POSSIBLE WORLDS:

WHAT THEY ARE GOOD FOR AND WHAT THEY ARE

Alexander Robert Pruss

B.Sc. (hon.), University of Western Ontario, 1991

Ph.D., University of British Columbia, 1996

Submitted to the Graduate Faculty of

Arts and Sciences in partial fulfillment

of the requirements for the degree of

Doctor of Philosophy

University of Pittsburgh

2001

[committee signature page goes here]

POSSIBLE WORLDS: WHAT THEY ARE GOOD FOR AND WHAT THEY ARE

Alexander R. Pruss, Ph.D.

Advisor: Nicholas Rescher

University of Pittsburgh, 2001

This thesis examines the alethic modal concepts of possibility and necessity. It is argued that one cannot do justice to all our modal talk without possible worlds, i.e., complete ways that a cosmos might have been. I argue that not all of the proposed applications of possible worlds succeed but enough remain to give one good theoretical reason to posit them. The two central problems now are: (1) What feature of reality makes correct alethic modal claims true and (2) What are possible worlds?

David Lewis makes possible worlds be concretely existing universes. Unfortunately, I show Lewis’s account involves set-theoretic, ethical, inductive and probabilistic paradoxes, and commits Lewis to an objectionable form of primitive modality that governs the choice of the counterpart relation. The most promising contemporary alternatives to Lewis’s theory have been the worlds of Adams and Plantinga constructed out of Platonic entities such as maximal collections of consistent propositions. However, these approaches fail to provide a satisfactory answer to the question of what makes true modal claims true. I also criticize some alternative accounts.

Finally, I discuss and combine two historical approaches. The first is an Aristotelian approach that says a non-actual event is possible is to say that some actual substances could have initiated a causal chain that could lead up to the event in question. However, it can be shown that some plausible global possibility claims can be made true on this account only if there is a necessarily existent first cause (or aggregate of first causes) capable of initiating very different universes. On the other hand, Leibniz made possible worlds be ideas in the mind of an omniscient necessarily existent deity. Leibniz fails to explain what it is that makes these possible worlds possible, but if we were willing to combine his story with the conclusion drawn from the Aristotelian one, we could get the following story: Possible worlds are ideas in the mind of an omniscient deity and what makes them possible is that this deity has the Aristotelian capability of initiating causal chains that can lead to them being actualized.

Preface

I would like to thank Robert Brandom, James Dreier, John Earman, Gregory Fitch, Richard Gale, Jeremy Heis, John Leslie, Kenneth Manders, David Manley, William McEnaney, Robert Koons, John Norton, Alvin Plantinga, Nicholas Rescher, Ernest Sosa, Donald Turner, Peter van Inwagen and two anonymous referees for encouragement, interesting discussions and/or comments on various parts of this project.

And above all, I would like to thank my wife and parents for all their loving encouragement in my studies.

Preface…….. vi

Table of Contents. vii

List of Tables. xii

Part I. Introduction. 1

Section 1 Generic definitions and basic modal realism.. 1

Section 2 Metaphysical versus logical possibility?. 5

Section 3 S5. 8

Section 4 Six kinds of views on possibility. 10

4.1 Parmenides, Spinoza, Leslie and Rescher 10

4.2 Leucippus, Democritus, Meinong, Lewis and Aristotle. 17

4.3 The linguistic view.. 20