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Stuart_Armstrong comments on Versions of AIXI can be arbitrarily stupid - Less Wrong Discussion

15 Post author: Stuart_Armstrong 10 August 2015 01:23PM

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Comment author: buybuydandavis 11 August 2015 07:48:02PM 5 points [-]

then an AIXI that follows one prior can be arbitrarily stupid with respect to another.

Yet another application of David Wolpert's No Free Lunch theorems.

We have dubbed the associated results NFL theorems because they demonstrate that if an algorithm performs well on a certain class of problems then it necessarily pays for that with degraded performance on the set of all remaining problems.

https://en.wikipedia.org/wiki/No_free_lunch_theorem

Comment author: Stuart_Armstrong 11 August 2015 09:17:21PM 1 point [-]

Yes. The no free lunch theorems are powerful in theory, but almost pointless in practice. I was hoping that AIXI could evade them, even in theory, but it seems to not be the case.

Comment author: buybuydandavis 26 August 2015 02:51:15AM 0 points [-]

The point of the NFL theorems in practice is to keep you from getting your hopes up that you'll get a free lunch.

Comment author: Stuart_Armstrong 26 August 2015 01:44:45PM 0 points [-]

So the point of no free lunch theorems is to tell you you won't get a free lunch? ^_^

Comment author: buybuydandavis 27 August 2015 12:38:50AM 0 points [-]

There's another real point - focusing on your prior as "fit" to the problem/universe.

The space of possible priors Wolpert considered were very unlike our experience - basically imposing no topological smoothness on points - every point is a ball from the urn of possible balls. That's just not the way it is. Choosing your prior, and exploiting the properties of your prior then becomes the way to advance.