I'm reading David Lewis' book, "Plurality of Worlds," 1986.

Lewis argues that not only are all possible worlds possible, but that 
they actually exist. He does not claim they exist in any form we can 
visit, or communicate with, but that the most economical philosophical 
position to take is that they do in fact actually exist.

Lewis is no dummy. (Actually, Lewis _was_ no dummy: he died last year. 
My very usage above and here of the present tense, as in  "Lewis argues 
that," is a kind of illustration of the weaker form of the plurality of 
worlds thesis. Namely, it is natural to take a subjunctive ("had it been 
the case") or possible world and treat it as a real world. For example, 
arguing in the present tense by treating even a dead author as if he 
were amongst us: "Aristotle tells us..."

This is recommended reading for Everything subscribers. Lewis has _some_ 
familiarity with the MWI, and cites Niven's 1968 story, "All the Myriad 
Ways." But Lewis is not depending on a physics interpretation for his 
thesis, although the physical (MWI or Tegmark/Schmidhuber/etc.) theories 
of mulitiple realities would fit in as a subset of Lewis' plurality of 

The strong form of the thesis, that all possible (no violations of 
logic) worlds have actual existence is dubbed "modal realism." A weaker, 
more common-sensical form is called "ersatz modal realism." This is form 
in which we can temporarily instantiate possible worlds, as, for 
example, "In a world where Microsoft had never existed, the software 
industry probably would have...."

I'm deliberately not choosing whether to believe the strong form, though 
it seems natural that everyone would believe the weak form. What's more 
useful at this point is to learn the methods of reasoning these analytic 
philosophers use, at least in this one world of counterfactuals and 
possible worlds. (As you may recall, I am very interested in the links 
between possible worlds and toposes, notably that the natural logic of a 
possible worlds model is Heyting logic. A colleague/fellow writer of 
Lewis's is Saul Kripke, who did interesting work in the 1960s showing 
the connections between Intuitionism and possible worlds semantics.)

The weaker form of modal reality is used by nearly every person to 
describe the possible worlds of the future: "Tomorrow it may rain." "I 
can see a world in which peace exists." And so on for millions of 
examples. We also use possible worlds semantics when discussing 
alternative theories of how things happened: "Maybe the way it worked 
was like this..." and "Had the Second World War not happened, the atom 
bomb wouldn't have been developed."

Alas, David Lewis uses some math in his books, especially in an earlier 
one I also have called "Counterfactuals," but the math is somewhat 
lacking in its generality. (Reading "Counterfactuals," 1973, all I could 
think was "This man is reinventing parts of point set topology! Give him 
a copy of Kelley's "General Topology" and let him use the accepted 
jargon for his inventions.")

It seems to to me that there are several communities (worlds) of writers 
and researchers in this general area of "multiple realities" and they 
have only tangential and fleeting communcation with each other:

* the "traditional" world of MWI, Everettistas, consistent histories, De 
Witt, Hartle, etc., with newcomers like Tegmark and Schmidhuber (adding 
models of computation)

* the philosophical world of Kripke, Lewis, Montague, and others, with a 
focus on possible worlds semantics, reasoning in different worlds, 
ontology, etc. This world somewhat intersects the above world through 
the medium of fiction. Novels are examples of possible worlds, and "What 
if" novels are a staple of science fiction. Larry Niven, Phil Dick, Rudy 
Rucker, Greg Egan, and many others have even written thoughtfully about 
what MWI means. Their novels discuss possible worlds and are in fact 
themselves possible worlds.

* the world of topos theory and study of synthetic realities derived 
from propositions, with the work of Lawvere, Johnstone, Kock, and 
others. This world intersects the possible worlds semantics world 
through the work of Kripke, as I said. It also is beginning to intersect 
the MWI world through the work of Chris Isham -- cf. that streaming 
video presentation I mentioned, at the URL:

Now it is my current interest to unify these three worlds, at least in 
my own mind.

I do recommend taking a look at David Lewis's work. It's a bit off the 
beaten track for most MWI thinkers, but it clearly deals with the same 
general ideas. And it offers new language and new tools.

--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

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