On the Plurality of Worlds

Citation

Lewis, D. (2001). On the Plurality of Worlds. Wiley-Blackwell. (First published by Blackwell, 1986.)

Chapter Summary

The book has a preface and four chapters, about 270 pages in all with the list of works cited and the index; the chapters differ greatly in length, with the first (pp. 1–95) and the fourth (pp. 192–263) the longest. The publisher describes it as a defence of modal realism, the thesis that our world is one of a plurality of worlds and that the individuals in it are a few of all the inhabitants of all the worlds, with Lewis arguing that the philosophical utility of the thesis is a good reason to believe it.

Chapter 1: A Philosophers' Paradise

The longest chapter states modal realism and then shows what the theory does. It opens with the thesis of the plurality of worlds: worlds are many and varied, isolated from one another, with no spatiotemporal relations between things in different worlds, and not different in kind or in manner of existing from ours. The case for believing it is that the hypothesis is serviceable, and the chapter's sections work through the uses: modality, treated as quantification over worlds; closeness of worlds, which supports an analysis of counterfactuals and raises questions about similarity orderings; content, in which knowledge and belief are characterised by the worlds and individuals compatible with them; and properties, which are taken as sets of their instances across worlds, so that properties which happen to have the same instances in this world are still distinct. Three further sections ask what makes things part of one world (Isolation), take up Concreteness, and ask what Plenitude requires, for which Lewis proposes a principle of recombination. The last section gives an indexical analysis of "actual."

Chapter 2: Paradox in Paradise?

Replies to objections that the quantity of worlds that must be postulated makes modal realism incoherent, including arguments from set theory. The section titles also take up how we can know about other worlds, whether the theory leads to scepticism or to indifference, whether it loses arbitrariness, and the "incredulous stare."

Chapter 3: Paradise on the Cheap?

Considers "ersatz" views that try to supply the same explanatory power without a plurality of concrete worlds. Sections treat linguistic ersatzism, pictorial ersatzism and magical ersatzism after a general account of the ersatzist programme. Lewis argues that each approach fails, for different reasons.

Chapter 4: Counterparts or Double Lives?

Develops counterpart theory, on which an actual object exists only in this world and has counterparts in other worlds that resemble it in important respects. After a section on good and bad questions, the sections argue against overlap between worlds, against trans-world individuals, against haecceitism and against constancy.

Key Concepts

Chapter 1: Modal realism at work

• Modal realism: the thesis that there are many concrete possible worlds, of which ours is one.
• Worldmates: things related spatiotemporally; Lewis uses this relation to say what makes things part of the same world.
• Principle of recombination: parts of different possible worlds can be patched together to yield another possible world, expressing the plenitude of possibilities.
• Indexical analysis of actuality: "actual" refers to the world of the speaker, as "here" refers to the speaker's place.

Chapters 2–3: Objections and rivals

• Incredulous stare: the reaction that so many concrete worlds cannot be believed.
• Ersatzism: accounts that replace concrete worlds with abstract representations, such as sets of sentences, pictures or primitive "magical" relations.

Chapter 4: Counterparts

• Counterpart theory: an object exists in one world only and has counterparts, not identical copies, in other worlds.
• Haecceitism: the view that worlds can differ purely in which individual plays which role.

Critical Analysis

Strengths

• A cost-benefit argument stated openly. The first chapter lists what possible worlds do for modality, counterfactuals, content and properties before objections are taken up, so the reader sees exactly what the theory is meant to buy.
• Systematic replies to rivals. Chapters 2 and 3 take on the paradoxes of quantity, the epistemological worries, and three families of ersatz theory, so the book handles its main competitors in turn.
• Short sections with descriptive titles. The table of contents is detailed enough that a reader can enter at a single question, such as properties, actuality or haecceitism.
• A clear authorial position. Lewis states the radical thesis plainly, including the "incredulous stare," and then argues for it, which makes the structure easy to follow.

Limitations

• Heavy background assumed. The treatment uses modal logic, counterfactual semantics and possible-worlds talk throughout, so a reader without that vocabulary will struggle in Chapter 1.
• A single thesis. The book defends one position, so alternatives to Lewis's account of modality must be read elsewhere.
• Long technical stretches. Chapter 3's linguistic ersatzism section runs to over twenty pages and Chapter 4's discussions of overlap and haecceitism are similarly long, which can slow a reader who wants only the overall argument.
• Chapter 1 carries much of the weight. About a third of the book is spent on what the theory does, so readers wanting the objections must wait for Chapter 2.

Where It Fits in the Course

In the metaphysics list this is a specialist book on modality. The introductions, Loux and Crisp's Metaphysics: A Contemporary Introduction, van Inwagen's Metaphysics, Mumford's Metaphysics: A Very Short Introduction and Conee and Sider's Riddles of Existence, cover the central problems at different levels and prepare for Lewis; Conee and Sider include necessity among their topics. Kripke's Naming and Necessity is the natural companion: it gives the lectures in which essence and necessity are developed, and Lewis's counterpart theory in Chapter 4 offers a different account of what it is for an object to have properties necessarily.

Readers who need the logical vocabulary can use Sider's Logic for Philosophy, in the logic list, whose chapters on modal logic and counterfactuals cover possible-worlds semantics.

How to Use This Book

The book is for graduate students and advanced undergraduates who have taken a metaphysics course and know basic modal logic and possible-worlds semantics.

A practical reading plan:

• Core: Chapter 1, in particular 1.1 (the thesis), 1.2 (modality), 1.5 (properties), 1.6 (isolation) and 1.8 (plenitude), then the first two sections of Chapter 2.
• Rivals: Chapter 3, starting with 3.1 for the ersatzist programme and then whichever of linguistic, pictorial or magical ersatzism interests you.
• Counterpart theory: Chapter 4, beginning with 4.1 and then the sections against overlap and against haecceitism.
• Optional: the sections on closeness and content (1.3, 1.4) if counterfactuals or belief are your focus.

For each chapter write down what Lewis claims the theory explains and what each rival gives up, then ask whether the cost-benefit argument convinces you.