“Pragmatic FDT, and predictors as game theory” by Stuart_Armstrong

Decision theory is back in fashion (defining fashion as "one good post on a good EA blog"). Bentham's Bulldog (BB) has published a case against FDT (functional decision theory), contrasting rationalist enthusiasm with academic scepticism: "Academic decision theorists don't like the theory. The number of academic decision theorists who adopt it could be counted on one hand by someone missing four of their fingers."

I am, just barely, a published academic decision theorist, so you can keep a small finger to count me too. My position is that, though FDT may have problems with its definitions and under-definedness, we can build defined variants that achieve what FDT attempted to.

I want to do two things in this post. First, sketch a "pragmatic" version of FDT designed to sidestep the theoretical pitfalls that Will MacAskill and Wolfgang Schwarz identify. Second, take a closer look at what predictors actually do, and argue that whenever they make counterfactual predictions, decision theory shades into game theory -- which explains why EDT/TDT/UDT/FDT can look irrational in the odd branch. It's the old debate of "should you pay the blackmailer", dressed up in predictor garb.

Pragmatic FDT

MacAskill and BB both [...]

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Outline:

(01:26) Pragmatic FDT

(05:54) Exploitable isomorphisms

(07:10) How we identify likely-true isomorphisms

(10:27) Application to standard problems

(12:00) Discontinuity across a spectrum of predictors

(15:05) No advanced counterpossible theory required

(15:30) Predictors, counterfactuals, and game theory

(19:47) Conclusion

(21:10) Appendix: CDT can't believe in predictors

The original text contained 10 footnotes which were omitted from this narration.

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First published:
July 3rd, 2026

Source:
https://www.lesswrong.com/posts/SdGbWkCZgCN7EGBxM/pragmatic-fdt-and-predictors-as-game-theory-1

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Narrated by TYPE III AUDIO.