Comment author:Avo
14 September 2015 09:24:12PM
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-3 points
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I claim that self-sampling plays no essential role in DA logic.

If you think that self-sampling is essential, then you still must allow one of the bacteria in the petri dish to use DA logic about its own future. If you do not allow the biologist to use DA logic, then the bacterium and the biologist will make different predictions about the likely future. One of them must be more accurate than the other (as revealed by future events). If it is the bacterium that is more accurate, what prevents the biologist from adopting the bacterium's reasoning? I argue that nothing prevents it. And since, in the real world, biologists (and scientists of all fields) do not adopt DA logic, I claim that the most compelling reason for this is the invalidity of DA logic from the get-go.

Comment author:turchin
14 September 2015 09:57:09PM
2 points
[-]

(I did not downvote you)

Look, we could replace "self sampling" with "random". Random bacteria (from all existing bacteria on Earth) will not start exponential growth. Infinitely small subset of all bacteria will start it. There is no difference between prediction of statistic and biologist in this case.
DA is statistical argument. It just say that most of random bacteria will not start exponential growth. The same may be true for young civilizations: most of them will not start exponential growth in the universe. But some may be.

Comment author:Avo
14 September 2015 10:24:54PM
-2 points
[-]

So do you agree with me that, in the experiment I described (a biologist sets up a petri dish with a specific set of initial conditions, and wants to find out if a small bacteria colony will grow exponentially under those conditions), DA logic cannot be applied (by either the biologist or the bacterium) to judge the probable outcome?

Comment author:turchin
14 September 2015 10:44:56PM
2 points
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DA should be applied to the situation where we know our position in the set, but do not know any other evidence. Of course if we have another source of information about the set size it could overweight DA-logic. If in this experiment the substrate is designed to support bacterial growth, it have is very strong posteriory evidence for future exponential growth.

But if you put random bacteria on random substrate it most likely will not grow. In this case DA works.
DA here says that 1 bacteria most likely will have only several off springs, and it is true for most random bacteria on random substrates.

So, will DA works here or not depends of details of the experiment with you did not provide.

Comment author:Avo
14 September 2015 10:57:32PM
-1 points
[-]

OK, let me rephrase the question.

The biologist has never heard of DA. He sets up the initial conditions in such a way that his expectation (based on all his prior knowledge of biology) is that the probability of exponential growth is 50%.

Now the biologist is informed of DA. Should his probability estimate change?

Comment author:turchin
14 September 2015 11:12:44PM
2 points
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Probably, not, as he has a lot of information about the subject. DA is helpful in case if you don't have any other information about the subject. Also DA is statistical argument thereby it could not be disproved by counterexample. It is always possible to construct a situation where it will not work. Like some molecules in the air are not moving, despite the fact that median velocity is very high.

It may be used in such problems as bus waiting problem (variant of Laplace sunrise problem). If last bus was 5 minutes ago, want is the probability that it will come in next 1 millisecond, next 5 minutes? next 1 year?

Comment author:Avo
15 September 2015 06:14:20AM
-1 points
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At least some DA proponents claim that there should always be a change in the probability estimate, so I am pleased to see that you agree that there are situations where DA conveys no new information.

Comment author:turchin
15 September 2015 08:25:58AM
2 points
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The situation would change, if I were Adam, first man in the world, and a priory will be able to start exponential human growth with P= 50 per cent probability (or think so). After finding that I am Adam, I would have to update this probability to lower. The way I update depends of sampling method - SSA or SIA but both result in early doom. SSA says that I am in short world.
SSI said that my apriory estimation of universal distribution of short and long civilization may be wrong. It would be especially clear if apriory P would not 50 percent, but say 90 per cent.

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