- This book defends modal1 realism: the thesis that the world we are part of is but one of a plurality of worlds, and that we who inhabit this world are only a few out of all the inhabitants of all the worlds.
- I begin the first chapter ("Lewis (David) - A Philosopher's Paradise") by reviewing the many ways in which systematic philosophy goes more easily if we may presuppose modal2 realism in our analyses. I take this to be a good reason to think that modal3 realism is true, just as the utility of set theory in mathematics is a good reason to believe that there are sets. Then I state some tenets of the kind of modal4 realism I favour.
- In the second chapter ("Lewis (David) - Paradox or Paradise?"), I reply to numerous objections. First I consider arguments that modal5 realism leads to contradiction; and I reply by rejecting some premises that are needed to produce the paradoxes. Then I turn to arguments that modal6 realism leads to consistent but unwelcome views: inductive scepticism, a disregard for prudence and morality, or a loss of the brute arbitrariness of our world; and again I reply by finding premises to reject. Finally I consider the sheer implausibility of a theory so much at variance with commonsensical ideas about what there is; I take this to be a fair and serious objection, but outweighed by the systematic benefits that acceptance of modal7 realism brings.
- In the third chapter ("Lewis (David) - Paradise on the Cheap?"), I consider the prospect that a more credible ontology might yield the same benefits: the programme of ersatz modal8 realism, in which other worlds are to be replaced by 'abstract' representations thereof. I advance objections against several versions of this programme. I urge that we must distinguish the different versions, since they are subject to different objections; it will not do to dodge trouble by favouring abstract ersatz worlds in the abstract, without giving any definite account of them.
- In the fourth and final chapter ("Lewis (David) - Counterparts or Double Lives"), I consider the so-called 'problem of trans-world identity'. I divide it into several questions, some of them good and some of them confused, and I compare my counterpart-theoretic approach with some alternatives.
- Nowhere in this book will you find an argument that you must accept the position I favour because there is no alternative. I believe that philosophers who offer such arguments are almost never successful, and philosophers who demand them are misguided. I give some reasons that favour my position over some of its close alternatives. But I do not think that these reasons are conclusive; I may well have overlooked some close alternatives; and I do not discuss more distant alternatives at all. For instance, I do not make any case against a hard-line actualism that rejects any sort of quantification over possibilities. You will find it easy enough to guess why I would not favour that view; I have nothing new, and nothing conclusive, to say against it; so it would serve no purpose to discuss it.
- It may come as a surprise that this book on possible worlds also contains no discussion of the views of Leibniz. Is it that I consider him unworthy of serious attention? - Not at all. But when I read what serious historians of philosophy have to say, I am persuaded that it is no easy matter to know what his views were. It would be nice to have the right sort of talent and training to join in the work of exegesis, but it is very clear to me that I do not. Anything I might say about Leibniz would be amateurish, undeserving of others' attention, and better left unsaid.
- About twelve years ago, I gave my thesis a bad name. I called it 'modal9 realism'. Had I foreseen present-day discussions of what 'realism' really is, I would certainly have called it something else. As it is, I think it best to stick with the old name. But I must insist that my modal10 realism is simply the thesis that there are other worlds, and individuals inhabiting these worlds; and that these are of a certain nature, and suited to play certain theoretical roles. It is an existential claim, not unlike the claim I would be making if I said that there were Loch Ness monsters, or Red moles in the CIA, or counterexamples to Fermat's conjecture, or seraphim. It is not a thesis about our semantic competence, or about the nature of truth, or about bivalence, or about the limits of our knowledge. For me, the question is of the existence of objects - not the objectivity of a subject matter.
- At many points, I am greatly indebted to friends who have helped me by discussion or correspondence about topics covered in this book: especially Robert M. Adams, D. M. Armstrong, John G. Bennett, John Bigelow, Phillip Bricker, M. J. Cresswell, Peter Forrest, Allen Hazen, Mark Johnston, David Kaplan, Saul Kripke, Robert Stalnaker, Pavel Tichy and Peter van Inwagen.
- Part of this book was delivered as the John Locke Lectures at the University of Oxford in Trinity Term, 1984. I am most honoured by Oxford's invitation; and I am most grateful to Oxford for providing me with the occasion to write on modal11 realism more fully than I had done before, and also with a much-needed deadline. I am grateful to Princeton University for sabbatical leave, and to the National Endowment for the Humanities for financial assistance, during the year in which most of the book was written.
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