prase comments on 5 Axioms of Decision Making - Less Wrong

35 Post author: Vaniver 01 December 2011 10:22PM

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Comment author: taw 02 December 2011 09:54:06AM *  -1 points [-]

You must be willing to assign a probability to quantify any uncertainty important to your decision. You must have consistent probabilities.

  • What's your probability of basic laws of mathematics being true?
  • What's your probability of Collatz conjecture being true?

If you answered 1 to the first, and anything but 0 or 1 to the second, you're inconsistent. If you're unwilling to answer the second, you just broke your axioms.

Comment author: prase 02 December 2011 11:28:44AM *  1 point [-]

What's your probability of basic laws of mathematics being true?

If (basic laws = axioms and inference rules), meaning of "true" needs clarification.

What's your probability of Collatz conjecture being true?

0.64 (Here, by "true" I mean "can be proven in Peano arithmetics".)

Comment author: taw 02 December 2011 11:34:05AM -2 points [-]

0.64 (Here, by "true" I mean "can be proven in Peano arithmetics".)

Then you're enitrely inconsistent, since P(Collatz sequence for k converges) is either 0 or 1 for all k by basic laws of mathematics, and P(Collatz conjecture is true) equals product of these, and by basic laws of mathematics can only be 0 or 1.

Comment author: prase 02 December 2011 11:52:07AM *  2 points [-]

Why had you chosen Collatz conjecture to illustrate the fact (which already has been discussed several times) that uncertainty about mathematical statements introduces inconsistency of some sort? I am equally willing to put p = 0.1 to the statement "last decimal digit of 1543! is 7", although in fact this is quite easy to check. Just I don't want to spend time checking.

If for consistency you demand that subjective probabilities assigned to logically equivalent propositions must be equal (I don't dispute that it is sensible to include that to definition of "consistent"), then real people are going to be inconsistent, since they don't have enough processing power to check for consistency. This is sort of trivial. People hold inconsistent beliefs all the time, even when they don't quantify them by probabilities.

If you point to some fine mathematical problems with "ideal Bayesian agents", then I don't see how it is relevant in context of the original post.

Edit: by the way,

P(Collatz sequence for k converges) is either 0 or 1

sounds frequentistish.

Comment author: taw 02 December 2011 12:17:00PM 1 point [-]

I am equally willing to put p = 0.1 to the statement "last decimal digit of 1543! is 7", although in fact this is quite easy to check. Just I don't want to spend time checking.

What probabilities are are you willing to assign to statements:

  • 1543! = 1540 * (1543 * 1542 * 1541 * 1539!)
  • The last digit of "1540 * (1543 * 1542 * 1541 * 1539!)" is 0 and not 7

Bayesian probabilities don't give you any anchoring to reality, they only give you consistency.

If you're willing to abandon consistency as well, they give you precisely nothing whatsoever.

Probabilities are a tool for talking about uncertainty, they are not uncertainty, to think otherwise is a ridiculous map-territory confusion.

sounds frequentistish.

As ad hominem attacks go, that's an interesting one.

If there's one possible universe where Collatz conjecture is true/false, it is true/false is all other possible universes as well. There are no frequencies there, it's just pure fact of logic.

Comment author: prase 02 December 2011 01:06:35PM *  5 points [-]

The last digit of "1540 * (1543 * 1542 * 1541 * 1539!)" is 0 and not 7

Updated. (Didn't occur to me it would be so easy.)

Bayesian probabilities don't give you any anchoring to reality, they only give you consistency. If you're willing to abandon consistency as well, they give you precisely nothing whatsoever.

It is unnecessarily black-and-white point of view on consistency. I can improve my consistency a lot without becoming completely consistent. In practice we all compartmentalise.

Probabilities are a tool for talking about uncertainty, they are not uncertainty.

I did certainly not dispute that (if I understand correctly what you mean, which I am not much sure about).

As ad hominem attacks go, that's an interesting one.

The point was, subjective probability is a degree of belief in the proposition; saying "it must be either 0 or 1 by laws of mathematics" rather implies that it is an objective property of the proposition. This seems to signal that you use a non-subjectivist (not necessarily frequentist, my fault) interpretation of probability. We may be then talking about different things. Sorry for ad hominem impression.