Re: modal logic and possible worlds

From: Tim May <>
Date: Tue, 13 Aug 2002 15:51:49 -0700

On Tuesday, August 13, 2002, at 02:34 PM, Wei Dai wrote:

> Tim, I think I'm starting to understand what you're saying. However, it
> still seems that anything you can do with intuitionistic logic, toposes,
> etc., can also be done with classical logic and set theory. (I'm not
> confident about this, but see my previous post in reponse to Bruno.)
> Maybe
> it's not as convenient or "natural" in some cases (similar to how modal
> logic can be more convenient than explicitly quantifying over possible
> worlds even when they are equivalent), but if one is not already
> familiar
> with intuitionistic logic and category theory, is it really worth the
> trouble to learn them?

I don't know. One learns a field for various reasons. Clearly a lot of
people think "classical logic with the right exceptions and terminology"
serves them well. A lot of others, though not as many, think program
semantics and possible worlds semantics are best understood in the
"natural" logic of time-varying sets and topos theory. I'm in the
latter category (no pun intended).

I also don't know what your goals are, despite reading many of your
posts. If, for example, you are looking for tools to understand a
possible multiverse, or how multiverses in general might be constructed,
I'm not at all sure any such tools have ever existed or _will_ ever
exist, except insofar as tools for understanding toposes, lattices, etc.

This is quite different from understanding, say, general relativity,
where the tools of differential geometry and exterior calculus are
immediately useful for understanding and for calculations.

The MWI/Tegmark/Egan stuff is very far out on the fringes, as we know,
and there is unlikely to be anything one can do calculations of. Still,
it seems likely that a _lot_ of mathematics is needed...a lot more math
than physics, almost certainly.

Modal logic seems to me to be _exactly_ the right logic for talking
about possible states of existence, for talking about possible worlds,
for talking about branching universes. So the issue is not "But can't I
find a way to do everything in ordinary logic?" but is, rather, to think
in terms of modal logic offering a more efficient "basis" (in the
conceptual vector space), a basis with a smaller semantic gap between
the formalism and the hypothesized world.

(You might also want to take a look at the paper by Guts, a Russian, on
a "Topos-Theoretic Model of the Deutsch Multiverse." Available at the
usual site.)
> For example, posets can certainly be studied and understood using
> classical logic. How much does intuitionistic logic buy you here?

Posets can certainly be studied with classical logic. However, posets
fail the law of trichotomy, that two things when compared by some
ordering result in one of three outcomes: A is less than B, A is greater
than B, or A is equal to B. This is the "common sense" comparison of
objects, one with a "linear" or "total" order. However, posets are
partially-ordered precisely because they don't follow the law of

Is one more natural than the other? More common? More useful?

I have my own beliefs at this point.

A book I strongly recommend, though it is difficult, is Paul Taylor's
"Practical Foundations of Mathematics." 1999. (I buy many books not to
read straight through, but to consult, to draw insights and inspirations
from, and to let me know what I need to learn more of. This is one of
those books. The first 175 pages, which I've been reading from, is
making more and more sense....the terms become familiar, I see
connections with other areas, and by a process akin to "analytic
continuation" the ensemble of ideas becomes more and more natural.
Beyond these pages, though, it's mostly incomprehensible.)

I recommend this book for the broad insights I am gaining from it, but
not as any kind of manual for tinkering with multiverses! (insert silly
smiley as one sees fit)

My conclusion from Tegmark's paper, which dovetailed with Egan's
treatment of "all topologies models" in "Distress," was that to make
progress a lot of math needs to be learned. Which is my current approach.

These are not my only inspirations. Indeed, I came to join this list
after becoming fascinated (again, after a nearly 28-year absence) in
topology, algebra, and the physics of time and cosmology.

Seen this way, category and topos theory are worth studying for their
own sake. I don't think it is likely that "every conceivable universe
with consistent laws of mathematics has actual existence" (to nutshell
my understanding of Tegmark's theory) is actually true (whatever that
means). Nor do I take Schmidhuber's "all running programs" notion very
seriously. Interesting ideas to play with, and to use some tools on.

Strangely, then, I view these notions as places to apply the math I'm
learning to. And I'm thinking small, in terms of simple systems. A paper
I have mentioned a couple of times is directly in line with this
approach: Fotini Markopoulou's "The internal description of a causal
set: What the universe looks like from the inside."

Here's the paper number and abstract:

From: Fotini Markopoulou <>
Date (v1): Tue, 17 Nov 1998 19:28:10 GMT (41kb)
Date (revised v2): Thu, 18 Nov 1999 17:32:41 GMT (42kb)

The internal description of a causal set: What the universe looks like
from the inside

Authors: Fotini Markopoulou
Comments: Version to appear in Comm.Math.Phys. (minor modifications). 37
pages, several eps figures
Journal-ref: Commun.Math.Phys. 211 (2000) 559-583

We describe an algebraic way to code the causal information of a
discrete spacetime. The causal set C is transformed to a description in
terms of the causal pasts of the events in C. This is done by an
evolving set, a functor which to each event of C assigns its causal
past. Evolving sets obey a Heyting algebra which is characterised by a
non-standard notion of complement. Conclusions about the causal
structure of the causal set can be drawn by calculating the complement
of the evolving set. A causal quantum theory can be based on the quantum
version of evolving sets, which we briefly discuss.
--end of excerpt--

Take a look at these papers (hers, the Guts paper, the various Baez,
Smolin, Crane, etc. papers). All free. Some are introductions. All have
a fair amount to say about the nature of reality. The stuff on causal
sets (lattices and posets) is of direct relevance to several areas of
modern physics. The relevance to MWI and Tegmark-style meta-branchings
seems clear to me.

As far as the math of nonstandard logic goes, I think the most
interesting application within our lifetimes will come with AI.

--Tim May
(.sig for Everything list background)
Corralitos, CA. Born in 1951. Retired from Intel in 1986.
Current main interest: category and topos theory, math, quantum reality,
Background: physics, Intel, crypto, Cypherpunks
Received on Tue Aug 13 2002 - 15:58:06 PDT

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