CarmendeMacedo comments on Rationality Quotes July 2011 - Less Wrong

2 Post author: Normal_Anomaly 03 July 2011 06:41AM

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Comment author: orthonormal 07 July 2011 04:04:14PM 28 points [-]

Suppose I think, after doing my accounts, that I have a large balance at the bank. And suppose you want to find out whether this belief of mine is "wishful thinking." You can never come to any conclusion by examining my psychological condition. Your only chance of finding out is to sit down and work through the sum yourself. When you have checked my figures, then, and then only, will you know whether I have that balance or not. If you find my arithmetic correct, then no amount of vapouring about my psychological condition can be anything but a waste of time. If you find my arithmetic wrong, then it may be relevant to explain psychologically how I came to be so bad at my arithmetic, and the doctrine of the concealed wish will become relevant — but only after you have yourself done the sum and discovered me to be wrong on purely arithmetical grounds. It is the same with all thinking and all systems of thought. If you try to find out which are tainted by speculating about the wishes of the thinkers, you are merely making a fool of yourself. You must first find out on purely logical grounds which of them do, in fact, break down as arguments. Afterwards, if you like, go on and discover the psychological causes of the error.

C.S. Lewis, "Bulverism"

(It's not exactly correct- evidence of bias is some evidence against a belief- but not always as strong of evidence as it's assumed to be.)

Comment author: CarmendeMacedo 10 July 2011 09:24:25AM 1 point [-]

Why should evidence of bias be some evidence against a belief? This would be like magic: using someone's failure of rationality to learn something about the world, which is absurd. (Example: Federer's wife is very confident that he will win, because she is biased in his favor. Does this give me any reason to bet against Federer? Obviously not.)

Comment author: Oscar_Cunningham 10 July 2011 10:01:00AM 9 points [-]

If you find out that someone believes A then that's evidence for A, so your beliefs change away from the priors. If you subsequently find that the person is likely biased then your beliefs return some way toward your priors. So finding out about the bias was in some sense evidence about A.

Comment author: Tesseract 10 July 2011 06:35:56PM *  3 points [-]

To be precise, knowing that someone is biased towards holding a belief decreases the amount you should update your own beliefs in response to theirs — because it decreases the likelihood ratio of the test.

(That is, having a bias towards a belief means people are more likely to believe it when it isn't true (more false positives), so a bias-influenced belief is less likely to be true and therefore weaker evidence. In Bayesian terms, bias increases P(B) without increasing P(B|A), so it decreases P(A|B).)

So CarmendeMacedo's right that you can't get evidence about the world from knowledge of a person's biases, but you should decrease your confidence if you discover a bias, because it means you had the wrong priors when you updated the first time.

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