Radford Neal

Wiki Contributions

Comments

Sorted by

Then you know that someone who voiced opinion A that you put in the hat, and also opinion B, likely actually believes opinion B.

(There's some slack from the possibility that someone else put opinion B in the hat.)

Wouldn't that destroy the whole idea? Anyone could tell that an opinion voiced that's not on the list must have been the person's true opinion.

In fact, I'd hope that several people composed the list, and didn't tell each other what items they added, so no one can say for sure that an opinion expressed wasn't one of the "hot takes".

I don't understand this formulation. If Beauty always says that the probability of Heads is 1/7, does she win? Whatever "win" means...

OK, I'll end by just summarizing that my position is that we have probability theory, and we have decision theory, and together they let us decide what to do. They work together. So for the wager you describe above, I get probability 1/2 for Heads (since it's a fair coin), and because of that, I decide to pay anything less than $0.50 to play. If I thought that the probability of heads was 0.4, I would not pay anything over $0.20 to play. You make the right decision if you correctly assign probabilities and then correctly apply decision theory. You might also make the right decision if you do both of these things incorrectly (your mistakes might cancel out), but that's not a reliable method. And you might also make the right decision by just intuiting what it is. That's fine if you happen to have good intuition, but since we often don't, we have probability theory and decision theory to help us out.

One of the big ways probability and decision theory help is by separating the estimation of probabilities from their use to make decisions. We can use the same probabilities for many decisions, and indeed we can think about probabilities before we have any decision to make that they will be useful for. But if you entirely decouple probability from decision-making, then there is no longer any basis for saying that one probability is right and another is wrong - the exercise becomes pointless. The meaningful justification for a probability assignment is that it gives the right answer to all decision problems when decision theory is correctly applied. 

As your example illustrates, correct application of decision theory does not always lead to you betting at odds that are naively obtained from probabilities. For the Sleeping Beauty problem, correctly applying decision theory leads to the right decisions in all betting scenarios when Beauty thinks the probability of Heads is 1/3, but not when she thinks it is 1/2.

[ Note that, as I explain in my top-level answer in this post, Beauty is an actual person. Actual people do not have identical experiences on different days, regardless of whether their memory has been erased. I suspect that the contrary assumption is lurking in the background of your thinking that somehow a "reference class" is of relevance. ]

Answer by Radford Neal30

I re-read "I Robot" recently, and I don't think it's particularly good. A better Asimov is "The Gods Themselves" (but note that there is some degree of sexuality, though not of the sort I would say that an 11-year should be shielded from).

I'd also recommend "The Flying Sorcerers", by David Gerrold and Larry Niven. It helps if they've read some other science fiction (this is sf, not fantasy), in order to get the puns.

How about "AI scam"? You know, something people will actually understand. 

Unlike "gas lighting", for example, which is an obscure reference whose meaning cannot be determined if you don't know the reference.

Sure. By tweaking your "weights" or other fudge factors, you can get the right answer using any probability you please. But you're not using a generally-applicable method, that actually tells you what the right answer is. So it's a pointless exercise that sheds no light on how to correctly use probability in real problems.

To see that the probability of Heads is not "either 1/2 or 1/3, depending on what reference class you choose, or how you happen to feel about the problem today", but is instead definitely, no doubt about it, 1/3, consider the following possibility:

Upon wakening, Beauty see that there is a plate of fresh muffins beside her bed. She recognizes them as coming from a nearby cafe. She knows that they are quite delicious. She also knows that, unfortunately, the person who makes them on Mondays puts in an ingredient that she is allergic to, which causes a bad tummy ache. Muffins made on Tuesday taste the same, but don't cause a tummy ache. She needs to decide whether to eat a muffin, weighing the pleasure of their taste against the possibility of a subsequent tummy ache.

If Beauty thinks the probability of Heads is 1/2, she presumably thinks the probability that it is Monday is (1/2)+(1/2)*(1/2)=3/4, whereas if she thinks the probability of Heads is 1/3, she will think the probability that it is Monday is (1/3)+(1/2)*(2/3)=2/3. Since 3/4 is not equal to 2/3, she may come to a different decision about whether to eat a muffin if she thinks the probability of Heads is 1/2 than if she thinks it is 1/3 (depending on how she weighs the pleasure versus the pain). Her decision should not depend on some arbitrary "reference class", or on what bets she happens to be deciding whether to make at the same time. She needs a real probability. And on various grounds, that probability is 1/3.

But the whole point of using probability to express uncertainty about the world is that the probabilities do not depend on the purpose. 

If there are N possible observations, and M binary choices that you need to make, then a direct strategy for how to respond to an observation requires a table of size NxM, giving the actions to take for each possible observation. And you somehow have to learn this table.

In contrast, if the M choices all depend on one binary state of the world, you just need to have a table of probabilities of that state for each of the N observations, and a table of the utilities for the four action/state combinations for the M decisions - which have size proportional to N+M, much smaller than NxM for large N and M. You only need to learn the N probabilities (perhaps the utilities are givens).

And in reality, trying to make decisions without probabilities is even worse than it seems from this, since the set of decisions you may need to make is indefinitely large, and the number of possible observations is enormous. But avoiding having to make decisions by a direct observation->action table requires that probabilities have meaning independent of what decision you're considering at the moment. You can't just say that it could be 1/2, or could be 1/3...

So how do you actually use probability to make decisions? There's a well-established decision theory that takes probabilities as inputs, and produces a decision in some situation (eg, a bet). It will (often) produce different decisions when given 1/2 versus 1/3 as the probability of Heads. Which of these two decisions should you act on?

That argument just shows that, in the second betting scenario, Beauty should say that her probability of Heads is 1/2. It doesn't show that Beauty's actual internal probability of Heads should be 1/2. She's incentivized to lie.

EDIT: Actually, on considering further, Beauty probably should not say that her probability of Heads is 1/2. She should probably use a randomized strategy, picking what she says from some distribution (independently for each wakening). The distribution to use would depend on the details of what the bet/bets is/are.

Load More