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Copernican Reasoning About Intelligent Extraterrestrials: A Reply to Simpson


Copernican reasoning involves considering ourselves, in the absence of other information, to be randomly selected members of a reference class. Consider the reference class intelligentobservers. If there are extraterrestrial intelligences (ETIs), taking ourselves to be randomly selected intelligent observers leads to the conclusion that it is likely the Earth has a larger population size than the typical planet inhabited by intelligent life, for the same reason that a randomly selected human is likely to come from a more populous country. The astrophysicist Fergus Simpson contends that this reasoning supports the claims that the typical planet inhabited by ETIs is smaller than Earth (radius \(\approx\) 5000 km; cf. Earth’s radius = 6371 km) and that the typical ETI is significantly larger than us (\(\approx\) 314 kg, the size of an adult male grizzly bear). Simpson’s applications of Copernican reasoning are novel and exciting. They should be of interest to philosophers concerned with Richard Gott’s delta t argument, the N = 1 problem in astrobiology, limited principles of indifference, and probabilistic epistemology in general. While we agree with Simpson about the qualitative direction of his conclusions, we take issue with his presentation of precise quantitative results because his methods (1) display bias, (2) ignore other variables contributing to population size, (3) commit an equivocation, and (4) conceal their dependence on arbitrary assumptions.

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The authors would like to thank Nicholas Horton, Brian Kierland, Amanda Landi, Bradley Monton, Dan Nielsen, Oliver Xue, and the philosophy department at Boise State University for invaluable conversations related to this paper. In addition, we are grateful for the thoughtful and accurate feedback we received from the reviewers for this journal.

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Correspondence to Samuel Ruhmkorff.

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Ruhmkorff, S., Jiang, T. Copernican Reasoning About Intelligent Extraterrestrials: A Reply to Simpson. J Gen Philos Sci 50, 561–571 (2019).

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  • Copernican Principle
  • Intelligent extraterrestrials
  • Aliens
  • Richard Gott
  • Fergus Simpson