On the Plurality of Worlds

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Summary: Modal Realism -What's it all about (Alfie)?
Review: Crudely this book has been written to coordinate Cantor's transfinite number and set theory with philosophy.

In English, this means that if space is infinite, then there are infinite worlds. This is rather like thinking of matter, and the creation of worlds, as outcomes of an infinite Las Vegas slot machine (my analogy-not his!) Some worlds will have someone called Hubert Humphrey who wins a US election, for example. But he will not be the same as the Hubert Humphrey who occupies this planet with us. (see chapter on Transworld Individuals, ie there
arent any people who exist in more than one world at a time, however similiar they are to other ones.

This is a lot of fun, iof you enjoy strange ideas and understand words bigger than marmelade. I used the dictionary more times than I have since reading Spot the Dog and Janet and John Book 1. The book is studied as part of the foundation course in Philosophy at Harvard, according to some pages I read on the Internet.

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Summary: OK, I changed my mind
Review: First of all, it's not about the semantics of modal logic, it's about the metaphysics of modality, in a way that rationalists will hate: we see it as providing an unworkable substitute for modal claims, and dodging serious issues about the ground of necessity. Over half of the book (parts 2 and 3) is of technical interest at best -- the infamous linguistic/pictorial/magical trichotomy is a particularly dire piece of navel-gazing. The book stimulated a good deal of interest in various ways of eliminating apparent existential implications of theories; I must say that I've found some of this machinery helpful in my own work on mind, but probably the interest to serious metaphysics is minimal. More peripheral issues: while the material on persistence through time is interesting and important, the attempts to eliminate belief de re fail and are motivated by a misunderstanding of psychological explanation. The material on properties is not backed up by the most solid argument, though over all of course the writing is beautiful and the arguments tricky. The judgment of history is still in the making, of course; in about twenty years, the dust will have settled and we'll be able to judge the merit of the work more dispassionately.

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Summary: More Reviewers Need to Read the Book
Review: It should be noted, contrary to a previous reviewer, that David Lewis' account of modal realism is completely incompatible with the 'many worlds' interpretation of quantum mechanics. If you are looking for a philosophical foundation for that project look elsewhere. Lewis defends the view that there are infinitely many concrete possibly worlds that are **non-actual (from the perspective of our world), and both spaciotemporally and causally isolated from all other worlds**. This latter condition clearly does away with any likeness between the many worlds interpretation of quantum mechanics and genuine realism interpretation of possible worlds; if the other worlds aren't causally related to ours then what is their theoretical utility in explaining causal processes of this world--i.e. in doing this-wordly physics? In summary, Lewis' possible worlds don't interact empirically with ours and therefore they are of no interest or theoretical utility to this-worldy physicists.
Having gotten that topic out of the way, I would also like to give my opinion of the book. Extremely intelligent people can brow beat you into believing just about anything. Reading Lewis is tiring even for the most brilliant--at least tiring if you resist, if you go along with him it is more like unmitigated intellectual paradise.
I believe that modal realism is false. The argument for this belief, however, can only start where On the Plurality of Worlds leaves off; the book will serve as a prolegomena for all future modal reductionism and metaphysics until a more ontologically tenable account that overcomes Lewis' objections is advanced. At this point I do not think it is clear that such an account has been advanced, and accordingly this book is still a great place to start. The literature is immense and growing rapidly though. Keep your eyes peeled. Some attractive ideas have been presented by Theodore Sider (see particularly: "The Ersatz Pluriverse", Journal of Philosophy, June 2002). Still, you will probably need to read this book to understand the context of much of the discussion; it all tends to revolve around Lewis' arguments.

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Summary: Most significant contribution to metaphysics in many years
Review: It's obvious to me that some of these reviewers don't understand either what Lewis was doing in this book or the standard philosophical response to it. He wasn't arguing that there are multiple universes connected to ours in some way explorable through science. Those would be parts of this world, in Lewis' sense. The worlds he's talking about are possible worlds. They aren't actual. That is, they don't exist in any way spatiotemporally or causally connected with the actual world. And yet he says they're as concrete as we are.

Philosophers' responses to this view are incredibly interesting. They think the idea is nuts, and yet they have no way to resist the conclusion that he gives compelling arguments that his view solves numerous philosophical problems that no one has been able to deal with in a perfectly satisfactory way. This doesn't convince many people that his view is correct, but the response has been pretty strong among those who want to use his system without thinking that it's true. They call it a modal fiction, and the view is called fictionalism. This is becoming incredibly influential among metaphysicians.

Aside from all that, most metaphysicians today recognize this book as just incredibly fruitful and creative in bringing together so many different strains in metaphysics. He deals with so many problems in such a lucid way that the book serves to introduce many problems in metaphysics, making advances in the discussion even apart from the contribution of his main thesis.

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Summary: A brilliant defense of an unorthodox view of modal logic.
Review: Lewis argues that the semantics of modal logic are best served by postulating the existence of a set of possible worlds, so that our modal operators refer to states of affairs within those worlds. He argues by analogy with numbers that the causal independence of those worlds is no obstacle to their theoretical usefulness, and offers detailed criticism of several attempts to simulate modal semantics without postulating the worlds as really existing. His positive argument for his own view primarily involves looking at other issues in modal logic from the perspective of his modal realism, showing how his view can help shed light on those issues. His discussion of those issues is of great independent interest even to those skeptical of his modal realism.

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Summary: A philosopher's paradise--in inspired lunacy
Review: Lewis' work always produces mixed feelings in me. There is a long list of good things to be said on his behalf, and, so far as I can see, only one really bad thing. But the bad thing is bad enough to taint my otherwise unalloyed (and profound) respect and admiration for this superb thinker, quite probably the best philosopher alive, and almost certainly the best I've ever read. (Aaron, if you are reading this, you were right about your teacher; he deserves all the plaudits you kept showering on him, and for all I know he may even be right about the crazy things he says.)

First, the pros. Lewis offers a modal metaphysics that is a) technically brilliant, elegant, and well-motivated, b) capable of providing reductive analyses of a vast range of otherwise obscure notions, and c) the best game in town if considered simply on it theoretical merits. He shows how his modal realism can be used to analyze modality (necessity, possibility, and the like, as well as restricted modalities) and mental and semantic content, and to make room for properties and counterfactual claims. He shows how to dissolve the debate among essentialists and anti-essentialists using counterpart theory, how to avoid various apparently serious objections to modal realism, and how to understand the debates about de re modality and "transworld identity". He offers a clear account of ersatzism, especially of the linguistic variety, which (I agree with him in holding) is the only real alternative to his modal realism among the theories so far offered, and is as such clearly an inferior theory. He offers a devastating argument against "magical ersatzism", probably the most commonly held view on modality apart from linguistic approaches. The argument is decisive, I think, and shows that approaches like those of Plantinga or Stalnaker cannot succeed. His modal realism has its limitations; because he cannot help himself to worlds bigger than a certain size and shape, and because he has all the worlds as concrete objects existing "side by side", so to speak, he can't countenance the full range of possibilities that an ersatzist can, and his analyses come apart at the edges. (For instance, the thesis of the plurality of worlds itself is, on Lewis' account, necessarily false; thus he must believe in necessarily false truths. Again, he cannot admit the possibility of two spatiotemporally disconnected universes, or worlds that differ as to which non-spatiotemporal abstracta exist in them.) When these defects are compared against the defects of Lewis' competitors, however, his view emerges as the clear winner.

Nonetheless, in the end all of this theoretical advantage is not enough. The thesis of the plurality of worlds is simply incredible. Indeed, I would go so far as to say that its falsehood is something like a Moorean fact; it disagrees so badly with common sense that no philosophical considerations in its favor could constitute an adequate reason for accepting it. Lewis's methodology is right; he proceeds in theorizing by weighing the costs of departure from pretheoretical opinion against the benefits in theoretical economy and utility. Unfortunately, this method is here being used by the wrong person; for all his brilliance, Lewis just doesn't have the *good sense*, or wisdom, to see how crazy his theory is--and this is the only thing that lessens my admiration for him. Faced with the evident inadequacies of all the competitor accounts that Lewis surveys, I am forced to make the choice Lewis says I will be forced to make:

"Paradise on the cheap, like the famous free lunch, is not to be had. Make of this what you will. Join the genuine modal realists; or foresake genuine and ersatz worlds alike."

Since I cannot join the modal realists (literally, "cannot"; I am psychologically incapable of it), I choose to foresake possibilia altogether. Ersatz or concrete, they must go; and if I can think of no alternative to these approaches--which are after all the only ones now on offer--I shall simply suspend judgement on whether there are really possible worlds of *any* sort, and on their nature and how we can do what we do with them. But I insist on maintaining the hope that some future theorist will tell us how we can help ourselves to possibilia, in a way that we can live with--for it seems absurd to give up the benefits of talking in terms of possibilia just because we have no adequate and credible metaphysics of them.

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Summary: Philosophers Need to Learn More Physics
Review: This is in memory of David Lewis, who has recently and unexpectedly passed away. I had the wonderful opportunity to meet David Lewis and hear him speak at a conference I attended, and was completely in awe of the enormity of the contributions he has made to analytic philosophy. While I am more of a continental philosopher, David Lewis' contributions to the discipline of philosophy as a whole cannot be ignored. His work simply cannot be recommended highly enough.