Reality and Humean Supervenience: Essays on the Philosophy of David Lewis

129 Pages·2001·12.506 MB·English

by  Gerhard Preyer, Frank Siebelt

Reality and Humean
Studies in Epistemology and Cognitive Theory
Supervenience
Series Editor: Paul K. Moser, Loyola University of Chicago
Metaepistemology and Skepticism
Essays on the Philosophy
by Richard A. Fumerton, University of Iowa
Warrant in Contemporary Epistemology: Essays in Honor of Plantinga 's
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Theory of Knowledge
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A Defense of the Given
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Pragmatism and Realism
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Realism/Antirealism and Epistemology
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Belief and Knowledge: Mapping the Cognitive Landscape
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Illusions of Paradox: A Feminist Epistemology Naturalized
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ROWMAN & LITTLEFIELD PUBLISHERS, INC. Contents
Published in the United States of America
by Rowman & Littlefield Publishers, Inc.
4720 Boston Way, Lanham, Maryland 20706
http://www.rowmanlittlefield.com Preface Vil
12 Hid's Copse Road, Cumnor Hill, Oxford OX2 9JJ, England Chapter 1
Reality and Humean Supervenience:
Copyright © 200 I by Rowman & Littlefield Publishers, Inc. Some Reflections on David Lewis's Philosophy
Gerhard Preyer and Frank Siebelt
D. M. Armstrong's article, "Going through the Open Door Again: Counterfactual versus
Singularist Theories of Causation," appears in this volume by the kind permission of
John Collins, Ned Hall, and L. A. Paul, editors of Causation and Counterfactuals (The PART I MODAL REALISM
MIT Press, forthcoming). Chapter 2
Island Universes and the Analysis of Modality
Chapter 11 is from Michael Tye, Consciousness, Color, and Content, (Cambridge, MA: Phillip Bricker 27
The MIT Press, 2000), pages 3-20.
Chapter 3
Time Travel Fiction
All rights reserved. No part of this publication may be reproduced, stored in a retrieval
system, or transmitted in any form or by any means, electronic, mechanical, John Bigelow 57
photocopying, recording, or otherwise, without the prior permission of the publisher. Chapter 4
Counting the Cost of Modal Realism
British Library Cataloguing in Publication Information Available
Peter Forrest 93
Chapter 5
Library of Congress Cataloging-in-Publication Data
Against Against Overlap and Endurance
Reality and Humean supervenience: essays on the philosophy of David Lewis I edited Paul Teller 105
and introduced by Gerhard Preyer and Frank Siebel!. Chapter 6
p. cm - (Studies in epistemology and cognitive theory)
The Case for Perdurance
Includes bibliographical references and index.
Harold W. Noonan 123
ISBN 0-7425-1200-2 ( alk. paper)-ISBN 0-7425-1201-0 (pbk.: alk. paper)
I. Lewis, David K., 1941-2. Realism. 3. Supervenience (Philosophy) I. Preyer,
Gerhard. IL Siebel!, Frank. III. Studies in epistemology and cognitive theory PART II PHYSICALISM, CAUSATION, AND CONDITIONALS
(Unnumbered) Chapter 7
Naturalism for the Faint of Heart
8945 .L455 R43 2001 Daniel Bonevac 143
191-dc21
Chapter 8
00-059162
Going through the Open Door Again:
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Counterfactual versus Singularist Theories of Causation
eTM D. M. Armstrong 163
The paper used in this publication meets the minimum requirements of American
National Standard for Information Sciences-Permanence of Paper for Printed Library
Materials, ANSI/NISO 239.48-1992.
vi Contents
Chapter 9
On Forward and Backward Counterfactual Conditionals
Jonathan Bennett 177
PART III REDUCTION OF MIND
Chapter 10 Preface
Multiple Reference, Multiple Realization, and the Reduction of Mind
Terence Horgan 205
Chapter 11 Frugal Humean ontology has always conceived our world to be made up of indi
Knowing What It Is Like: vidual perceptions: sensual patterns that we receive by means of our internal and
The Ability Hypothesis and the Knowledge Argument external senses and the basic regularities among them. Twentieth-century descen
dants of this idea, such as Russell's theory of logical atomism or Wittgenstein's
Michael Tye 223
ontology of facts, added logic and language to approach, thereby partially remov
Index 239 ing classical Humeanism's enclosure within a solipsist psychology. But whether
Humeanism was psychologically based, as in Hume's original writings, or seman
About the Contributors 245 tically based, as in Russell's or Wittgenstein's work, it was still the presumption
that problems in philosophical ontology could be addressed without paying too
much tribute to the development of physical science. Contemporary Humeanism
in philosophical ontology shows a more open-minded attitude toward physical
science, which becomes apparent especially in the important philosophical contri
butions which comprise the work of David Lewis.
If asked what Humeanism means today there is no other philosopher to turn to
whose work covers such a wide range of topics from a unified Humean perspec
tive. The core of Lewis's many contributions to philosophy, including his work in
philosophical ontology, intensional logic and semantics, probability and decision
theory, and topics within philosophy of science as well as a distinguished philoso
phy of mind, can be understood as the development of a philosophical position that
is centered around his conception of Humean supervenience. If we accept the
thesis that it is physical science and not philosophical reasoning that will eventu
ally arrive at the basic constituents of all matter pertaining to our world, then
Humean supervenience is the assumption that all truths about our world will
supervene on the class of physical truths in the following sense: There are no
truths in any compartment of our world that cannot be accounted for in terms of
differences and similarities among those properties and external space-time rela
tions that are fundamental to our world according to physical science.
Although Lewis upholds not only physicalism but also Humean supervenience,
both theses should nevertheless be distinguished as they are logically independent.
One can be a physicalist without being forced to accept Humean supervenience,
and for anti-physicalists, Humean supervenience is still a philosophical option. If
all else supervenes on an arrangement of fundamental properties being both physi
cal and localized, both theses would be true. But if all else supervenes on physical
properties that are not localized, only physicalism would be true. An example
would be a world wherein it is not possible to decompose entangled quantum
superpositions into local parts. And if all else supervenes on fundamental proper-
Preface ix
viii Preface
Not every philosopher would go so far as to agree with Lewis that we can make
ties that are localized but not physical, then Humean supervenience is true but not
sense of some of these stories without hiding subtle logical inconsistencies. In
physicalism. An example would be a world in which some fundamental properties
chapter 3, John Bigelow points out that time travel stories or fictions must draw a
are extremely unlike those properties in our current physics. According to Lewis
far-reaching distinction between the idea that time passes and a radical
there might be a world where properties of particles that make up living or con
spatialization of time. He confronts Lewis with this distinction, arguing that such
scious matter never occur in dead matter.
stories are inconsistent if we accept a radical spatialization of time. Although some
In chapter 1, Gerhard Preyer and Frank Siebelt stress the important conse
of the stories do appear to be internally self-consistent if they are told against a
quences that the thesis of Humean supervenience holds for all major aspects of
metaphysical background that spatializes time, nonetheless the appearance of
Lewis's philosophy. In doing so the authors show how Lewis's thesis can serve as
internal self-consistency is always, in the end, an illusion. There is no possible
a foundation for modem physicalism, including the philosophy of modality, the
world without time for Bigelow and there is no time that does not pass. Yet, if time
theory of decision, the mind-body problem, and even central aspects in ethical
were spatialized then it would not be something which passes, and this raises
theory. Both authors argue that with his thesis ofHumean supervenience, Lewis's
questions about the underlying metaphysics of spatialized time. The result of
varied contributions in many areas of contemporary philosophical thought are
Bigelow's analysis is that a spatialzing metaphysics is internally inconsistent, and
nevertheless directed toward a systematic and unique philosophical position.
The first impression when confronted with this thesis is that Humean super therefore every time travel story is essentially inconsistent.
In chapter 4, Peter Forrest confronts Lewis's rich ontology with a dilemma that
venience imposes constraints on theorizing about philosophical ontology only as
it reaches out for distinctions pertaining to our world. Yet our modal notions of arises when he tries to accommodate two principles that are in outright logical
necessity and possibility, central to our notions of laws, causation, and counter conflict. The one is well known under the heading of Occam's razor: entia non
factual reasoning, are nevertheless concerned because Lewis opts for a realist sunt multiplicanda sine necessitatis. (Don't multiply entities without good reason!)
conception of logical space. In his acclaimed theory of modal realism he argues for The other stems back to Hume, and is introduced by Forrest as Hume's razor
(HUR): Don't multiply necessities without good reason. The more exotic the
a realist theory of modality in which our world is only one world among an infinite
plurality of possible worlds that comprise all logical space, where those worlds are possibilia the more modal realists commit an offense against Occam's razor, but if
united in more or less the same spatiotemporal way as Lewis thinks our actual modal realists restrict the possibilia they multiply necessities and so offend (HUR).
One possible solution Forrest discusses might be to distinguish two versions of
world is united. Of course his conception of logical space owes much to those
constraints that are imposed on modal reasoning by his thesis of Humean Occam's razor. The first could be that actual entities are not to be multiplied more
supervenience, which can be seen in many arguments in the essays that make up than is necessary, the second concerns all entities there might be. He calls the first
special Occam's razor (SOR), the second general Occam's razor (GOR). Modal
part I on his theory of modal realism.
realists could argue then that GOR has no force. Yet if GOR is totally rejected,
As far as our notion of modality is concerned, it follows from Humean princi
modal realists could multiply qualitatively identical copies of worlds in logical
ples of plentitude that island universes are possible. Physical reality might contain
space, without incurring any cost, which doesn't look very promising. Forrest
absolutely isolated parts. This causes trouble for Lewis's modal realism if it is
therefore argues that modal realists should accept all three razors, but, when con
required for entities to be spatiotemporally related in order for them to be parts of
flict arises, they should insist that HUR has priority over GOR. Forrest assumes
the same world. Phillip Bricker illustrates how the modal realist might have a way
out in his discussion of this topic. In chapter 2, he begins by developing a different that modal realists handle the threat of Occam's razor in this way. But then modal
concept of actuality, which he calls absolute actuality, while arguing for its superi realism carries costs in two ways. First, modal realism incurs a GOR cost if possi
ority over Lewis's conception. He goes on to revise the standard analysis of mo ble worlds are multiplied without reducing the number of necessities. Second,
modal realism incurs a HUR cost if they multiply necessities, which in fact comes
dality as individual quantifiers and takes modal operators to be plural quantifiers
over possible worlds. In Bricker's view this could solve the problem of island down to restricting possibilities. Forrest proposes that for modal realists there is no
universes within a modal realist theory of logical space. It also leads to an unquali straightforward solution to this dilemma.
In chapter 5, Paul Teller examines Lewis's arguments against overlap, the
fied principle of compossibility that also allows for the possibility of there being
thesis that no two distinct worlds share any parts, and against endurance, the thesis
nothing at all. And, as Bricker ventures to show, the thesis of the identity of indis
cernible worlds could then be decisively refuted. that things persist through time by being wholly present at different times. In
Another aspect of Lewis's theory of modal realism relates to the question of Teller's view both arguments have the same form. They rely on the assumption
how to make sense of time travel stories. Do such stories represent genuine possi that modal realism, which allows for overlap, must come to strange conclusions
bilities, are they logically consistent? Lewis thinks that at least some of these when considering accidental intrinsics of individuals, for example the accidental
stories do not lead to outright contradictions within his theory of modal realism. characteristic of Ernest, that he was born with six fingers on his left hand. If the
x Preface Preface xi
very same person could also be part of another possible world, where he has five former taken to be causes of effects that never occur, the latter as events that do
fingers on his left hand how could they be one and the same person? They obvi not occur but nevertheless have positive effects. Preventions bring about the ab
ously don't share all the same properties. Isn't the sharing of all properties a neces sence of the event prevented, omissions are absences that cause something positive
sary condition to allow for talk of identity? The argument easily carries over to to occur. Other problematic cases for Lewis's theory have to do with the notion of
identity over time, where we would have one and the same whole person present indeterministic or probablistic causation, where we have a cause occurring, pro
at different times, while nevertheless having different properties at those times. ducing an effect which had some positive chance x to happen. In such a case, if the
For Teller, the serious problem with the notion of endurance and overlap is not cause hadn't occurred, the effect would still have some very slight chance of also
change of properties, but change of parts. Teller argues for a version of a relational occurring, which runs against the counterfactual analysis. For Armstrong causation
conception of properties that holds firm against Lewis's arguments that no rela is essentially singularist, a matter of this causing that, where nothing else besides
tional analysis of properties makes sense. the cause and the effect is involved. Such a notion of causation is an essentially
In chapter 6, Harold W. Noonan, on the other hand examines, and, for the most anti-reductionist one, a notion that cannot be analyzed further into something else.
part, defends Lewis's views on the nature of identity over time. Noonan argues Armstrong then tries to show that a singularist theory of causation can handle
that one of Lewis's arguments for the thesis that persisting things are perdurers cases of preventions and omissions as well as cases of probabilistic causation.
rather than endurers is virtually irresistible and that there are other considerations In chapter 9, Jonathan Bennett addresses Lewis's analysis directly. Lewis's
in favor of this view. He ventures to show that the arguments that have been put theory of causality rests on his modal semantics for counterfactuals. For Lewis,
forward against Lewis's view are answerable, and that the most seemingly difficult counterfactuals of the form A > B in which the consequent pertains to a time later
arguments to answer are the ones against which a defender of Lewis's metaphysics than that of the antecedent rest on a notion of closeness according to which the
is already prepared. The conclusion of his discussion is that the burden of proof closest A-worlds are ones that are exactly like the actual world until shortly before
rests squarely on the shoulders of Lewis's opponents. the time of the antecedent and then become unlike it through either an indetermi
Another aspect of Lewis's Humeanism relates to his strong denial of any nacy or a small miracle. Such counterfactuals are usually discussed under the
necessary connections in rebus, which of course was one of the cornerstones in name forward conditionals. Bennett supports Lewis's general account of those
Hume's classical epistemology and metaphysics. Denying necessary connections types of counterfactuals and criticizes two rival accounts (one of them his own)
between distinct existences imposes constraints on our notions of laws, causation, that provide for the unrestricted use of backward counterfactuals in which the time
and counterfactuals. For it implies the contingency of laws and thereby if one of the consequent is earlier than that of the antecedent. Bennett aims to show that
accepts the law-based approach to causation and for the most part of the only legitimate backward counterfactuals are the ones yielded by Lewis's
counterfactuals, then one must accept the contingency of causation and theory of forward counterfactuals; that is, A > Earlier is true only if Earlier is part
counterfactuals as well. Still, law and causal and counterfactual statements all of every legitimate point of departure connecting a fork from the actual world to A.
carry rather strong modal implications, so there is a problem in trying to combine Another aspect of Lewis's theory is that his notion of closeness is not antecedent
Humean supervenience with those aspects of modality, which we find implied by dependent, because he wants the closest A-worlds to be ones that separate from the
them. Some of those questions that arise from trying to integrate these modal actual world shortly before the time of A. Bennett then discusses whether the
aspects of our world with the broadly physicalist picture implied by Lewis's thesis conditions that Lewis employs in his analysis to secure his understanding of close
of Humean supervenience are addressed in the essays of part II. ness can be made to succeed against rival antecedent relative accounts of close
In Lewis's version of physicalism the problem is how modal truths supervene ness.
on nonmodal ones. In chapter 7, Daniel Bonevac proposes a strategy by which the Part III addresses questions pertaining to Lewis's physicalist-reductionist
notion of supervenience can stand alongside the notion of law and causality. He approach toward mental properties and the mind as such. In his view, mental
distinguishes a weak and a strong notion of supervenience and argues that the properties are type-identical to neurophysical properties. As with many
modal weakly supervenes on the nonmodal. His arguments rest on a novel concep reductionist approaches that accept physicalist accounts of type-identities between
tion of faint-hearted conditionals that, providing suitable conditions obtain, allow the mental and the physical we have the problem of multirealization: mental prop
for modal properties like dispositions, capacities, and so forth to be supervenient erties that are realized by different neuro-physiological properties. If multi
on occurrent properties. Whether his argument succeeds depends on his account of realization is accepted as possible for mental states, it would make talk of identity
conditionals and his thesis that modal notions, with the help of those types of problematic, as identity is logically an equivalence relation. In Lewis's prominent
conditionals, are to some extent translatable into nonmodal ones. solution to the problem, examples of putative multiple realization are treated like
In chapter 8, D. M. Armstrong points to several problems in Lewis's counter cases of kind-restricted property identity.
factual analysis of causation. There are cases of prevention and omission: the
xii Preface Preface xiii
In chapter 10, Terence Horgan argues that Lewis's answer to the problem of <www.rz.uni-frankfurt.de/protosociology>, who initiated this book. We would
multirealization is not applicable to what he calls strong multiple realization of also like to mention Laura Padgett and Georg Peter who helped form this book's
mental properties and that a credible theory of mind should accommodate this final shape.
potential phenomenon. Horgan proposes certain modifications in Lewis's larger
metaphysical framework, especially in regard to his treatment of mental-state Gerhard Preyer, Frank Siebelt
names. Horgan argues, contrary to Lewis, for a nonreductive materialist theory of J.W. Goethe-Universitiit, Frankfurt am Main
mind by showing that such a theory could accommodate cases of strong multiple
realization.
For a physicalist like Lewis, the only information we can gain about our world
is physical information, informational truth is therefore confined to physical truth.
Given that hypothesis, how could a physicalist account for those types of informa
tion that we receive when experiencing, for example, the taste of olive oil for the
first time? We might have all kinds of physical information about it, that it is
green, that it is made of olives and so on. Yet by tasting it for the first time, do we
not gain new information, new knowledge about phenomenal characteristics that
could not be had by way of assembling all physical truths about it? Isn't knowing
what it's like to taste olive oil also a type of knowledge, different from mere
physical knowledge? This famous objection against a physicalist construal of
knowledge is usually called the Knowledge Argument. Lewis has rejected the
assumption of phenomenal knowledge and thereby denies the Knowledge Argu
ment. Instead he has argued for an ability interpretation of putative phenomenal
knowledge. For him, knowing what it is like, for example, to taste olive oil is
know-how and not a special unphysical type of knowledge. By tasting olive oil
one gains abilities, for example, the ability to recognize olive oil in meals. If true,
the ability interpretation rules out any kind of phenomenal knowledge. In fact,
phenomenal knowledge is thereby consistent with physicalism because it neither
concerns phenomenal characteristics nor does it have the structure of genuine
propositional knowledge. Moreover, it is consistent with physicalism because it is
only knowledge in disguise, as when we speak of knowing how to swim or how to
sail a boat.
In chapter 11, Michael Tye claims that the standard objections to the ability
interpretation fail. He argues that the ability hypothesis is wrong and that
phenomenal knowledge still poses problems for physicalism. As a solution Tye
proposes an alternative account of knowing what it is like that ties it conceptually
to both knowing-that and to knowing-how. Given his proposal, he maintains that
one can develop a satisfactory solution to the knowledge argument against
physicalist accounts of knowledge.
Together these essays challenge aspects of Lewis's philosophy, posing new
questions and opening up new points of view in one of contemporary philosophy's
most ingenious and independent authors. We wish to thank our contributors and all
those whose help has made this collection possible, especially David Lewis for his
encouragement and detailed commentary, Ernest Sosa and Paul Moser for their
expertise and suggestions, D. M. Armstrong in Australia and the board of the
periodical, Protosociology An International Journal ofI nterdisciplinary Research
Chapter 1
Reality and Humean Supervenience:
Some Reflections on David Lewis's Philosophy
Gerhard Preyer and Frank Siebelt
Contrary to classical Humeanism, David Lewis asks us as philosophers to accept
the view that it is physical science that will giving us a complete as well as an
explanatory adequate description of our world in the long run. "The world is as
physics says it is and there is no more to say. World history written in physical
language is all of world history," to quote one of his most celebrated remarks.1 Of
course that does not strip philosophical ontology of its a priori conceptual
enterprise. But still, accepting that hypothesis imposes a massive constraint on any
conceptual enterprise that addresses ontological questions. To outline those
restrictions and the way in which Lewis deals with the intriguing philosophical
questions sprouting from it, we arrive at the core of Lewis's philosophy.
First of all, this constraint restricts our notion of truth in ontology, for it
implies-via descriptive completeness-the thesis that to be true in our world is to
be couchable in principle by physical means. Furthermore, by means of explana
tory adequacy it also implies that truths so couched are explanatory adequate in
principle as well. If all truths pertaining to our world are ultimately physical truths,
physical science should be able to give us an inventory of the basic constituents of
our world. One aspect of that task then is for physical science to shape at least a
tentative answer as to what properties and relations we have to accept as making
up for objective distinctions and similarities in our world. Physical science would
have to provide an inventory of perfectly natural properties and relations by which
we can characterize differences and similarities of all matters of fact as we find
them in our world.2
Thus stated, the notion of naturalness of properties is primitive. There is no
further explanation of what constitutes the presence of the same natural property
in different places. Explanation stops here. Lewis sets out on his ontological enter
prise with the assumption that the basic furniture of our world can be conceived as
spatiotemporal bits or points, or their material occupants, instantiating arrange
ments of fundamental localized intrinsic properties and external space-time rela
tions between them.3 What these localized physical properties finally are is for
physical science to determine. They could be properties like spin, charge, or fla-
2 Gerhard Preyer and Frank Siebelt Reality and Humean Supervenience 3
vor, as recent developments in physical science suggest. Physicalism as a philo both should nevertheless be distinguished because physicalism and Humean
sophical conception is what we are talking about here. As such there is Lewis's supervenience are logically independent. 7 If all else supervenes on an arrangement
thesis that what makes up for truth in any compartment of our world should in the of fundamental properties being both physical and localized, what Lewis holds
long run be found among those basic arrangements of natural properties and exter true for our actual world, both theses would be true. But if all else supervenes on
nal relations. Physicalism thus understood is not merely a strong thesis, for it physical properties that are not localized, Humean supervenience would be false
implies that all putative truths pertaining to our world must ultimately be physical while physicalism true. We can think of a world wherein it is not possible to
truths, otherwise we would have to deny them their truth status. Physicalism then decompose entangled quantum superpositions into local parts. Vice versa, if all
is not merely a philosophical doctrine but rather a whole philosophical program else supervenes on fundamental properties that are localized but not physical then
that needs to be clarified and defended against many contemporary misunderstand only physicalism would be wrong: not Humean supervenience. An example would
ings. be a world in which fundamental properties are extremely unlike those properties
in our present day physics. According to Lewis there might be a world where
properties of particles that make up sensible living matter never occurs in dead
Humean Supervenience matter.
Yet, if both theses are true, then given the class of basic localized natural
For Lewis, the thesis of the descriptive completeness and explanatory adequacy of properties and external relations in our world, as well as any other class of prop
physical science is not a necessary truth. Some physicalists think that way, but e1iies-for example, the set of vital properties by which we characterize living
they face a serious modal objection. Why can't there be worlds in which organisms-the set of mental properties by which we characterize sensible living
physicalism is false, worlds in which we could have disembodied spirits or Carte organisms including ourselves, then any true worldly difference stated in terms of
sian egos, or worlds in which mind-body dualism is true, so that truths about those vital or mental properties should actually be determined by differences
minds in them are truths that cannot be couched in terms of physical truths about within arrangements of those fundamental and localized properties and relations.
those worlds? There might also be worlds in which we would find additional "A supervenience thesis is a denial of independent variation .... To say that so
nonphysical properties completely alien to our actual world. Lewis is willing to and-so supervenes on such-and-such is to say that there can be no difference in
accept that modal objection against necessitarian conceptions.4 Physicalism respect of so-and-so without difference in respect of such-and-such."8 And it is
therefore might possibly be false if we think of other worlds. In light of that fundamental that Hu mean supervenience thus understood should be regarded as an
argument physicalism should best be understood as a contingent ontological asymmetrical relation.9
hypothesis, for all we know to be true for our world so far, but possibly false if we What makes this supervenience relation a Humean one? Hume's legacy lies
think of others. most apparently in his denial of necessary connections between distinct existences.
There is one major unifying theme that emerges from Lewis's recent contribu Translating this anti-necessitarian approach within Lewis's version of physicalism,
tions to philosophical ontology and that lies at the center of his conception of the Humean aspect lies in his denial of any strong conception of natural laws and
physicalism. This is his thesis of Humean supervenience:5 if physicalism, as out causality. There might be universal regularities in our world, there might be causal
lined above, is true for our world then all truths about it will supervene on the class relations between what happens in one region of space-time and what happens in
of physical truths in the following sense. There are no truths in any compartment another, and supervenience could of course be law-based. But none of those regu
of our world that cannot be accounted for in terms of differences and similarities larities and causal relations exhibit strong modal constraints on what could or
among those basic patterns of localized arrangement of properties and external could not happen. For a law to hold in our world for Lewis simply means that a
space-time relations that physical science will in its progress discover as being certain pattern can be found in the arrangement of natural properties that pertains
basic and fundamental to our world. This does not imply that physical science has to our world. And the fact that a certain pattern is present and no other cannot be
already reached this objective of a completed theory. What it has discovered up to further explained. It could perhaps be explained if Lewis's notion of natural prop
now is subject to future corrections and expansion.6 Yet, for Lewis there is hope erties allowed for primitive dispositions or primitive propensities or chances which
that the whole truth about how our world is constituted is physical truth and can be impose some kind of necessity on a pattern to be present. Lewis's notion of prop
embedded within a completed theory in more or less the same way as present-day erties, however, excludes such an un-Humean way out. Every perfectly natural
physics suggests. property is intrinsic for him, which is to say that the presence of any natural prop
What we have said so far seems to imply that physicalism and Humean erty at a particular place does not imply any other natural property to be present at
supervenience are merely two characteristics of a single philosophical thesis. any other place. Nor does the presence of any natural property, when we think of
Although Lewis upholds not only physicalism but also Humean supervenience,
4 Gerhard Preyer and Frank Siebelt Reality and Humean Supervenience 5
probabilistic dispositions like propensities or chances, imply any tendency for proposal to reduce possible worlds to something else."11 Modal realists accept
some other natural property to be present at any other place. ontological commitment when quantifying over them in modal semantics. Possible
If natural properties also included primitive dispositions, propensities, or worlds, although merely "possible" from the standpoint of our world, so they
chances, natural laws would be metaphysically necessary .10 Humeanism by con argue, are nevertheless as real as sticks and stones and so are their inhabitants,
trast claims that all natural laws and causal relations do not restrict genuine possi possibilia, so to say. In fact, those entities should not be taken as fictions, they are
bilities. Therefore, any form of Humeanism must strongly oppose any notion of exactly what we talk about when we say that something is necessarily or merely
natural properties that allows propensities, dispositions, or chances to be ontologi possibly true.
cally primitive. Whatever basic pattern of natural properties characterizes a space Besides this ontological dispute, there is a much more important question to
time region of our world, this pattern does not necessitate (or necessitate to a which modal realism can deliver an answer. In Lewis's view modal realism can
certain degree if chances are involved) anything that happens in another region. give us the much needed ontological framework in which we can formulate a
Nor does it prove any happening in any other compartment impossible, even if unified theory for the concept of logical space. Within Lewis's theory of modal
causal and lawful relations are involved. Humean supervenience takes everything realism the structure of logical space can be characterized in terms of space-time
that happens to be the case in our world to happen contingently so. It is this aspect or space-time analogous and mereological relations only. To begin with, Lewis's
of Lewis's physicalism that makes it Hu mean through and through. theory provides us with an indexical analysis of actuality .12 On his view to say that
something is actual attributes no special ontological status to it. To be actual just
means to be spatiotemporally related to us. Being spatiotemporally related to us is
a necessary and sufficient condition for being actual, but that is all there is to
Modal Realism, Laws, and Causation
actuality. Second, Lewis gives us an analysis of how to explain the intuitive appeal
of the idea that our world is in some sense unique. Uniqueness can mean mereo
Humean supervenience is one unifying element in Lewis's physicalist approach,
logical maximality. Our world could be characterized mereologically as the maxi
the other is his modal realism. Modal realism should primarily be seen as a spe
mal sum or fusion that is arrived at by starting with us and adding all those entities
cific answer to the problem of how to understand our common idioms of possibil
that are spatiotemporally related to us.
ity and necessity. While many philosophers regard quantifying over possible
Lewis's indexical analysis is meant to be exhaustive in the sense that every
worlds within the classical analysis of the sentential modalities "It is possible that"
entity we regard as actual should be captured by this approach, for being spatio
and "It is necessary that" to be noncommital to these entities, it is not so with
temporally connected is an equivalence relation. This gives us the worldmate
Lewis. When we provide Leibniz-style truth-conditions for "It is necessary that p"
relation for our world defined completely in terms of space-time relatedness. "If
by saying, roughly, that pis true in all possible worlds, and require at least some
two things are spatiotemporally related, then they are worldmates. The converse is
worlds in which p is true for the truth of "It is possible that p," should we take
much more problematic. Yet that is more or less the doctrine I propose. Putting the
apparent quantification over possible worlds at face value, just as we quantify over
two halves together: things are worldmates iffthey are spatiotemporally related. A
sticks and stones? That is, are possible worlds and possibilia real, do they exist, as
world is unified, then, by the spatiotemporal interrelation of its parts."13 We could
we think of sticks and stones in our actual world?
say therefore xis a worldmate of y iff xis spatiotemporally connected toy, which
Modal anti-realists argue vehemently for a negative answer. Possible worlds
implies via his maximality assumption that x and y must both be parts of the same
are useful fictions for them and by being mere fictions they should not find a place
world. (Of course we are not spatiotemporally related to our world as a whole,
alongside sticks and stones in our conception of what to take as real entities wor
only to proper parts of it.)
thy of quantification. Instead they understand quantifying over possible worlds to
If we accept Lewis's thesis of a plurality of worlds, then this analysis will lead
be simply a semanticist's instrument that can be successfully employed in spelling
us to a startlingly simple noncircular identification principle of what it is to be
out truth-conditions within modal semantics. Asked about the status of those
merely a logical possibility in logical space from our this-worldly point of view:
instruments themselves they provide ersatz concepts like maximal consistent sets
everything x that is not our worldmate is merely possible relative to our world.
of sentences or propositions, concepts that are less problematic ontologically but
Following these steps, what characterizes entities like the winged horse or
rich enough formally to spell out full-blooded truth theoretical semantics. Modal
Siegfried's dragon as being mere possibilia is their not being spatiotemporally
realists, on the other hand, are sceptics regarding those reductionist attempts to
related to any part of our world. Lewis holds that possible worlds don't differ in
explain worlds away. "I take them to be respectable entities in their own right.
kind from our actual world. We are asked to think of them as individuated in
When I profess realism about possible worlds, I mean to be taken literally. Possi
logical space by being maximally unified in more or less the same spatiotemporal
ble worlds are what they are and not some other thing. If asked what sort of thing
and mereological way as Lewis thinks our actual world is. (Or in a space-time
they are, I cannot give the kind of reply my questioner probably expects: that is, a
6 Gerhard Preyer and Frank Siebelt Reality and Humean Supervenience 7
analogous way, if we are not willing to eliminate real possibilities by taking over recombination gives us worlds in which duplicates of world-bound individuals
too much physical structure of our space-time to be constant over all logical coexist together as worldmates.
space.) In this respect Lewis's modal realism is a rich ontology only quantitatively, Given a more or less unrestricted principle of recombination for logical space,
insofar as it postulates a large number-measured at least by the infinite cardinal it follows that laws of nature cannot be strictly necessary. Otherwise many
:1 -the infinite cardinal of the set of all subsets of the real numbers-of possible recombinations have to be ruled out a priori. And given Lewis's supervenience
2
worlds. Qualitatively, it doesn't posit any entities to be individuated via new kinds. assumptions, natural laws must also supervene on the arrangement of localized
"My realism about possible worlds is merely quantitatively, not qualitatively, natural properties. Usually truths of law are understood to be truths that lie at the
unparsimonious. You believe in our actual world already. I ask you to believe in heart of our modal conception of dispositions and counterfactuals, causation, and
more things of that kind, not in things of some new kind."14 capacities. As to the analysis of laws, Lewis favors a version of the systematic
Continuing from these initial steps we arrive at an identification principle for regularity approach developed by J. S. Mill and F. P. Ramsey, in which lawmakers
possibilities that is applies to all of logical space, that is, it applies to all those are contingent regularities fulfilling additional constraints of integration in deduc
maximally spatiotemporally unified regions in them (that is, the worlds): for every tively closed systems of truths pertaining to our world. For Lewis, a law of nature
possible world w and every part x of w, every entity y that is not spatiotemporally holding at our world is a true regularity that appears either as a theorem or as an
related to xis merely possible relative tow. As an important corollary in his the axiom in each of those deductively closed systems that comprise all truths in our
ory, possible worlds are completely isolated from each other. Worlds neither world while achieving the best balance of simplicity in axiomatization and strength
overlap in logical space by sharing identical parts nor will inhabitants of one world in information content.16
be externally cross-worldly related to inhabitants of another. There is no space However, being a law is also a modal notion, laws sustain counterfactual and
time and causal route leading either way from one world to any other. Worlds causal conclusions while contingent regularities do not. But iflawhood is a contin
might be internally related though, they could share all or some of their intrinsic gent property how is the special modality of laws derived, if being a law doesn't
and extrinsic properties.15 Worlds could be duplicates of one another, there could impose a priori constraints on recombination within logical space? Lewis's answer
be similarities among them to every degree. But for worlds to be internally related is that differences and similarities of law between worlds are important aspects
in that way there need not be a space-time or causal route between them. In fact no that contribute to a large extent to overall similarity and difference. "Since a differ
such route could exist in Lewis's theory, for if it could, at least one part of one ence in laws would be a big difference between worlds, we can expect that worlds
would have to be spatiotemporally related to at least some part of another, which with the same laws as a world i will tend to be closer to i than worlds at which the
would contradict the initial assumption that we speak of different worlds. laws of i hold only as accidental generalizations, or are violated, or-worse
Generalizing a little, if there were such cross-worldly relations other than still-are replaced by contrary laws .... the laws of i will hold throughout many
internal ones within logical space, these would impose severe restrictions as to of the spheres around i, and thus will tend to be cotenable with counterfactual
what is and what is not a possibility in logical space. To limit possibilities then suppositions."17 It is because laws have their status only as being integrated in a
means to limit the worlds that exist in logical space. Stating the point differently, whole cluster of generalizations that they contribute holistically in a dominant way
to cut down logical possibilities is to increase the logical necessities to be pre to the overall similarity of worlds. It is from this aspect that the special modality of
served throughout logical space, which in fact, in Lewis's theory, comes down to laws stem. In contrast, isolated accidental generalizations cannot impose similarity
minimizing the number of worlds there are in logical space. Lewis, as a Humean, constraints between worlds to such an extent that counterfactual conclusions can
strongly denies that there can be necessary connections between distinct reasonably be backed, and this is why accidental generalizations lack what we
existences, and worlds are prime examples of distinct existences. Instead he opts regard as a specific modal force of laws.
for an unrestricted Humean principle of recombination, which allows all distinct As to the notion of causality, Lewis understands it, in contrast to Hume, more
existences to be recombined in every gerrymandered way, so that anything can or less as a relation between singular events. Here again Lewis's modal realism
coexist with anything else, provided all entities occupy different space-time posi plays an important theoretical role, for he analyses causal relatedness in terms of
tions. Of course this can't be taken literally, as it would be in conflict with Lewis's a special relation of counterfactual dependence, and not as most Humeans would
denial of any genuine overlap of worlds. If Siegfried's dragon and the winged like to have it in terms of de facto regularities. Hume himself already had some
horse are not worldmates in one world, they can't exist together in Siegfried's or thing similar in mind when he claimed that if the cause had not occurred, the effect
in the winged horses or in any third world. Roughly, this means that every collec would never had existed. This Humean idea is where Lewis's counterfactual
tion of distinct existences will have duplicates or counterparts all existing together analysis starts.18 Event e* depended causally on event e in the actual world means
in some world, that is, there is a world having all those as parts. Thus understood roughly for Lewis that event e* was counterfactually dependent on event e. Had
the captain of the Exxon Valdez not consumed as much alcohol as he did, the