Copernican Reasoning About Intelligent Extraterrestrials: A Reply to Simpson

  • Samuel RuhmkorffEmail author
  • Tingao Jiang


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.


Copernican Principle Intelligent extraterrestrials Aliens Richard Gott Fergus Simpson 



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.


  1. Cassidy, E. S., West, P. C., Gerber, J. S., & Foley, J. A. (2013). Redefining agricultural yields: From tonnes to people nourished per hectare. Environmental Research Letters, 8(3), 034015. Scholar
  2. Diamond, J. (1997). Guns, germs, and steel: The fates of human societies. New York: W. W. Norton & Co.Google Scholar
  3. Gott, J. R. (1993). Implications of the Copernican principle for our future prospects. Nature, 363(6427), 315–319.CrossRefGoogle Scholar
  4. Gott, J. R. (1994). Future prospects discussed. Nature, 368(6467), 108.CrossRefGoogle Scholar
  5. Gott, J. R. (1996). Our future in the universe. In V. Trimble & A. Reisenegger (Eds.), Clusters, lensing, and the future of the universe (pp. 140–151). San Francisco: Astronomical Society of the Pacific.Google Scholar
  6. Gott, J. R. (2001). Time travel in Einstein’s universe. Boston: Houghton Mifflin.Google Scholar
  7. Main, D. (2015). Aliens are enormous, science suggests. Newsweek, April 5. Accessed 12 July 2019.
  8. Monton, B., & Kierland, B. (2006). How to predict future duration from present age. The Philosophical Quarterly, 56(222), 16–38.CrossRefGoogle Scholar
  9. PBL Netherlands Environmental Assessment Agency. (2018). History database of the global environment. Accessed 12 July 2019.
  10. Peters, R. H., & Raelson, J. V. (1984). Relations between individual size and mammalian population density. The American Naturalist, 124(4), 498–517.CrossRefGoogle Scholar
  11. Pontzer, H., Raichlen, D. A., Wood, B. W., Mabulla, A. Z. P., et al. (2012). Hunter-gatherer energetics and human obesity. PloS One, 7(7), e40503. Scholar
  12. Simpson, F. (2015a). The nature of inhabited planets and their inhabitants. arXiv:1503.07804v1.
  13. Simpson, F. (2015b). The size distribution of inhabited planets. Monthly Notices of the Royal Astronomical Society: Letters, 456(1), L59–L63. Scholar
  14. Simpson, F. (2017a). The longevity of habitable planets and the development of intelligent life. International Journal of Astrobiology, 16(3), 266–270. Scholar
  15. Simpson, F. (2017b). Bayesian evidence for the prevalence of waterworlds. Monthly Notices of the Royal Astronomical Society, 468(3), 2803–2815.CrossRefGoogle Scholar
  16. Walpole, S. C., Prieto-Merino, D., Edwards, P., Cleland, J., et al. (2012). The weight of nations: An estimation of adult human biomass. BMC Public Health., 12(1), 439. Scholar

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© Springer Nature B.V. 2019

Authors and Affiliations

  1. 1.Division of Social StudiesBard College at Simon’s RockGreat BarringtonUSA

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