Fwd: Bad Bayesian - no biscuit! (was A New Year's gift for Bayesians)

From: Eliezer Yudkowsky (sentience@pobox.com)
Date: Wed Jan 19 2005 - 12:48:01 MST

attached mail follows:

[I'm back from holidays having done some reading relevant to some
recent discussion on this list. Along with part of Popper's _Realism
and the Aim of Science_, I reread a little Penguin book on some of
Feynman's lectures entitled _The Meaning of It All_ containing a
chapter entitled the Uncertainty of Science.

In his, imo, well written, and enjoyable essay A Technical Explanation
of Technical Explanation, http://yudkowsky.net/bayes/technical.html
Eliezer writes (page 32 of 36 when I printed it out) :

    "Imagine that you wake up one morning and your left arm
     has been replaced by a blue tentacle. The blue tentacle
     obeys your motor commands - you can use it to pick up
     glasses, drive a car, etc. How would you explain this
     hypothetical scenario? Take a moment to ponder this
     puzzle before continuing."

So I did imagine it. I imagined it in good faith, and I imagined it
consistent with a spirit of exploration and good will built that Eliezer
had established through the early part of his essay.

Where Eliezer had placed "spoiler space", I stopped reading and I
wrote down my explanation. (I'd been reading with pen in hand and
making critical notes in the margin.) It seemed to be fair and
scientific to provide an answer *before* reading on so as not to
contaminate the experiment. )

I wrote (and I quote):

    "I'd "explain" it provisionally as some surprising organisation
    of people had entered my house and replaced my arm whilst
     I slept with technology I didn't know existed.

    I'd be bewildered. Frightened even. But I'd not think "magic"
    had occurred".

And then, with the heightened curiosity of one who has escalated
their commitment I went back to see what Eliezer the Bayesian,
Eliezer the spreader-of-analogical-probability-clay-mass would have

And he'd written this.

   "How would I explain the event of my left arm being replaced
    by a blue tentacle? The answer is that I wouldn't. It isn't going
    to happen."

Email perhaps can't convey my exact reaction to that but here's
the comments I wrote in the margin.

"No. No. No. You cheated Eliezer. You cheated !  
You can't assign a probability of zero ! *
Not fair!! You said it did happen. Your being dishonest with the
data to say its not going to happen." 
Most of what I know of Bayesian reasoning I know as a result of
reading Eliezer's two essays on it.  So perhaps if my understanding
of Bayesian reasoning or inference is wrong I can escape by
blaming Eliezer for it :-) 
I suspect, on the basis of those two essays that I am a Bayesian
although I didn't know I was and so I haven't been calling myself
one. The merit I see in the Bayesian approach is that it manages
uncertainties more carefully and consistently then most people do
intuitively. [And boy does the world need that].
So, it's 2005.  I'm a Bayesian. And as long as I'm wearing metaphorical
teeshirts I'm also a Bright.  
Brett Paatsch
*  Eliezer's assigning a probability of zero to observed facts however
unlikely those facts might have been a priori is the reason for my
heading this post Bad Bayesian - no buscuit.
PS: Happy new year.

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