MugaSofer comments on Problematic Problems for TDT - Less Wrong
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Newcomb:
Prisoner's Dilemma:
Do you see the contradiction here?
Newcomb, Alice: The simulation's source code and available information is literally exactly the same as Alice's, so if Alice 2-boxes, the simulation will too. There's no way around it. So Alice one-boxes.
Newcomb, Bob: The simulation was in the situation described above. Bob thus predicts that it will one-box. Bob himself is in an entirely different situation, since he can see a source code difference, so if he two-boxes, it does not logically imply that the simulation will two-box. So Bob two-boxes and the simulation one-boxes.
Prisoner's Dilemma: Alice sees Bob's source code, and summarizes it as "identical to me except for a different comment". Bob sees Alice's source code, and summarizes it as "identical to me except for a different comment". Both Alice and Bob run the same algorithm, and they now have the same input, so they must produce the same result. They figure this out, and cooperate.
Ignore Alice's perspective for a second. Why is Bob acting differently? He's seeing the same code both times.
Don't ignore Alice's perspective. Bob knows what Alice's perspective is, so since there is a difference in Alice's perspective, there is by extension a difference in Bob's perspective.
Bob looks at the same code both times. In the PD, he treats it as identical to his own. In NP, he treats it as different. Why?
The source code that Bob is looking at is the same in each case, but the source code that [the source code that Bob is looking at] is looking at is different in the two situations.
NP: Bob is looking at Alice, who is looking at Alice, who is looking at Alice, ...
PD: Bob is looking at Alice, who is looking at Bob, who is looking at Alice, ...
Clarifying edit: In both cases, Bob concludes that the source code he is looking at is functionally equivalent to his own. But in NP, Bob treats the input to the program he is looking at as different from his input, whereas in PD, Bob treats the input to the program he is looking at as functionally equivalent to his input.
But you said Bob concludes that their decision theories are functionally identical, and thus it reduces to:
And yet this does not occur in NP.
EDIT:
The point is that his judgement of the source code changes, from "some other agent" to "another TDT agent".
Looks like my edit was poorly timed.
One way of describing it is that the comment is extra information that is distinct from the decision agent, and that Bob can make use of this information when making his decision.
Oops, didn't see that.
What's the point of adding comments if Bob's just going to conclude their code is functionally identical anyway? Doesn't that mean that you might as well use the same code for Bob and Alice, and call it TDT?
In NP, the comments are to provide Bob an excuse to two-box that does not result in the simulation two-boxing. In PD, the comments are there to illustrate that TDT needs a sophisticated algorithm for identifying copies of itself that can recognize different implementations of the same algorithm.
Do you understand why Bob acts differently in the two situations, now?