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I'm very grateful to receive such long and thoughtful responses from some of the world's most creative and influential moral philosophers. Since I largely agree with Jeff McMahan and Larry Temkin, I will devote most of my scarce space to Rob Sparrow.
Sparrow earlier claimed that since women gestate and live longer, enhancers are committed to parents conceiving only girls. To avoid this absurdity, we must reject enhancement and endorse what Sparrow calls “therapy”. I noted we first need to know what “therapy” means, and devised various clarificatory cases. This central aspect of my paper remains unaddressed, with Sparrow admitting his view is ill-defined.1 He also grants his reductio is inapplicable to some enhancement doctrines, implying he provided no case uniquely favouring therapy.
Other enhancement doctrines may avoid Sparrow's reductio for different reasons. For example, John Harris noted that enhancers are not committed to enhancing regardless of consequences, including extinction. Sparrow then denies extinction-avoidance provides reasons for individuals. I argued that (unless non-contribution involves disproportionally high costs) it is wrong for individuals to contribute to disasters like extinction or climate change. Sparrow now admits this but claims parents may permissibly do so for their children's sake.1 Their children presumably can reciprocate, so family emissions can grow with everybody permissibly taking for others what it remains impermissible for those others to take for themselves. Since this is implausible, Sparrow's argument fails.
Sparrow also claimed that a non-individual concern to prevent extinction is akin to Nazi eugenics, and comparable to creating people with barely worth-living lives to feel superior to them. Commenting on the case, I claimed that “the avoidable creation of lives barely worth living is itself abhorrent”.2 McMahan assumed I meant ‘creating such lives is never permissible’,3 but by “avoidable” I meant ‘when creating better …