outweighed by the population increase) we are forced to prefer Z, a world of hundreds of billions of people all living lives barely worth living, to A. Even if we do not hold that coming to exist can benefit someone, we still must at least admit that Z is no worse than A.<ref name=":6" />

Parfit and other philosophers have explored a number of potential ways to avoid this "repugnant conclusion", many of which are discussed in ''Reasons and Persons''. Parfit himself spent significant time trying to find a coherent ethical theory that would avoid it.<ref name=":6" /> One potential objection challenges what life in the A-world would be like and whether life in the Z-world would differ very much from a normal privileged life. Another one proposes that movement from the A-world to the Z-world can be blocked by discontinuity. [[Negative utilitarianism|Negative utilitarians]] may reject the premise of maximizing happiness, emphasizing the converse, the minimization of suffering.<ref name=":8" /> Philosopher [[Larry Temkin]] challenges the assumption that the "better than" relation is [[Transitive relation|transitive]].<ref>{{Cite journal |last=Temkin |first=Larry S. |date=1987 |title=Intransitivity and the Mere Addition Paradox |url=https://philpapers.org/rec/TEMIAT |journal=Philosophy and Public Affairs |volume=16 |issue=2 |pages=138–187}}</ref> Others proposed a minimal threshold of liberties and primary social goods to be distributed, or suggested to adopt a [[Deontology|deontological]] approach that looks to values and their transmission through time.<ref name=":8">Various authors of essays in ''The Repugnant Conclusion,'' eds. Jesper Ryberg and [[Torbjörn Tännsjö]] ([[Kluwer Academic Publishers]], 2004) {{ISBN|978-1402024726}}; and, Young Kim, [https://rowman.com/ISBN/9781498516518/Justice-as-Right-Actions-An-Original-Theory-of-Justice-in-Conversation-with-Major-Contemporary-Accounts Justice as Right Actions: An Original Theory of Justice in Conversation with Major Contemporary Accounts] ([[Rowman & Littlefield|Lexington Books]], 2015) ch.10 ( {{ISBN|978-1-4985-1651-8}}).</ref> [[Michael Huemer]] and [[Torbjörn Tännsjö]] endorse the conclusion, considering that the repugnance comes from intuitions that should be revised.<ref>{{Cite web |last=Matthews |first=Dylan |date=2019-06-05 |title=The case for adding more and more people to the Earth |url=https://www.vox.com/the-highlight/2019/6/5/18617894/repugnant-conclusion-population-growth-philosophy |access-date=2025-01-26 |website=Vox |language=en-US}}</ref><ref>{{Cite journal |last=Huemer |first=Michael |date=2008 |title=In Defence of Repugnance |url=https://philarchive.org/archive/HUEIDO |journal=Mind}}</ref> In 2021, a number of philosophers said that avoiding the repugnant conclusion was receiving excessive focus and should not be considered a necessary condition for an adequate theory of [[population ethics]].<ref>{{Cite journal |last=Zuber |first=Stéphane |last2=Venkatesh |first2=Nikhil |last3=Tännsjö |first3=Torbjörn |last4=Tarsney |first4=Christian |last5=Stefánsson |first5=H. Orri |last6=Steele |first6=Katie |last7=Spears |first7=Dean |last8=Sebo |first8=Jeff |last9=Pivato |first9=Marcus |last10=Ord |first10=Toby |last11=Ng |first11=Yew-Kwang |last12=Masny |first12=Michal |last13=MacAskill |first13=William |last14=Lawson |first14=Nicholas |last15=Kuruc |first15=Kevin |date=13 April 2021 |title=What Should We Agree on about the Repugnant Conclusion? |url=https://www.cambridge.org/core/journals/utilitas/article/what-should-we-agree-on-about-the-repugnant-conclusion/EB52C686BAFEF490CE37043A0A3DD075 |journal=Utilitas |language=en |volume=33 |issue=4 |pages=379–383 |doi=10.1017/S095382082100011X |issn=0953-8208|hdl=10852/90849 |hdl-access=free }}</ref>

Against average utilitarianism, Parfit also argues that if all that matters is average happiness, then the best population could be arbitrarily small, for example containing only a few extremely happy individuals. Moreover, this would imply that adding people with lives worth living would make the world worse if these are less happy than the average.<ref name=":4" />{{Rp|420}}