If the subject is a contrarian, the desired X doesn't exist. But this doesn't prevent the GLUT from predicting all actions correctly.

We are imagining the following GLUT entry:

"The subject is presented with evidence E and the [possibly false] fact that the predicted action for this entry is X" -> The subject will do action Y.

X need not be equal to Y. It may be that there is no X for which the predicted action Y equals the action X that we told the subject we predicted.This is trivially possible if the subject is a contrarian who will always do the opposite of what you tell him he will do. But the GLUT is still completely accurate! We simply lied to the subject about what the GLUT predicted. We told him we predicted he would do X on being told he would do X, but actually we knew he would do Y.

Your actions are compact and continuous, thanks to the laws of physics. If the GLUT outputs a prediction in a way that is also compact and continuous, either because it follows the laws of physics or you just programmed it that way, then there's at least one fixed point where he'll take the action he sees. Text output is discrete, and thus this wouldn't work, but there are other ways of doing this. For example, you could show him video of what he'll do.

If so, this seems to say something interesting about limitations on what a simulation can do, but I'm not sure exactly what.

It says that it's not necessarily possible to make a self-fulfilling prophesy. You can predict what someone will do without input from you, but you may or may not be able to make an accurate prediction given that you tell them the prediction.

There is not a general solution.

Suppose that the human is just a mathematical function, where the input is a real number x and his output is a real number y. The person is the function y=f(x); and (in the non-self-referential case) an accurate GLUT just predicts: if the person is given the number x, then he will give you the number y.

In the second case, where the person is told what the GLUT says, he is now a function of two variables h(x,y)=z. The person is given the number x, and told that the GLUT predicts that he will output y, and then he outputs z. In order for the GLUT to be an accurate predictor we must have y=z, but that is not possible in general. For instance, consider h(x,y)=y+1; the person always outputs a number that is 1 more than what the GLUT said he would output.

The problem that the GLUT-maker faces is that she knows the human's function, h(x,y)=z, and she must create a GLUT function, y=g(x), such that g(x)=h(x,g(x)) for all x. I presume that there is math out there which says what features h(x,y) must have in order for this to be possible, but I don't know it.

Is the fact that the simulated subject is a human important for the proposed thought experiment, besides that it activates all sorts of wrong intuitions about free will and makes the lookup table unimaginably huge or even infinite?

Is it clear that an accurate X exists?

It is not, why should it be? By assumption the subject does whatever the GLUT predicts but it doesn't follow that the GLUT includes a proposition "if the subject is confronted with the information that the GLUT predicts that he will do X, he will do X".

At this point you are not talking about a simulation of a person, but rather a simulation of the entire world. Confronted with the knowledge that a GLUT exists predicting I will do anything, I will resolve to pick some relatively trivial decision each day, write down 10 choices, numbered 0 to 9, and then choose the item based on the first numerical digit I see written out on the 11th page of the first section of the New York Times for that day, excluding page numbers or references to page numbers. At that point, the simulation of me has to simulate the production of the next day's New York Times, which is tantamount to having to simulate the entire world.

Not that that is necessarily any harder to do than a complete and accurate simulation of an individual person, mind you.

This dilemma could not possibly exist because: the map must be smaller than the territory. (Has this concept ever been formalized anywhere on lesswrong before? I can't seem to find it. Maybe it should?).

Every time you add a scenario of the form:

"The subject is confronted with the evidence that his wife is also his mother, and additionally with the fact that this GLUT predicts he will do X"

Where the GLUT itself comes into play, you increase the scenarios that the GLUT must compute by one.

Now the GLUT needs to factor in not just situation "n," but also situation "n + GLUT." The GLUT that simulates this new "n + 1" complex of situations is now the new "GLUT-beta," and it is a more-complex lookup table than the original GLUT.

So, yes, GLUT-beta can simulate "person + interaction with GLUT" and come up with a unique prediction X.

What GLUT-beta CANNOT DO is simulate "person + interaction with GLUT-beta" because "person + GLUT-beta" is more complex than "GLUT-beta" by itself, in the same way that "n + 1" will always be larger than n. The lookup table that would simulate "person + interaction with GLUT-beta" would have to be an even more complex lookup table...call it "GLUT-gamma," perhaps.

The problem is that a Giant Lookup Table is a map, but it is not a very compressed map. In fact, it is the least compressed map that is possible. It is a map that is already as complex as the territory that it is mapping.

A Giant Lookup Table is like making a 1:1 scale model of the Titanic. What you end up with is just the original Titanic. Likewise, a 1:1 map of England would be...a replica of England.

Now, a 1:1 Giant Lookup Table can still be useful because you can map from one medium to another. This is what a Giant Lookup Table does. If you think of learning a foreign language as learning a Giant Lookup Table of 1:1 vocabulary translations (which is not a perfect description of foreign language learning, but just bear with the thought-experiment for a moment), we can see how a Giant Lookup Table could still be useful even though it is not compressing the information at all.

So your 1:1 replica of the Titanic might be made out of cardboard rather than steel, and your 1:1 map of England might be made out of paper rather than dirt. But your 1:1 cardboard replica of the Titanic is still going to have to be hundreds of meters in length, and your 1:1 map of England is still going to have to be hundreds of kilometers across.

Now, let's say you come to me wanting to create a 1:1 replica of "the Titanic + a 1:1 replica of the Titanic." Okay, fine. You end up basically with two 1:1 replicas of the Titanic. This is our "GLUT-beta."

Now, let's say you demand that I make a 1:1 replica of "the Titanic + a 1:1 replica of the Titanic," but you only have room in your drydock for 1 ship, so you demand that the result be compressed into the space of 1 Titanic. It cannot be done. You will have to make the drydock (the territory) larger, or you will have to compress the replicas somehow (in other words, go from a 1:1 replica to a 1:2 scale-model).

This is the same dilemma you get from asking the GLUT to simulate "person + GLUT." Either the GLUT has to get bigger (in which case, it is no longer simulating "person + itself," but rather, "person + earlier simpler version of itself"), or you have to replace the 1:1 GLUT with a more efficient (compressed) prediction algorithm.

Likewise, if you ask a computer to perfectly, quark-by-quark, in 1:1 fashion, simulate the entire universe, it can't be done because the computer would have to simulate "itself + the rest of the universe," and a 1:1 simulation of itself is already as big as itself, so it in order to simulate the rest of the universe, it has to get bigger, in which case it has more of itself to model, so that it must get bigger, in which case it has even more of itself to model, etc. in an infinite regress.

The map must always be less than or equal to the territory (and if the map is equal to the territory, then it is not really a "map" as we ordinarily use the term, but more like 1:1 scale model). So all of this can be simplified by saying:

The map must be smaller than the territory.

Or perhaps, this saying should be further refined to:

The map must be less complex than the territory.

That is because, after all, one could create a map of England that was twice as big as the original England. However, such a map would not be able to exceed the complexity of the original England. Everywhere in the original England where there was 1 quark, you would have 2 quarks on your map. You wouldn't get increasingly complex configurations of quarks in the larger map of England. If you did, it would not be a faithful map of the original England.

"The subject is confronted with the evidence that his wife is also his mother, and additionally with the fact that this GLUT predicts he will do X". Is it clear that an accurate X exists?

You mean, he is confronted with the statement that this GLUT predicts he will do X. That statement may or may not be true, depending on his behavior. He can choose a strategy of either always choosing to do what is predicted, always choosing to do the opposite of what is predicted, or ignoring the prediction and choosing based on unrelated criteria. It is possible to construct a lookup table containing accurate predictions of this sort only in the first and third cases, but not in the second.

The subject is confronted with the evidence that his wife is also his mother, and additionally with the fact that this GLUT predicts he will do X.

In what situation does the GLUT predict that he will do X? Let's say, for example, that the GLUT predicts: "Confronted with the evidence that his wife is also his mother, the subject will do X." When the subject is confronted with the evidence, and also with the GLUT's prediction, he does Y. But the GLUT is not wrong, because it predicted his actions under a different set of information than he actually had. Alternatively, let's define A as: "evidence that his wife is also his mother, and that when shown evidence A, does X". There is no particular reason that there has to exist an X such that it is possible to construct A.