A.K.A. talking about just this one world is like saying the only beer that exists is the beer in my fridge

Philosopher David Lewis's On the Plurality of Worlds, published in 1986, is a defense of modal realism, the thesis that any possible world exists and is as real as our own actual world. In The Plurality of Worlds, the idea is is that we're supposed to adopt this worldview because of its utility, the way mathematicians deploy set theory. "The benefits are worth their ontological cost," Lewis writes. "Modal Realism is fruitful; that gives us good reason to believe that it is true." In modal realism, the language of modality reflects the actual: anything that could exist does and therefore discussions about what might or must be are simplified. A lot of people, for the sake of this simplification, invoke possible worlds purely in an abstract sense, but Lewis insists that these worlds inhabit actual walk-around-in-it space.

You know that book in Donnie Darko? Imagine it exists and takes itself absolutely seriously. That's On the Plurality of Worlds. You'll be nodding along to a fairly cogent argument and suddenly - wait a second! - you're reading something like

Likewise, if there is no trans-world causation, there is no trans-world travel. You can't get into a 'logical-space ship' and visit another possible world. You could get into what you confusedly think is a logical-space ship, turn the knob, and disappear. And a perfect duplicate of you at your disappearance, surrounded by a perfect duplicate of your ship, could appear ex nihilo at some other world. Indeed, there are plenty of worlds where aspiring logical-space travellers disappear, and plenty of worlds where they appear, and plenty of qualitative duplications between ones that disappear and ones that appear. But none of this is travel unless there is one surviving traveller who both departs and arrives. And causal continuity is required for survival; it is a principal part of what unifies a persisting person. ... No trans-world causation, no trans-world causal continuity; no trans-world causal continuity, no survival; no survival, no travel. All those people in various worlds who meet their ends in 'logical-space ships', as well as the more fortunate ones who appear ex nihilo in such ships, are sadly deluded.
It's a lovely fever dream, but... huh? Lewis's description of how spacetime might work in one of his infinitely many possible worlds is also pretty illustrative of the book's content and style:
Time might have the metric structure of the real line, as we normally suppose. And yet there might be infinitely many world-like epochs one after the other. Each might be of finite duration; but their finitude might be hidden from their inhabitants because, as the end of an epoch approaches, everything speeds up. Suppose that one generation lives and dies in twelve months, the next in six, the next in three, . . . so that infinitely many generations fit into the last two years of their epoch. Similarly, world-like regions of finite diameter might be packed spatially, with shrinkage as things approach the edge.

Lewis called the traditional response to his philosophy "the Incredulous Stare", since it's exactly what he'd get.

It's ultimately a compelling book, but could never be my thing. I don't know if it's anyone's thing. Lewis has a lot of admirers but not so many followers, it seems. Paying for systematic ease of use with the coin of common sense is a sucker's deal.

On the Plurality of Worlds in poetry

There is a collection by poet-mathematician Jacques Roubaud called Sur la pluraliti des mondes de Lewis that ironizes and explores the limitless allowances of Lewis's philosophy, mining possible worlds for poetic fodder with which to come to terms with the death of his wife. From the extracts I've seen, Roubaud draws very directly on On the Plurality of Worlds, appropriating Lewis's terminology and examples. For instance, compare the following with the first passage I quoted earlier:
There is no transworld travel

Nor any transworld traveler dragging his world along with him

There is neither revoking of duration, nor consciousness before time, nor any special way of surviving the sudden lack of direction

I wake up at night, I see the backside of the world

but this is not the way to reach you

All quotations from the following:
David Lewis, On the Plurality of Worlds (1986). Oxford: Basil Blackwell Ltd.
Jacques Roubaud, Sur la pluraliti des mondes de Lewis (1995). Dalkey Archive Press.

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