Anthropics over-simplified: it’s about priors, not updates

I’ve ar­gued that an­thropic rea­son­ing isn’t magic, ap­plied an­thropic rea­son­ing to the Fermi ques­tion, claimed that differ­ent an­thropic prob­a­bil­ities an­swer differ­ent ques­tions, and con­cluded that an­throp­ics is pretty nor­mal.

But all those posts were long and some­what tech­ni­cal, and needed some fa­mil­iar­ity with an­thropic rea­son­ing in or­der to be ap­plied. So here I’ll list what peo­ple un­fa­mil­iar with an­thropic rea­son­ing can do to add it sim­ply[1] and eas­ily to their pa­pers/​blog posts/​dis­cus­sions:

  1. An­throp­ics is about pri­ors, not up­dates; up­dates func­tion the same way for all an­thropic prob­a­bil­ities.

  2. If two the­o­ries pre­dict the same pop­u­la­tion, there is no an­thropic effect be­tween them.

Up­dat­ing on safety

Sup­pose you go into hid­ing in a bunker in 1956. You’re not sure if the cold war is in­trin­si­cally sta­ble or un­sta­ble. Stable pre­dicts a chance of nu­clear war; un­sta­ble pre­dicts a chance.

You emerge much older in 2020, and no­tice there has not been a nu­clear war. Then, what­ever an­thropic prob­a­bil­ity the­ory you use, you up­date the ratio

Pop­u­la­tion balancing

Sup­pose you have two the­o­ries to ex­plain the Fermi para­dox:

  • The­ory 1 is that life can only evolve in very rare con­di­tions, so Earth has the only life in the reach­able uni­verse.

  • The­ory 2 is that there is some dis­aster that reg­u­larly obliter­ates pre-life con­di­tions, so Earth has the only life in the reach­able uni­verse.

Since the to­tal pop­u­la­tion pre­dicted by these two the­o­ries is the same, there is no an­thropic up­date be­tween them[2].


  1. Th­ese points are a bit over-sim­plified, but are suit­able for most likely sce­nar­ios. ↩︎

  2. If you use a refer­ence class that doesn’t in­clude cer­tain en­tities—maybe you don’t in­clude pre-mam­mals or be­ings with­out cen­tral ner­vous sys­tems—then you only need to com­pare the pop­u­la­tion that is in your refer­ence class. ↩︎

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