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From: Matthew Donald <mjd1014.domain.name.hidden>

Date: Sat, 23 Sep 2000 10:54:58 +0100 (BST)

I wrote

*> The correct (objective) a priori probability of our observations is
*

*> defined by whatever is the true physical theory.
*

Nick Bostrom <nick.domain.name.hidden> replied

*> Sounds like a contradiction! Objective probabilities aren't a
*

*> priori - they depend on what physics happens to be true in the
*

*> actual world, and that's something we only learn from experience.
*

I am using ``a priori probability'' here as a synonym for ``objective

probability''. I think that this is fairly common among physicists,

although I realise that this may be confusing to philosophers,

especially those raised on Kant. I would use ``propensity'' as a third

synonym.

The important point is that such probabilities are independent of

our beliefs. In my theory, they can be defined as probabilities

given the universal wavefunction and perfect knowledge of the past

history of an observer. As I have a theory in which observers are

finitely defined, such probabilities make sense and depend only on

the (objective) laws of physics (i.e. the Hamiltonian of quantum

field theory). I think Bostrom is pointing out that these laws of

physics are not ``a priori'' in the philosophical sense.

I wrote

*> As a first approximation to the correct probabilities, however, it
*

*> is not reasonable to guess that we are about as likely as most
*

*> observers like us. This is the self-sampling assumption.
*

I meant ``not unreasonable'' of course. Sorry.

Bostrom wrote

*> Quantum theory is irrelevant to the Adam&Eve example, because
*

*> that is a thought experiment, so I can simply postulate that
*

*> quantum physics is false in their world. It's important to
*

*> distinguish between methodological or epistemological rules
*

*> (SSA), and empirical hypotheses (quantum theory). The former can
*

*> be applied to the latter, but mixing them up can easily result in a
*

*> category mistake.
*

I don't think that the Adam and Eve example is of any interest

except in as far as it sheds light on the doomsday argument as it

applies to us in the real world. If, as I would contend, the

correctness of quantum theory changes how one should understand

the example, then I think it is highly relevant.

I wrote

*> Nevertheless, Eve still must expect to be typical as an observer.
*

*> Otherwise, she has no reason to believe her physical theories. Her
*

*> birth rank puts her in the position of someone who has tossed a
*

*> fair coin 1,000 times and it has always come up heads. But she is
*

*> tossing a million other coins at the same time so it isn't really
*

*> that surprising.
*

Bostrom replied

*> For all she knows, she may simply be one out of two humans -
*

*> pretty typical.
*

Eve should find her birth rank surprising because of her (assumed)

knowledge of human biology. The question is whether she can

deduce her own infertility from that surprise.

But my point is that if she also wants to believe that her physical

theories are correct, then she has to believe in her statistical

sampling methods and in the results provided by others.

Bostrom also wrote

*> Let's assume the MWI is false.
*

Under any interpretation of quantum theory, it really does look as if

dice are being thrown. In my opinion, because of the way that

instabilities are magnified in the normal functioning of the

human brain, those dice affect every aspect of our behaviour on any

timescale above a millisecond.

If, as in the Bohm interpretation, probabilities are purely

epistemic, Eve will constantly acquire information which

would previously have been much more difficult to predict (given

only her past awareness) than a birth rank of 2 in a species of

trillions. In this situation, I suppose that the first part of my

argument would be that Eve has discovered lots of facts (in

particular, facts about the current state of her brain) which

were astonishingly unlikely given her prior history. She would

therefore be unwise take any particular one of those facts too

seriously.

Even in classical terms, future neural processing is quite

unpredictable from current neural processing.

The second part of my argument is that one cannot separate birth

rank from the other facts about Eve's mental life. Birth rank is not

the sort of fact that one could imagine Eve ever being asked to

guess, as if she had no idea of the answer. As long as she has been

aware, she has had lots of clues that her birth rank was

expectional, just as we have always had lots of implicit clues of

our birth rank (e.g. the taste of powdered baby milk).

Matthew Donald (matthew.donald.domain.name.hidden)

web site:

http://www.poco.phy.cam.ac.uk/~mjd1014

``a many-minds interpretation of quantum theory''

***************************************************

Received on Sat Sep 23 2000 - 02:56:38 PDT

Date: Sat, 23 Sep 2000 10:54:58 +0100 (BST)

I wrote

Nick Bostrom <nick.domain.name.hidden> replied

I am using ``a priori probability'' here as a synonym for ``objective

probability''. I think that this is fairly common among physicists,

although I realise that this may be confusing to philosophers,

especially those raised on Kant. I would use ``propensity'' as a third

synonym.

The important point is that such probabilities are independent of

our beliefs. In my theory, they can be defined as probabilities

given the universal wavefunction and perfect knowledge of the past

history of an observer. As I have a theory in which observers are

finitely defined, such probabilities make sense and depend only on

the (objective) laws of physics (i.e. the Hamiltonian of quantum

field theory). I think Bostrom is pointing out that these laws of

physics are not ``a priori'' in the philosophical sense.

I wrote

I meant ``not unreasonable'' of course. Sorry.

Bostrom wrote

I don't think that the Adam and Eve example is of any interest

except in as far as it sheds light on the doomsday argument as it

applies to us in the real world. If, as I would contend, the

correctness of quantum theory changes how one should understand

the example, then I think it is highly relevant.

I wrote

Bostrom replied

Eve should find her birth rank surprising because of her (assumed)

knowledge of human biology. The question is whether she can

deduce her own infertility from that surprise.

But my point is that if she also wants to believe that her physical

theories are correct, then she has to believe in her statistical

sampling methods and in the results provided by others.

Bostrom also wrote

Under any interpretation of quantum theory, it really does look as if

dice are being thrown. In my opinion, because of the way that

instabilities are magnified in the normal functioning of the

human brain, those dice affect every aspect of our behaviour on any

timescale above a millisecond.

If, as in the Bohm interpretation, probabilities are purely

epistemic, Eve will constantly acquire information which

would previously have been much more difficult to predict (given

only her past awareness) than a birth rank of 2 in a species of

trillions. In this situation, I suppose that the first part of my

argument would be that Eve has discovered lots of facts (in

particular, facts about the current state of her brain) which

were astonishingly unlikely given her prior history. She would

therefore be unwise take any particular one of those facts too

seriously.

Even in classical terms, future neural processing is quite

unpredictable from current neural processing.

The second part of my argument is that one cannot separate birth

rank from the other facts about Eve's mental life. Birth rank is not

the sort of fact that one could imagine Eve ever being asked to

guess, as if she had no idea of the answer. As long as she has been

aware, she has had lots of clues that her birth rank was

expectional, just as we have always had lots of implicit clues of

our birth rank (e.g. the taste of powdered baby milk).

Matthew Donald (matthew.donald.domain.name.hidden)

web site:

http://www.poco.phy.cam.ac.uk/~mjd1014

``a many-minds interpretation of quantum theory''

***************************************************

Received on Sat Sep 23 2000 - 02:56:38 PDT

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