lukstafi comments on Counterfactual Calculation and Observational Knowledge - Less Wrong

11 Post author: Vladimir_Nesov 31 January 2011 04:28PM

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Comment author: Tyrrell_McAllister 01 February 2011 12:36:43AM *  3 points [-]

In what way, if any, is this problem importantly different from the following "less mathy" problem?

You have a sealed box containing a loose coin. You shake the box and then set it on the table. There is no a priori reason for you to think that the coin is more or less likely to have landed heads than tails. You then take a test, which includes the question: "Did the coin land heads?" Fortunately, you have a scanning device, which you can point at the box and which will tell you whether the coin landed heads or tails. Unfortunately, the opaque box presents some difficulty even to the scanning device, so the device's answer is right only 99% of the time. Furthermore, its errors are stochastic (or even involve quantum randomness), so, for any given coin-in-a-box, the device is probably correct but has a chance of making an error. You point the scanning device at the box and observe the result (it's "heads").

Then, unsurprisingly, Omega appears and presents you with the following decision. Consider the counterfactual world where the coin landed the same as it did in your world, but where the scanning device displayed "tails" instead of "heads", after you pointed it at the box. You are to determine what Omega writes on the test sheet in that counterfactual world.

Comment author: lukstafi 01 February 2011 01:09:25AM 0 points [-]

In my opinion the original post (barring the later comment by the author) does not imply that Q is the same in the real world and in the counterfactual world. Am I wrong here?

Then, if Omega is a trustworthy all-powerful device, it would not construct a counterfactual that is straight-out impossible just to play a trick on me. Therefore I conclude that the counterfactual amounts to running an identical scanner another time and getting a different result. But now I no longer think that it is an independent copy of the scanner -- actually it is completely dependent (it is determined to return a different answer), so I no longer think that the conclusion about the coins is fifty-fifty, but that we shouldn't update.

Comment author: Tyrrell_McAllister 01 February 2011 01:34:39AM 2 points [-]

In my opinion the original post (barring the later comment by the author) does not imply that Q is the same in the real world and in the counterfactual world. Am I wrong here?

I have been assuming that Q is the same complicated formula in both worlds.

Comment author: lukstafi 01 February 2011 03:42:29AM 1 point [-]

I have been assuming that Q is the same complicated formula in both worlds.

Of course it is the same formula. And it is the same calculator as well.

Comment author: Vladimir_Nesov 01 February 2011 02:29:41PM 0 points [-]

I have been assuming that Q is the same complicated formula in both worlds.

This is correct. Clarified in the post.