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We should note that in the second and third cases it can also happen that one can’t improve their axiom set to cover more even when using a statistical axiom set. One such case would be when the perceived laws of the universe change in unpredictable ways from day to day (of course, this can happen without any change in the actual axiom set for the universe).
Let us denote by \definitie{observable description} of a possible universe $U$ for some intelligent beings $B$ inside $U$ any axiom set that can make all possible predictions for $U$ from the point of view of the beings $B$. [TODO:
Make sure these things work This is not a good definition, I should clarify what happens if there can be a universe $U$ in which there are predictions that can't be covered by any countable axiom set. Maybe I should use only universes for
nondeterministic universes] which I can make those, but then ours may not be such a universe. Or maybe I should use the paragraph below as a definition.]
For Then, for the given intelligent beings we would say in the first case that their universe has a
finite \definitie{finite observable
description description} and in the second and third case that it has an
infinite \definitie{infinite observable
description. description}. Of course, a possible universe $U$ could have multiple types of intelligent beings, each type perceiving the universe in a different way. Because of this difference in perception, for some intelligent beings the universe $U$ may have a finite observable description while for others it may have an infinite observable description.
If we can have infinite descriptions, then the set of optimal systems of axioms would have the same cardinality as the set of real numbers $\reale$. Indeed, for the planets-on-a-line example above, we could select any subset of planets and get an universe with an optimal set of axioms that is distinct from any other subset. The set of all subsets of $\naturale$ has the same cardinality as $\reale$, so the set of optimal systems of axioms would have at least this cardinality. On the other hand, each system of axioms is written using an at most countable set of symbols, so there can't be more than $\reale$ systems of axioms. [TODO: Add a footnote or a chapter at the end with more mathematical justification for this. Maybe add there the half-proof given
here.] here. Maybe I shouldn't bother.]
\section{Description probabilities}
These observable descriptions For the purpose of this paper, let us denote by \definitie{finite property} of
possible worlds are general enough and different enough that it's hard to say something
about them, except that they make sense in a mathematical way. Still, given any property
$P$ we could try to see what is the chance of that
it's true in the set something which can be written using a finite number of words. Since we will use only finite properties here, let us drop "finite" and call any of
observable descriptions. them simply \definitie{property}.
If our universe is not designed, then any possible universe could have existed (and maybe all These observable descriptions of possible
universes actually exist). Focusing only on universes which have a space-time worlds are general enough and
in which intelligent beings can exist, if we would want different enough that it's hard to
pick a random one for a reasonable definition of random, each universe would have say something about them, except that they make sense in a
zero probability of being chosen. If mathematical way. Still, given any property $X$ we
further restrict these universes to ones which allow a predictive system of axioms for the entire universe\footnote{The entire universe is required here for simplicity, but one could
also work when only a part of the universe can have a predictive system of axioms.}, then each system of axioms is as likely try to
be randomly picked as any other, so each has a zero probability. I argue that, even more, see what is the
systems of axioms chance that
would be produced by the intelligent beings it's true in
that universe (in the
sense mentioned above) have each a zero probability. In other words, any reasonable probability over these axiom systems is continuous. set of observable descriptions.
[TODO: Delete if I am happy with the paragraph before this one. In order to compute the probability of If our universe is not designed, then any possible universe could have existed (and maybe all possible universes actually exist). Focusing only on universes which have a
property $P$ space-time and in which intelligent beings can exist, if we would
need want to pick a
statistical distribution random one for
the set a reasonable definition of
observable descriptions. We random, each universe would have
no good way a zero probability of being chosen. If we further restrict these universes to ones which allow a predictive system of
choosing among axioms for the
many possible distributions, entire universe\footnote{The entire universe is required here for simplicity, but
we one could also work when only a part of the universe can
still find interesting things without much choosing. Let us note that there is have a
class predictive system of
distributions which stands out as being reasonable for this purpose: axioms.}, then each system of axioms is as
long likely to be randomly picked as
we don't see any
reason to prefere other, so each has a
specific description over all zero probability. I argue that, even more, the
others, our only choice is to use continuous distributions, systems of axioms that
is, distributions for which would be produced by the
probability of intelligent beings in that universe (in the sense mentioned above) have each a zero probability. In other words, any
given observable description reasonable probability distribution over these axiom systems is
zero.] continuous.
[TODO: I stopped rewriting here.] While this is designed such that we can't directly say anything about a specific observable description, we can say things about what has a real chance of being true for a random observable description. First, let us note that any property that is true for only one description has a zero probability (i.e. it is false virtually everywhere). Even more, any property which is true for a countable number of descriptions has a zero probability. This means that any property with a non-zero probability is for sure true for an uncountable number of descriptions. Of course, there may be properties which are true for an uncountable number of descriptions and still have a zero probability.
While this is designed such that we can't directly say anything about a specific observable description, we can say things about what has a real chance of being true in a random observable description. First, let us note Now, it could happen that
for any description there is a
zero-probability property
that which is true for
only one description has a zero probability (i.e. that description, making it
likely that we can't find anything interesting this way. Fortunately, this is
false virtually everywhere). Even more, any not true. Indeed, a property
which is
true for written using a
countable number of descriptions finite alphabet and has a
zero probability. This means that any property with a non-zero probability finite length, so there is
for sure true for an uncountable at most a countable number of
descriptions. Of course, there may such properties. Let $Y$ be
properties which are true for an uncountable number this set. Then $P(Y)$, the probability of the set of
observable descriptions
and still have a zero probability. for which at least one proposition in $Y$ is true, is the sum of the probabilities of all elements in $Y$, so $P(Y) = 0$.
Now, it could happen We can then say that for
any description there is a zero-probability property which is true for that description, making it likely that we can't find anything interesting this way. Fortunately, this is virtually all descriptions [TODO: Make sure I want descriptions here and not
optimal axiom sets or something. Probably I want descriptions], only properties with non-zero probability are true.
Indeed, a property is written using a finite alphabet and has a finite length, so there is at most a countable number of such properties. Let $Y$ be this set. Then $P(Y)$, This means that, if the probability of
the set of observable models for which at least one proposition in $Y$ our world being designed is
true, is the sum of non-zero, the
probabilities of all elements in $Y$, so $P(Y) = 0$. only rational choices are that either our world is designed or only non-zero probability properties are true.
We can then say that for virtually all models, only properties with non-zero probability are true. This means that, if the probability of our world being designed is non-zero, the only rational choices are that either our world is designed or only non-zero probability properties are true. Now, let us return to the issue of observable
models descriptions being finite or infinite. With an finite alphabet, only a countable set of models have a finite observable description. Then the "has a finite description"
proposition property [TODO: is it a property of the description or of the universe? Does that match what I said above about only inferring things about descriptions? Ah, it's a property of an axiom set, I think.] is a zero-probability one, so either our universe is designed, or at any point in time there will be an
infinite number of things that we didn't axiomatize yet about our universe but we think that they are important. [TODO: It's probably better to say that there will be an important part of our universe that we can observe but can't
model. Also, since model no matter how hard we
chosed the precision and prediction error in an arbitrary way, this part that can't be modelled is visible at any "zoom" level.]
[TODO: Find the right term for "has a finite description" thing. Is it property? Is it proposition? How are these terms used in philosophy?] try.
\section{Approximations}