(Partial support for this work was provided by the National Science
Foundation under Grants No. SES-8606472 and No. SES-8705469. I thank

especially Don Howard, Ernan McMullin, and Abner Shimony for comments
on an earlier draft of this essay.)

The purpose of the papers in this volume is to discuss the hard questions
and hard choices that recent quantum physics has

presented for philosophy in general, not just for the philosophy of
science. The authors examine what has been established, what

options are still available, and what revisions, radical or otherwise,
may be necessary in our philosophical views. Since the volume

is intended for philosophers in general, and not just for experts in
the foundational problems in quantum theory, the papers are not

thick with technical details. A central development to which all of
these papers are in some way related is Bell's theorem. There is

an enormous literature on the technical aspects of Bell's theorem and
on the foundational problems of quantum mechanics (see,

for example, Ballentine 1987). My task here is to provide some introductory
material that will help the reader new to this subject

to understand the subsequent papers. Let me be explicit in stating
that the tale which follows is not always chronologically faithful

to the historical record nor is it in all details a literal transcription
from the original papers cited. By way of orientation, I begin

with a general and somewhat loose overview of the subject and then
proceed to define terms and concepts more precisely.

*1. A little history*

While it is true that interest in the interpretative problems of quantum
mechanics received major impetus from the

seminal paper of John Bell (1964) on the Einstein-Podolsky-Rosen (EPR)
paradox, there was life in the field before Bell-and before

EPR too (see, for example, Wheeler and Zurek 1983). As early as 1913,
before Bohr's paper of that year on the semiclassical model

of the hydrogen atom had appeared in print, Rutherford pointed out
a problem for causality in Bohr's model. Bohr had postulated

that the frequency v of light emitted by an electron in its transition
from an initial energy level E,n to a final level E,, (figure 1) is

given by

To Rutherford, it appeared as though the electron would have to know
to what energy level it was going before it could

decide what frequency it should emit (Hoyer 1981, 112). By 1917, Einstein
wanted to know how, in Bohr's model, the photon

decided in what direction it should move off (figure 2). Schr6dinger
attempted a largely classical interpretation of his own

equation, but Max Born (1926) proposed a consistent statistical interpretation
of quantum mechanics. Determinism, in the sense

of our being able to predict the unique outcome of a measurement on
an event-by-event basis, was gone from the formalism,

although Einstein and Schr6dinger struggled (Przibram 1967) against
what became codified as the "Copenhagen" interpretation of

quantum mechanics. True, the majority of physicists (if they chose
to think about the issue at all) believed that atomic events

could not, even in principle, be predicted on an event-by-event basis.
Still, one could (and some notables did) question the

completeness of quantum mechanics, asking whether there might not exist
a successor theory which could, in principle, make such

event-byevent predictions. In fact, von Neumann ([1932] 1955, 313-328)
offered a "proof" that such "hiddenvariables" theories

could not exist. Much later, Bell (1966) did address the question of
the relevance of that "proof."

In 1935, Einstein, Podolsky and Rosen (EPR) published a paper in which
they questioned the completeness of quantum

mechanics. That is, they asked whether one could be certain, on physical
grounds, that more could be specified (or known) about a

system than could be predicted with certainty by the formalism of quantum
mechanics. By means of a specific thought experiment,

they argued that the incompleteness of quantum mechanics was entailed
by the formalism of quantum mechanics itself, along with

entirely plausible assumptions excluding action at a distance ( "locality")
and about the reality (or definiteness) of a physical

quantity independent of our choice to observe it. Their argument has
been lucidly discussed by Shimony (1978). We do not

consider the original EPR thought experiment here. For pedagogical
purposes, there is a simpler one due to Bohm (1951). The EPR

paper did not offer any alternative theory to quantum mechanics, nor
did it mention hidden variables. Nevertheless, the additional

parameters that would be necessary to give a complete specification
of the state of a system have subsequently come to be referred

to as "hidden variables" and any theory encompassing such parameters
as a "hidden-variables theory."

….

*2. Bell's theorem*

Prior to Bell's 1964 paper, the question of whether or not there could
exist a deterministic hiddenvariables theory with no

instantaneous action at a distance seemed incapable of resolution.
Of course, no one had succeeded in writing down an empirically

adequate example of one. But, that did not prove that one could not
exist. After all, if a student fails to solve a difficult homework

problem, the reason could be that he or she lacks the wit to do it
or, indeed, it could be a problem with no solution. In the absence

of a successful deterministic, local, hidden-variables theory, discussion
of the possibility of such a theory could appear to be little

more than idle argument appropriate only for a free Saturday afternoon
or for cocktail parties. Bell's paper changed that in a

dramatic fashion. The strength of a theorem is inversely proportional
to the strength of the assumptions it makes. That is, if you

assume a lot and prove a little, no one is particularly impressed.
But if you (apparently) assume practically nothing and obtain a

remarkable result, that is impressive.

In effect, Bell (1964) argued that determinate (i.e., predetermined
prior to the measurement) projections for the spins of

the electrons and locality are incompatible with the (spin) correlations
predicted by quantum mechanics.

….

In fact, the actual experiment (in a real laboratory with real equipment)
is much more difficult to do than my rather glib

characterization in figure 3 might lead one to expect. A detailed discussion
of the experimental situation can be found in the

comprehensive review article by Clauser and Shimony (1978) and in Redhead
(1987b). There also exists a general, less technical

review by Shimony (1988). Such experiments have been earned out, some
of the latest and most convincing being those by

Aspect, Grangier, Dalibard, and Roger (1981, 1982) in Paris. The empirical
results are representable, well within the limits of

experimental error, by the simple distributions.

….

The logical skeleton of the argument is that the assumptions of locality
and determinism, plus the actual experimentally observed

distributions of the real world, have produced the contradiction of
Eq. (7). Although one can, in principle, attempt to undermine

the empirical leg of the triad upon which this argument rests (cf.
Clauser and Shimony 1978), each successive experiment

forecloses more such possible loopholes and makes such a line of attack
ever less plausible. So, the arrow of modus tollens appears

more reasonably directed at the assumptions of locality and/or determinism.
We have purposely not gone into the details of the

argument by which Bell passed from Eq. (3) to the (Bell) inequality
of Eq. (4) because we want to focus on the logical structure of

the argument. In the appendix to this essay, the reader can find a
simple proof of a contradiction like Eq. (7). So, Bell's remarkable

result, or theorem, is that no deterministic, local hiddenvariables
theory can account for the empirical result of the experiment. It

is worth emphasizing that these types of correlations are a pervasive
feature of the quantum world. They are not peculiar to the

Bohm-EPR class of experiments alone. However, the Bohm-EPR configuration
is in a sense the "simplest" one yet known which

exhibits these "mysterious" quantum correlations.

….

Let me stress two points here. First, Bell never wrote down a single
local, deterministic theory. Rather, he proved, without ever

having to consider any dynamical details, that no such theory can in
principle exist. The entire class was killed at a stroke-a classic

"no-go" theorem. Second, Bell's theorem really depends in no way upon
quantum mechanics. It refutes a whole category of

(essentially) classical theories without ever mentioning quantum mechanics.
And it turns out that the experimental results not

only refute the class of local, deterministic theories but also agree
with the predictions of quantum mechanics. (That is, a

straightforward application of the rules of quantum mechanics does
lead to the results of Eqs. [5].) Abner Shimony (1984b, 35) has

appropriately given the name "experimental metaphysics" to this type
of definitive empirical resolution of what appears to be a

metaphysical question.**
**

*3. Some distinctions*

In my presentation thus far, I have been rather cavalier in oversimplifying
the issues and in conflating terms that must be carefully

distinguished. So I now turn to the purpose of the subsequent papers
in this volume and to some of the work that the authors have

done in recent years. Today, when one looks back at Bell's original
paper and at some of the early responses to it, one is struck by

at least two facts. First, the paper contains a modicum of mathematical
formalism. Depending upon one's level of mathematical

sophistication, the proof may not be immediately transparent and one
can wonder whether something has gone awry in those pages

and symbols. After all, the result is so remarkable: it forces us to
face indeterminism and/or nonlocality in principle. Could the

proof be flawed? As often happens with great discoveries, proofs are
subsequently fashioned which make the important result seem

almost self-evident. Bell's theorem was no exception. Eventually, there
were picture proofs and nonmathematical discussions

(d'Espagnat 1979; Mermin 1981a, 1985) of Bell's result and of the quantum-mechanical
riddles it makes us face. While such

discussions are nontechnical, they can remain rather long and involved.
The reader's eyes may glaze over before the end. However,

if one is willing to pay the price of a little algebra-really, only
about six lines of arithmetic-one can immediately go from Bell-type

premises and a requirement of empirical adequacy to a contradiction
like 1 > 2 (Stapp 1971, 1979; Redhead 1987a). The

mathematics is so simple and brief you are certain no error has been
made. You think you understand it all! (The details of such a

proof are given in an appendix to this paper.)

So then, first, the formalities or manipulations
in the proofs were greatly simplified. But then, the second, and in many

ways more difficult, phase began-unpacking the assumptions and the
meanings of the terms used in these proofs and coming to

some understanding of just what the implications are. This is a job
that philosophers are particularly well equipped to do. The

terms `reality', `determinism', and `causality' cannot be used interchangeably
and one must be especially careful to distinguish

between locality and reparability. Perhaps a few sketchy definitions
will help for a start:

Arthur Fine (1984a, 1984b) and Don Howard (1985, 1987) have provided a useful perspective for several of these issues by theirreality- existence of an objective, observer-independent world (often closely related to determinate values)determinism- sufficient information at to allows prediction of a specific result at a later time tcausality- a specific preceding event (or "cause") for every effect - a concept familiar from prequantum, classical theorieslocality- no influence transmitted faster than lightseparability- spatially separated systems always have independently definable properties and existence (and these

properties exhaust the description of any system made up of these subsystems).

careful and enlightening historical reconstructions of Einstein's views on locality and reparability, bringing out essential differences

here between Einstein and Bohr. Henry Folse (1985), Don Howard (1986), and Dugald Murdoch (1987) have done similar work in

reconstructing Bohr's philosophy of science. Furthermore, as we indicated previously in Eq. (3), a crucial mathematical step in the

usual proof of Bell's theorem is the factorization of a certain expression for joint probabilities. A long debate has arisen as to the

physical warrant for this step. This factorizability (or "Bell" locality or statistical independence) is not implied by the first signal

principle of relativity ( "Einstein" locality). Michael Redhead (1983) and Linda Wessels (1985) have analyzed in detail the assump

loin the present context, the term `determinism' is usually predicated of a theory, as in a deterministic theory. In quantum field

theory, `causality' is used in a sense rather different from (but related to) the classical cause-effect one. (See Gushing, 1986, for a

fuller discussion of the meaning of the term `causality' in modem theoretical physics.) The reader should be warned that the terms

`locality' and `reparability' are the most problematic as far as universally-agreed-upon definitions are concerned. The ones I give

here alert the reader to a distinction between these terms. However, each author below must be checked carefully for his or her

own precise use of these terms. It is also true historically that the evolution of an explicit distinction between those two terms was

a long time in coming. (See Howard [1985] and Folse [this volume] for careful discussions of this issue.) Furthermore, we

distinguish among different types of locality and nonlocality. Finally, d'Espagnat (1984) treats the issues of reality and of

separability carefully and at great length.

....

Another insightful observation about the meaning of the Bell inequality was made by Fine (1982b). He argued that Bell

inequalities of the type in Eq. (4) above are the necessary and sufficient conditions for the existence of a deterministic

hidden-variables model which will produce the joint distributions for the Bohm (EPR) experiment of figure 3. But the existence of

such a complete set of state variables A is equivalent to a common-cause explanation (in the common past of the parts of the

system to be observed) for these distributions or experimental outcomes. Knowing that there is such an empirically applicable test

for the possibility of a common-cause explanation will prove important for the discussions which follow in subsequent papers in

this volume.

*4. Philosophical implications*

We can now ask just what the implications of all of this are for our
view of the physical world. Thus far we have pointed out

certain restrictions on allowable world views (or representations of
reality) that are demanded by quantitative relations (the Bell

inequalities) containing only empirically measurable distributions
of experimental results. In a sense, the tone has been negative

since we have stressed what type of theories or explanations are not
possible. Must we, for example, abandon belief in an

observer-independent reality? Or, as David Mermin has put it, "Is the
moon there when nobody looks?" We have shown what

cannot work rather than exploring some theory or explanatory framework
that is successful in reproducing the results of experiment.

Of course, we do have an empirically adequate theoretical
framework within which to organize the observational datanamely,

quantum mechanics. However, this enormously empirically successful
theory has difficult interpretative problems associated with it.

Henry Stapp (1979, 14) makes a point similar to Mermin's when he characterizes
our immediate reaction to a literal acceptance of

some of the more extreme interpretations of quantum mechanics:

One objection to this view is that it seems excessively anthropocentric, at least if consciousness is reserved for human beingsThat is, our most successful theory of processes at the microlevel, namely quantum mechanics, poses serious problems for scientific

and higher creatures. Before the appearance of such creatures the world would be synthesizing endless superposed

possibilities, with nothing actual or real, waiting for the first conscious creature to occur among the possibilities. Then a

gigantic collapse would occur. Similarly, the Martian landscape would be nothing but superimposed possibilities until

Mariner landed and some observer in Houston viewed his TV screen. Then suddenly the rocks and boulders would all snap

into their observed places. This view seems to assign a role to such observers that is out of proportion to their place in the

world they create.

realism (which requires roughly and at a minimum that our scientific theories are to be taken as giving us literally true descriptions

of the world).

Bas van Fraassen (1982a; this volume) has argued that the experimental violation of the Bell inequality tells against

scientific realism. That is, if scientific realism does not work at the microlevel, then it cannot be generally valid. In a provocative

article, Asher Peres (1985) has posed yet another quantum paradox "as a challenge to those physicists who claim that they are

realists" (p. 201). His conclusion at the end of that article (p. 205) is that "Any attempt to inject realism in physical theory is bound

to lead to inconsistencies. "(At the 1986 Quantum Measurement Theory Conference (Greenberger 1986) in New York City, I

mentioned to Peres that his position appeared to be an instrumentalist one. He replied with no apparent discomfort that others had

told him that before. For a physicist's statement on an instrumentalist interpretation of quantum mechanics, see Peres (1988).)

At the other end of the spectrum from van Fraassen
(1980) or Peres on views of scientific realism, we find Ernan McMullin

(1984) who points to the great success structural theories have enjoyed
in several sciences (such as chemistry, astrophysics, geology,

and genetics) in taking a starkly realistic view of the entities contained
in those theories. It is in regard to the interpretation of the

ontologies underlying mechanical theories (whether classical or quantum)
that problems most often arise (McMullin 1989).

McMullin recommends treating these theories as a special class and
considers as inappropriate the demand for a realistic

interpretation of force or field that would be unproblematic for molecule
or gene or galaxy. He is willing to put aside for the present

certain difficult questions of a realistic interpretation for mechanics
(that is, classical mechanics, quantum mechanics, quantum field

theory-all of what would seem to many to be the foundations of physics):
"Because of its many special features, mechanics is quite

unsuitable as a paradigm of science generally, though philosophers
are wont to overlook this" (1984, 10). Rather than being the

paradigm of natural science, much of physics becomes, at least in the
context of this issue, an anomaly. It appears that McMullin

restricts consideration to cases that satisfy, in some broad sense,
Newton's Rule III of Reasoning in Philosophy in Book III of the

Principia (Newton [1726] 1934):14 "The qualities of bodies, which admit
neither intensification nor remission of degrees, and which

are found to belong to all bodies within the reach of our experiments,
are to be esteemed the universal qualities of all bodies

whatsoever. " This rule is often taken as saying that we may extrapolate
general features of the macroworld to the microworld. To

some, McMullin's circumscription of mechanics may be too costly a move
to make on behalf of scientific realism. Somewhere

between van Fraassen and McMullin, we find Heisenberg (1958, 185) with
his suggestion that we must admit a new class of physical

entity into our theories: potentia (Shimony 1978, 1986; Stapp 1979,
1985a). Bell (1984) has made a similar suggestion in speaking

of the "beables" of quantum field theory.

One of the most interesting philosophical questions,
perhaps, concerns the relations among empirical adequacy,

explanation, and understanding for quantum phenomena. Are explanation
and understanding really possible when a detailed causal

explanation is in principle impossible? In Bas van Fraassen's (1985)
terms, are the EPR correlations a mystery? Paul Teller has

suggested that relational properties of physical objects may not simply
supervene wholly upon nonrelational properties of localizable

individuals, but that a type of "relational holism" is essential in
which the objects have inherent relations among themselves. He

claims that this is "a holism we can understand" (Teller 1986, 73).
But is it? A central issue is whether or not we can truly

understand such descriptions of our world. These problems are forced
upon us, of course, when we take the present formulation of

quantum mechanics as exactly correct, needing no modification.

In the same vein, we can even ask whether all the
desiderata, which we may want in a theory that accords with the

phenomena of the real world, can be mutually compatible. Peres and
Zurek (1982) set up a triad (figure 5) involving the three

"wishes" of determinism, verifiability, and universality and argue
that no theory can, even in principle, satisfy simultaneously all

three demands. (By "verifiability" here they mean the freedom of choice
of an observer or experimenter to fix a given setting on, or

orientation of, a measuring device to test the predictions of a theory.)
We can have at best just any two of the three. In the end,

quantum mechanics may be the best theory it is possible to have.

We might question the value of discussing the implications
that essentially nonrelativistic quantum mechanics (say, the

usual Schrodinger equation) has for such issues, since the more complete
(and problematic) theory in use today is relativistic

quantum field theory. I have argued elsewhere in some detail (Gushing
1988) that the root of quantum paradoxes is the superposition

principle and that remains in any quantum theory. Michael Redhead and
PaulTeller (cf. Brown and Harre 1988) do believe that

quantum field theory introduces new philosophical problems which must
now be faced. These would, of course, be in addition to,

and not solutions of, the interpretative difficulties already presented
here.

This introductory essay may at least establish a prima facie case for
the relevance of quantum mechanics to general philosophical

issues related to epistemology and ontology. There are serious problems
here, not simply questions of mathematical formalism.

Hence, the rationale for this volume. Several years ago John Bell (1975,
98) made an observation about the understanding of our

world which quantum theory gives us:

The continuing dispute about quantum measurement theory is not between people who disagree on the results of simpleIn our search for a new understanding, we face the challenge characterized by Costa De Beauregard (1983, 515-516) in this way:

mathematical manipulations. Nor is it between people with different ideas about the actual practicality of measuring

arbitrarily complicated observables. It is between people who view with different degrees of concern or complacency the

following fact: so long as the wave packet reduction is an essential component, and so long as we do not know exactly when

and how it takes over from the Schrodinger equation, we do not have an exact and unambiguous formulation of our most

fundamental physical theory.

Hard paradoxes . . . are resolved only by producing a new and adequate paradigm, in Kuhn's words. In physics, this impliesThe papers in this volume are attempts to fashion an explanatory discourse with a view to producing an understandable

the production of a new mathematical recipe (e.g., Copernicus's heliocentrism, or Newton's inversesquare law) and tailoring

an explanatory discourse exactly fitting the mathematics (e.g., Einstein's interpretation of the Lorentz-Poincare formulas; or

still better, Minkowski's).

This sort of "explanation" is usually felt (and often for a long time) as itself paradoxical. Newton's action at a distance,

Einstein's "reciprocal" interpretation of the Lorentz contraction, have very often been deemed "hardly explanations at all."

What occurs in the "paradox and paradigm" peripateia (or, in Kuhn's words, in a "scientific revolution") is a victory of

formalism over modelism. In the EPR case we do have, since many years, the formalism. We are at home with it for

performing calculations, but not yet for viewing our world, and our relation to it.

view of our world. The ultimate goal is to construct a framework that is empirically adequate, that explains the outcomes of our

observations, and that finally produces in us a sense of understanding how the world can be the way it is. These are three linked but

distinct goals. It remains an open question whether all of these are simultaneously attainable.