To paraphrase Robin Hanson, rationality should exist.
Personally, in terms of utility functions, I'm inclined to give my own suffering positive utility (it's just, y'know, substantially lower than the utility of pleasure). But then, I write poetry on the beauty of the meaningfulness of pain. It's curious what a few years of systematically killing emotion will do to you when it reawakens with a vengeance.
Sure, if humans exist then rationality should exist.
But my question stands, why should humans exist? If I'm reading correctly your post assumes that a rapid cessation of humans would be somehow a bad thing.
Stop me if you feel this is entering into uselessly nihilistic territory and we can call it quits here.
This is the public group instrumental rationality diary for April 5-14.
Thanks to cata for starting the Group Rationality Diary posts, and to commenters for participating!
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