PhD-thesis on Observational Selection Effects

From: Bostrom,N (pg) <"Bostrom,N>
Date: Fri, 16 Jun 2000 19:14:29 +0100

Hi, I've just submitted my doctoral dissertation titled "Observational
selection effects and probability", which presents the first mathematically
explicit "observation theory". I use it to resolve a range of puzzles
related to the "anthropic principles", multiverse models, the Doomsday
argument, etc. The text is available at

as a MS-Word-file (77,000 words; 1.2 KB). The abstract is pasted below.

Feel free to download it if you think it might be interesting to you. Any
constructive comments, criticism or suggestions would be welcome. (I'm
planning to write this up as a book accessible to a wider interdisciplinary
audience, so your input would be come to good use.)

Most of you who get this message will have helped me in one way or another
during the PhD-process. I would like to say a warm Thanks! to all of you.

Best wishes,

Nick Bostrom
Dept. Philosophy, Logic and Scientific Method
London School of Economics
This thesis develops a theory of how to reason when our evidence has been
subjected to observational selection effects. It has applications in
cosmology, evolutionary biology, thermodynamics and the problem of time's
arrow, game theoretic problems with imperfect recall, the philosophical
evaluation of the many-worlds and many-minds interpretations of quantum
mechanics and David Lewis' modal realism, and even for traffic planning.
        After refuting several popular doctrines about the implications of
cosmological fine-tuning, we present an informal model of the observational
selection effects involved. Next, we evaluate attempts that have been made
to codify the correct way of reasoning about such effects - in the form of
so-called "anthropic principles" - and find them wanting. A new principle is
proposed to replace them, the Self-Sampling Assumption (SSA).
        A series of thought experiments are presented showing that SSA
should be used in a wide range of contexts. We also show that SSA gives
better methodological guidance than rival principles in a number of
scientific fields. We then explain how SSA can lead to the infamous Doomsday
argument. Identifying what additional assumptions are required to derive
this consequence, we suggest alternative conclusions. We refute several
objections against the Doomsday argument and show that SSA does not give
rise to paradoxical "observer-relative chances" as has been alleged.
However, we discover new consequences of SSA that are more counterintuitive
than the Doomsday argument.
        Using these results, we construct a version of SSA that avoids the
paradoxes and does not lead to the Doomsday argument but caters to
legitimate methodological needs. This modified principle is used as the
basis for a mathematically precise theory of reasoning under observational
selection effects. This "observation theory" resolves the range of
conundrums associated with anthropic reasoning and provides a general
framework for evaluating theories about the large-scale structure of the
world and the distribution of observers within it.
Received on Fri Jun 16 2000 - 11:44:40 PDT

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