Missing Shade of Blue
Posts: 62 Joined: Dec. 2008
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Keith and Bill,
My understanding of Bill's point is that he thinks the grue-bleen inductive agent must have all the same components as the blue-green agent plus something extra. This extra something is not subject to normative selection, so it will eventually deteriorate, leaving a regular blue-green agent. If we redescribe the situation in grue-bleen terms, then the verdict just reverses. Now it is the blue-green agent who has the something extra, and so she will eventually be whittled down to the grue-bleen agent (well, not literally her, but her descendants).
In either case, we will end up with the same sort of agent. Anything that could distinguish blue-green agents from grue-bleen ones must be some kind of idle non-adaptive dangler prior to 2012, so its going to disappear. Is this an accurate paraphrase of your argument, Bill? Now I hope you'll bear with me. The next section is kind of long, but I wanted to make sure the issues involved are clear. I feel like all of us are talking past each other a little bit.
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Now I think that there is no reason to think the response systems of the two agents must be additively related in this way. Let me give an example of a really simple blue-green inductive agent. When it sees a bunch of emeralds (pre-2012) it forms the belief "Emeralds are green." What I mean by "forms a belief" in this context is cashed out completely in terms of the following stimulus-response behavior.
The agent can bet a certain amount of resources for or against certain questions posed to it by some external tester (a proxy for the environment). Here are the responses to some questions:
"Will emerald x be green?" - Bet on yes. "Will emerald x be blue?" - Bet on no. "Will emerald x be grue?" - If x is discovered prior to 2012 bet on yes, else bet on no. "Will emerald x be bleen?" - If x is discovered prior to 2012 bet on no, else bet on yes.
Now we have a grue-bleen agent. When faced with the exact same initial set of data as the first agent it forms the belief "Emeralds are grue," cashed out in terms of this response table:
"Will emerald x be grue?" - Bet on yes. "Will emerald x be bleen?" - Bet on no. "Will emerald x be green?" - If x is discovered prior to 2012 bet on yes, else bet on no. "Will emerald x be blue?" - If x is discovered prior to 2012 bet on no, else bet on yes.
Note that both these agents are formally identical, but they differ in their semantic properties: they respond differently to different external stimuli. One of them will start losing resources in 2012.
Now if Bill is right, one/both of these agents has an idle dangler in its response system that will deteriorate before 2012. In this case, the formal identity of both agents means that if one of them has this dangler the other will too (there's no reasonable sense in which the response system of one of these agents is a proper sub-part of the response system of the other). And the plausible candidate for the dangler is the difference in their responses to grue and green questions. This difference will only manifest after 2012, so prior to that it is not subject to normative selection (on Bill's story). This difference in the responses will be whittled away until we are left with creatures who are neutral with respect to these inductive biases, which means they either: (a) make inconsistent bets post-2012 (betting yes on both green and grue for instance), (b) refuse to bet on post-2012 emeralds, or © develop some sort of probabilistic betting strategy that weighs both biases equally (so maybe half the population bets on green and the other half on grue).
But all of this relies on a premise: The part of the inductive bias that has to do with grue/green differences is purely additive. Removing it doesn't affect the functioning of the rest of the system. This is a big big assumption, though. It is quite possible that the cheapest sort of response system does distinguish grue and green. In other words, the cheapest way to buy inductive success in the present might involve committing the system to one particular inductive bias in the future. And it might be the case that while the cost of both the blue-green and grue-bleen inductive systems is equivalent prior to 2012 (by hypothesis), they are both cheaper than the sort of inductive system that does not choose between the biases.
Now even if you accept all this, you might say, "OK, so maybe it is possible that it's cheaper to have an post-2012 inductive bias rather than none at all. But it's still going to be the case that only one of these biases will survive. By hypothesis, the grue-bleen trait is neutral relative to the blue-green one, so we expect one of them to go to fixation just by random drift." But this is only relevant if the blue-green and grue-bleen populations interbreed. I agree that we should expect a uniform inductive bias within a breeding sub-population.
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