Chapter 1
Supervenience and Identity
1.1
Introduction
Like many other contemporary philosophers,
I have strong physicalist intuitions. I
am inclined to think that chemical phenomena, for example, are all at bottom physical,
even though chemists do not describe those phenomena in physical terms. What is more, I am inclined to think the same
about the phenomena studied by meteorology, biology, psychology, sociology and
the other so-called "special sciences".
My
aim in this initial chapter is to see how far such physicalist intuitions can
be supported by serious arguments. This
question is not as much discussed in the contemporary philosophical literature
as it might be. Of course many
philosophers with physicalist inclinations have formulated different possible
versions of physicalism, and explored the relations between them. And many other philosophers, with opposed
inclinations, have elaborated various non-physicalist views of psychology,
biology, sociology, and other special phenomena. But for the most part neither party has
paused to argue its case against the other.
The friends of physicalism tend simply to start with their physicalist
intuitions, and try to develop a theory which fits them. Their opponents dismiss those intuitions out
of hand as symptoms of an overblown admiration for science.
Not all philosophers treat physicalism as beyond debate in this
way. An increasing number of
contemporary thinkers are coming to recognize that there are plenty of
pertinent arguments that bear on the issue.1
Dogmatic physicalists and anti-physicalists alike will do well to attend
to these arguments. Anti-physicalists
will discover that physicalism is supported by premises which are difficult to
deny, even if you have little regard for science. And physicalists will find out why some
versions of physicalism are defensible, while others are not.
1.2
Supervenience
Let me start by trying to be a bit more
precise about what I mean by physicalism.
One simple way of formulating physicalism would be to require that all
special properties, like chemical, or biological, of psychological properties,
should be identified as types with physical properities, in the way that the
property of being hydrogen, say, can be identified with the physical property
of having atoms with one proton and one electron. But while such "type identities"
may be available within basic chemistry, they seem unlikely to characterize the
other special sciences. In particular,
it seems unlikely that psychological properties, such as being worried about
the future, for example, can be identified with any specific physical
properties, along the lines of having a certain arrangement of molecules in
your head. It is surely implausible to
suppose that all the different people who have ever been worried about the
future must have some intra-cranial molecular property in common. And, if that is not implausible enough, what
about the future brain-injured people who will have their damaged parts replaced
by miracles of silicon-based micro-technology, or the hominid but silicon-based
denizens of Proxima Centauri's third planet?
Presumably they will be able to worry about the future too. But they can't possibly share molecular
arrangements with the rest of us, given that we don't have any silicon in our
brains.
Fortunately for physicalism, type identity is not the only way in which
special properties can be viewed as essentially physical. An alternative way of formulating physicalism
is in terms of the supervenience of the special on the physical. Supervenience on the physical means that two
systems cannot differ chemically, or biologically, or psychologically, or
whatever, without differing physically;
or, to put it the other way round, if two systems are physically
identical, then they must also be chemically identical, biologically identical,
psychologically identical, and so on.
The advantage of formulating physicalism in terms of supervenience is
that, unlike type identity, this doesn't require that the same physical
property must determine a given special property whenever it is instanced. My worrying about the future might involve
one molecular arrangement, an arrangement such that that anybody who has it
will be worrying about the future; your worrying about the future might be
ensured by a different physical arrangement, but again one that suffices to
determine that all its possessors are worrying about the future; future brain-damaged patients and Proxima
Centaurians will have yet different such physical arrangements; and so on.
How satisfactory an explication of physicalism is the requirement of
supervenience on the physical? I shall
consider first whether supervenience is necessary for physicalism, second
whether it is sufficient.
On
the face of it, supervenience seems an obvious necessary condition for
physicalism in any given area: if two
chemical systems, say, can differ, even though they are physically identical,
then it would seem to follow that they must contain something non-physical.
However, an immediate qualification is needed. Suppose two chemical samples are physically
identical: they contain exactly the same
molecules and have exactly the same internal structure. Nevertheless one may be heavier than the
other, if one is one the earth and the other on the moon. So the heaviness of chemical systems does not
supervene on their physical characteristics.
Yet presumably we don't want on this account to regard physicalism as
refuted by the heaviness of chemical samples.
If anything supervenes on physical characteristics, surely heaviness
does.
The obvious response to this problem is to note that heaviness is a
relational property of chemical samples, depending not only on the intrinsic
features of the sample, but also on the features of another system, namely, the
surrounding gravitational field.
Accordingly, we should modify the requirement of supervenience, for
relational properties, so as to demand that such properties should supervene,
not on the internal physical characteristics of the system at issue, but rather
on those plus the physical characteristics of the relevant related system. If we do this, then the heaviness of chemical
samples is no longer a counter-example to physicalism: for the heaviness of a chemical sample
obviously does supervene on the internal physics of the sample plus the physics
of the surrounding gravitational field.
(Equivalently, if less naturally, we could say that the relational
properties of a system were not really properties of that system as such, but
only of some larger system incorporating the relevant related system, and then
require that such relational properties supervene on the physical properties of
the larger system.)2
Given this qualification about relational properties, I shall take it
henceforth that supervenience is a necessary condition for physicalism. But is supervenience sufficient for
physicalism? This is a rather more
tricky issue. In outline, we can see how
supervenience might suffice. Supervenience
says that, if two systems are physically identical, then they must also be
chemically identical, biologically identical, psychologically identical, and so
on. That is, the shared physical
features of these systems determine their special features. But how could this be so, if anything
non-physical were required for those special features?
Some care is needed, however, to make this line of thought
watertight. The issue depends on exactly
how we understand supervenience, and in particular on how strongly we read the
"determine" in "the shared physical features of these systems
determine their special features".
In due course we shall see that there is a weak reading of this
"determine" on which supervenience clearly does not suffice for
physicalism, and a stronger reading on which supervenience does provide a
satisfactory characterization of physicalism.
But let me not pause for these technicalities at this point. My primary interest in this chapter, as I
said, is not with the characterization of physicalism as such, but with the
possibility of arguments which support physicalist views. In line with this, it will make more sense
for me to fill in the details of what I mean by physicalism once we have see
what arguments are available, rather than before.
Perhaps it will be helpful to be graphic for a moment. The world recognized by physicalism is at
bottom a world consisting of physical facts, of particles and fields in motion
through space. At this basic level all
facts can be described by strictly physical terminology, like "mass",
"energy", and "position".
However, physicalism, as I am thinking of it, will also allow that we
often use non-physical terminology, like "sulphuric acid",
"thunderstorm", "elephant", and "thinking of the
future", to group and categorize large-scale arrangements of physical
facts. Moreover, physicalism allows that
such special terminology isn't just a shorthand for complex physical
properties: for, in those cases where
type identity fails, special categories cannot even in principle be specified
in physical terms. Nevertheless,
physicalists will say, the instances of any such special kind will still just
be complexes of physical stuff. For
supervenience, in an appropriately strong sense, implies that nothing more is
required for any special kind to be instanced than the physical facts should be
thus-and-so. After all, if anything more
were required, then presumably it would be possible for the special features of
two systems to differ even though they were physically identical, which is just
what supervenience rules out.
So
far we have been concerned only with what physicalism says. It remains to consider whether we should
believe it. In the rest of this chapter
I shall argue that physicalism is strongly supported by an important feature of
physical science, namely, the internal completeness of physics. However, before proceding, it will be helpful
briefly to consider a number of further preliminary points that may be worrying
some readers.
1.3
More Preliminaries
1.3.1
Some of you may feel uneasy about my brisk dismissal of the possibility
of type identities between physical categories and special categories. In particular, you may feel that if a special
subject matter is scientific enough to contain projectible laws, then it would
be surprising if its categories were not type identifiable with physical
categories. For why should we expect
special categories to conform to any stable regularities, if they are
determined by different physical structures on different occasions?
I
think this question points to a powerful, though not inescapable, argument for
type identity, and shall devote the next chapter to it. In this chapter, however, I shall focus on
the prior issue of whether we should accept physicalism, understood in terms of
supervenience. Once we have decided
this, we can then turn to the further issue of whether we should accept type
identity as well. My eventual conclusion
will be that type identity holds for some, but not all special sciences: more specifically it holds for those special
sciences that lack a teleological underpinning.
1.3.2
In this chapter, and in much of the rest of this book, I shall speak as
if our "common sense psychology", which attributes beliefs, desires
and other familiar states to people, is a "special science". But this is of course a contentious
assumption. Many philosophers view
everyday psychology as somehow incommensurable with science, as offering a
quite different kind of understanding from science. And other philosophers, while allowing that
folk psychology may have pretensions to science, hold that it fails miserably
to live up to them.
I
intend to by-pass this issue in most of the rest of this book, by stipulating
that, unless I say otherwise, my use of folk psychological talk is to be
understood as a place-holder for the true special science of psychology. So philosophers who think that folk psychology
is already a science can take my words at face value. On the other hand, those who think something
different is needed for a genuine cognitive science should simply understand my
psychological talk as referring indirectly to their own favoured cognitive
states. There remain the pessimists who
think that cognitive science of any kind is impossible, that there cannot be a
theory of our cognitive workings that stands to our physics and physiology as
meteorology, say, stands to the physics of the atmosphere. To these pessimists I simply concede that if
their bet about the future of cognitive science is right, then a number of the
issues I address in this book do not arise.
(Though in fact the issue of this chapter, the relationship between the
psychological and the physical3, arises not only for optimists who accept the
possibility of a high-level psychological science, but also for those
pessimists of a Wittgenstenian or Davidsonian bent who reject this possibility
but nevertheless uphold everyday psychology as a respectable but non-scientific
form of discourse. For they too need to
consider the relationship between psychological states and brain states. It is only pessimists who take the
eliminativist line and reject high-level psychological thinking of any kind who
can avoid addressing the mind-body problem.)
My
own view, for what it is worth, is that everyday psychology constitutes an
impressive theory from a scientific point of view, capable of improvement and
refinement, of course, and with a number of philosophically puzzling aspects,
but certainly containing a great deal of predictive information, and quite
probably giving some insight into the structure of our internal workings. I prefer to avoid, however, debates about
whether its undoubted imperfections merely mean it is a somewhat inaccurate
theory about real entities (like nineteenth-century atomic theory), or whether
they make it a false theory about imaginary entities (like the
eighteenth-century caloric theory of heat).
This issue would be hard enough to resolve if we knew the whole
psychological truth (though then it wouldn't matter very much). But, as it is, there are better things to
think about.4
1.3.3
Among the ways in which psychology is philosophically puzzling is that
it deals in propositional attitudes: its
explanations invoke beliefs, desires and other states which represent things as
being a certain way. (And we can expect
the states of any future cognitive science to be similarly representational.) In chapter 3 below I shall address the topic
of mental representation. At this stage
we need only note that representation complicates the issue of the
supervenience of the mental on the physical.
For, as a number of writers have observed5, there are many plausible
cases of two people having physically identical brains, and yet having
propositional attitudes with different representational contents. These examples imply that psychological
states individuated by representational content don't supervene on the physics
of the brain.
Physicalists about psychology have two options here. They can argue that any such
"broad" psychological state is really a kind of relational state, and
that therefore, in the way indicated earlier in this chapter, physicalism only
requires the supervenience of such states on the physical properties of some
larger system which includes the individual's brain as a part. Alternatively, they can argue that such broad
states are not really part of serious psychological theory, and therefore that
their non-supervenience is not a problem for physicalism about serious
psychology. In what follows I shall
defend the former line, in particular in sections 1.5 and 1.7 below.
1.4
The Completeness of Physics
Now for the arguments in favour of
physicalism. In what follows I shall
consider two different such arguments.
But both arguments will hinge on what I shall call "the
completeness of physics". So in
this section let me explain what I mean by this.
I
take it that physics, unlike the other special sciences, is complete, in the sense
that all physical events are either determined, or have their chances
determined, by prior physical events according to physical laws. In other words, we need never look beyond
the realm of the physical in order to identify a set of antecedents which fixes
the chances of any subsequent physical occurrence. A purely physical specification, plus
physical laws, will always suffice to tell us what is physically going to
happen, insofar as that can be foretold at all.
Note
that not all subject areas are complete in this way. For instance, meteorology is not
complete. Some weather phenomena arise
from antecedents which are not themselves weather phenomena. The beat of a butterfly's wing, students of
chaos tell us, can play a part in determining next week's cyclone. Less exotically, psychology is obviously not
complete, given that plenty of mental events result from non-mental ones, as
when I sit on a drawing pin and feel a pain.
But physics is special in this respect.
If we take any physical result, and look back in time to see what gave
rise to it, then, I say, prior physical factors will always suffice to give us
as full an explanation of that result as is possible.6
I
have stated the the completeness of physics baldly, as something to which all
will assent. But perhaps some readers
will have doubts. I can imagine two
possible sorts of worry. The first would
be a general worry that the completeness of physics is an empirical claim and
therefore inadmissible in a philosophical argument. I have nothing to say to this beyond the
points made earlier in the introduction to this book. The second worry would be more specific: even if empirical claims are admissible in
philosophy, is the completeness of physics really a well-supported empirical
claim? In particular, what exactly does
"physics" mean here? On some
perfectly natural ways of reading this term, the completeness of "physics"
seems false.7 However, let me postpone
discussion of this second worry to section 1.9 below. For the moment it will be helpful take the
completeness of physics at face value and see what would follow if it were
true. We will be better placed to
evaluate queries about it when we see how it matters to physicalism.8
1.5
The Manifestability Argument for Supervenience
Consider now the following argument for the
supervenience of psychology on physics.
Premise (1). According to the completeness of physics, the
chances of physical consequences are fixed, once physical antecedents are
given. So if two systems are physically
identical and in the same physical contexts, they will issue in the same
physical consequences or chances thereof.
Premise (2). Now add in the assumption, which I shall call
the "manifestability of the mental", that if two systems are mentally
different, then there must be some physical contexts in which this difference
will display itself in differential physical consequences, or at least in
differential chances for such consequences.
Conclusion.
It follows that mental differences without physical differences are
impossible. (1) tells us that physical
identity guarantees identity of physical consequences or chances thereof. And (2) tells us that mental difference
requires the possibility of different physical consequences or chances
thereof. So physical identity rules out
mental difference.9
The crucial idea here is that the completeness of physics leaves no room
for mental differences, or any other differences, to make a difference to
physical consequences, once physical antecendents are given. Physical categories by themselves always
suffice to fix the chances of physical consequences, without the help of mental
categories. So the only way for mental
differences to be manifestable is for them to have different physical bases.10
The two premises to this argument are the completeness of physics and
the manifestability of the mental. As I
said, I shall come back to the completeness of physics at the end of the
chapter. Here we need to consider the manifestability
of the mental. The most obvious argument
for this principle would be that mental differences must always be capable of
showing themselves in differential behaviour:
there certainly seems something initially odd about the idea of two
people who are mentally different, yet behave in the same way in all physical
contexts. (In this connection, note that
the manifestability principle is not the strong requirement that every
particular mental difference actually manifests itself in differential physical
consequences; just the weaker assumption
that, for any type of mental difference, there is some type of physical context
in which that difference would be physically manifested.)
If
this behavioural interpretation of the manifestability principle were
acceptable, then a strong version of the supervenience of the psychological on
the physical would follow, namely, the supervenience of psychological states on
brain states. For we could run the
argument as follows. Mental difference
require behavioural differences. But
behavioural difference are fixed specifically by prior brain states. So there can't be mental differences without
brain state differences.11
However, there are good reasons for denying that all psychological
states supervene on brain states I am
thinking here of the kind of "broad" propositional attitudes
mentioned in section 1.3.3. As we saw,
the distinguishing characteristic of broad attitudes is that individuals with
identical brains can fail to share them.
So it follows from the argument in the last paragraph that a
manifestability requirement in terms of behavioural displays is too strong a
requirement for broad attitudes. And
this is of course what we do find:
differences in in broad attitude don't automatically display themselves
in behavioural differences. To take a
familar example, consider Carl, who wants a glass of H2O, and Lrac, his
physically identical Twin Earth counterpart, who wants a glass of XYZ. They have different broad attitudes. But their behaviour, in the sense of the
physical movements of their bodies, will be the same in all physical contexts.
As
I observed in 1.3.3, the failure of broad attitudes to supervene on brain
states does not mean that physicalism is false.
For if broad states are relational states, then it will suffice for
physicalism that they supervene on the physics of the
individual-system-and-relevantly-related-systems, even if not on the physics of
the individual system alone. So it remains
possible that the general manifestability argument for supervenience might
still establish this weaker kind of supervenience for broad beliefs, even if
not supervenience on brain states. All
we need is a weaker manifestability premise to the effect that differences in
broad beliefs are somewhere manifested in physical consequences, even if they
are not manifested in behavioural consequences.
In
defence of this weaker version of the manifestability premise, note that a
mental difference which was not physically manifestable in any way would be
radically undetectable. We know that our
sense organs work by physical interaction with the environment, as do the
instruments and other aids by which we extend the power of our sense organs. So if two different mental states yielded
exactly the same physical manifestations in all contexts, then there would seem
no possibility of our ever finding out about their difference. Yet surely any real mental difference ought
to be somehow detectable, even if not behaviourally.
To
illustrate this point, note that even the broad mental difference between Carl
and his identical doppelganger Lrac will be distinguished by some differential
physical consequences. For this broad
mental difference depends on the relational difference that, where Carl is
surrounded by H2O, Lrac is surrounded by XYZ.
And this difference in their environments will obviously produce some
differential physical consequences by which we can distinguish the two cases.
I recognize
that this defence of the manifestability requirement, and hence of
supervenience, is less than fully principled.
For one thing, it leaves it open for opponents of physicalism to object
that it is possible that there be mental differences that are not in any way
detectable by human beings. More
pressingly, opponents of physicalism could also query whether our ability to
detect mental differences always depends on physical interaction with our
environments. Thus anti-physicalists
might argue that our access to conscious mental states in particular is
primarily via introspection, rather than via the normal five senses, and that
there is no immediate reason to suppose that the deliverances of introspection
are mediated by physical processes, however it may be with the other
senses. This would then open the way for
anti-physicalists to argue that conscious states might fail to supervene on
physical goings-on, and so that conscious differences need not manifest themselves
physically, and yet to hold that those differences could still be detectable,
via introspection: for example, they
could argue that you could be in just the same physical state at two different
times, and yet know introspectively that you were in pain at one time and not
at the other.
I
shall not attempt to plug this particular gap in the manifestability argument,
however. For there is a rather more
basic flaw in the argument, to which I shall now turn. To deal with this more basic flaw, we will
need to switch to a significantly different form of argument for
physicalism. Moreover, this alternative
form of argument will be immune to the anti-physicalist appeal to non-physical
introspective powers.
1.6
Manisfestabilty is Not Enough
To understand the more basic flaw in the
manifestability argument, recall how I earlier alluded to the possibility of
different ways of understanding supervenience, depending on how strongly we
read the "determine" in "physical features determine special
features" (or, equivalently, on how strongly we read the
"cannot" in "cannot differ in special features without differing
physically").
A
weak version of supervenience would understand these notions in terms of
natural necessity: that is, it would
take physical features to determine special features across all possibilities
where we hold the actual laws of nature fixed;
equivalently, it would say that any two systems governed by the actual
laws of nature cannot be different in any special respects without differing
physically.
A
strong version of supervenience would do it in terms of
"metaphysical" necessity rather than natural necessity: supervenience requires that physical nature
determines special nature across all possible worlds whatsoever; no two possible systems of any kind can be
different in some special respect without being physically different.
Now
only the stronger of these versions of supervenience constitutes a plausible
explication of physicalism. To see why,
we need only consider epiphenomenalism, the doctrine in the philosophy of mind
which holds that mental states "float above" the brain as distinct
conscious phenomena, not responsible for any physical effects themselves, but
nevertheless causally determined by the physics of the brain, and so incapable
of varying without physical variation.
Epiphenomenalism implies supervenience in the weak sense, since it
implies that, if we hold all natural laws constant (including in particular the
putative epiphenomenalist laws by which
by physical brain states cause conscious states) then physical nature
will determine mental nature: identical
physical brain states, plus the laws according to which physical brain states
cause conscious states, will ensure identical conscious states. But epiphenomenalism clearly isn't a
physicalist doctrine, since it explicitly specifies that conscious mental
properties are ontologically distinct from physical ones. So weak supervenience clearly does not
suffice for physicalism.
However, the same objection does not apply if we equate physicalism with
strong supervenience. For while
epiphenomenalism does imply that the mental is fixed by the physical across all
natural pssibilities, it does not imply that such brain-mind determination
holds across all possibilities, including those possible scenarios where these
epiphenomenalist brain-mind laws break down.
This is because epiphenomenalism insists that conscious properties are
ontologically distinct from physical ones.
So it implies that it is metaphysically possible, even if not
"naturally" possible, for a creature physically just like me, say, to
have different conscious states, or indeed to have no conscious states at
all. While such a creature would violate
epiphenomenalism's putative natural laws of mind-brain causation, and so fail
to be "naturally" possible, epiphenomenalism allows that these laws
themselves are not absolutely necessary, and so implies that such a creature is
metaphysically possible. Conversely, the
doctrine that such a creature is not
metaphysically possible would be inconsistent with epiphenomenalism's
distinction of conscious mind from physical brain, and so would constitute a
plausible explication of genuine physicalism.
After all, strong supervenience says that it is metaphysically quite
impossible for two beings to differ in some special property unless they differ
physically. But how could this be so
absolutely impossible, unless the special property was itself in some sense
itself physical? If the special property
weren't itself physical, then surely there would be metaphysical room, if not
natural room, for it to float free of the physical realm, in the way the
epiphenomenalist's conscious properties float free of physical properties in
worlds with different brain-mind causal laws.
So it looks as if strong supervenienceÑthat is, the denial of any
metaphysical room for special properties to float free of physical onesÑwill
indeed ensure that special properties are physical.12
To
return to the original issue of this section, the basic flaw in the
manifestability argument for physicalism is that it only constitues a good
argument for weak supervenience, not for strong supervenience, and so fails to
establish physicalism. The fault lies
with the manifestability premise, that is, the premise according to which mental
differences must manifest themselves in differential physical
consequences. For any version of this
premise strong enough to deliver genuine physicalism would blatantly beg the
question against non-physicalist views like epiphenomenalism.
Consider what epiphenomenalists would say about the manifestabilty
premise. They would happily allow that
mental differences will display themselves in differential physical
consequences as long as the laws by which brain states cause conscious states
are held constant: given these laws,
then different conscious states must have been caused by different physical
states, and we can expect these physical differences to have different physical
consequences. But epiphenomenalists will
point out that there is no need to expect this manifestability requirement to
hold up across all possible worlds, including worlds where the actual
brain-mind laws break down. After all,
if we allow, as epiphenomenalists will, that there are metaphysically possible
worlds in which I have physical duplicates with different conscious states, or
with no conscious states at all, then we will not expect these mental
differences, between me and my other-minded physical duplicates, to display
themselves in any differential physical consequences.
So
epiphenomenalist anti-physicalists will see no reason to concede that the
manifestability premise holds across all metaphysically possible worlds, even
if it holds in all naturally possible worlds.
And correspondingly, they will not view the manifestability argument as
providing any substantial reason to suppose that the mental supervenes on the
physical across all metaphysically possible worlds. They can allow that mental differences will
display themselves in differential physical consequences as long as we hold all
laws of nature fixed, and correspondingly concede the weak supervenience thesis
that mental differences without physical differences are naturally
impossible. However this, as we have
seen, falls short of physicalism proper.
Genuine physicalism requires strong supervenienceÑmental differences
without physical differences are metaphysically impossible. But epiphenomenalists will see nothing in the
manifestability argument to force them to this stronger claim, for they will
have no inclination to accept that all metaphysically possible mental
differences must display themselves in differential physical consequence.13
1.7
The Causal Argument for Physicalism
Let me now turn to a somewhat different argument
for physicalism, which I shall call "the causal argument". This argument, like the manisfestability
argument, will hinge on the completeness of physics. But instead of appealing to requirements on
the manifestation of mental states, it will appeal to the possession of causal
powers by mental states. This shift of
focus will yield a more effective line of reasoning against anti-physicalist
view like epiphenomenalism.
Thus consider the following premise, which
I shall call the "principle of mental efficacy":
Premise (3). Every mental occurrence causes some physical
effect.
Note now that, on just about any account of
causation14, the following is an immediate corollary of the completeness of
physics:
Premise (4). All physical effects have complete physical
causes ("complete" in the sense that those causes on their own
suffice by physical law to fix the chances of those effect).
Consider now some mental occurrence, and one of the physical effects
which are required by (3). For example,
suppose you decide to lift your arm, and as a result your arm rises15. By (4) this physical effect will also have a
complete physical cause, which will presumably involve the neuronal and other
physical antecedents of your arm rising.
So it follows that your arm rising has two causes: a mental cause, your decision, and also a
physical cause, your neurones firing.
Does this mean that such physical effects are always overdetermined,
like the death of the man who is shot and simultaneously struck by a random
bolt of lightning? This doesn't seem
right. After all, when an effect is
overdetermined by two causes, it follows that it would still have occurred if
either one of the causes had been absent:
the man would still have been killed by the lightning bolt even if he
hadn't been shot, and vice versa. But we
don't similarly want to say that your arm would still have gone up even if you
hadn't wanted to lift it, or, alternatively, even if different neurones had
fired in your brain.
The obvious conclusion is that your desire and your neurones are not two
independent causes, like the shot and the lightning bolt, but are in some sense
the same cause. We need somehow to
identify the mental cause with the physical cause, so as to avoid the
conclusion that the movement of your arm was overdetermined16.
Note how this argument differs from that in the last section. There the aim was to show that the physical
always co-varies with the mental, and the argument was that physical variation
is needed to produce the external evidence for mental variation; the trouble was that this argument only
established co-variation across naturally possible worlds, which was too weak
for physicalism. In this section the aim
has been to show that the mental is ontologically inseparable from the
physical, and the argument has been that such a separation would imply an
absurd proliferation of causal overdetermination; if this ontological inseparability does
follow, it will mean that there is no metaphysical room for mental properties
to float free of physical ones, and so will establish genuine physicalism.
It
might seem as if the causal argument begs the question against anti-physicalist
epiphenomenalism just as much as the manifestability argument. Epiphenomenalists, after all, will deny the
assumption of causal efficacy, just as they denied any strong manifestability
premise. So they will escape the causal
argument too. They don't need to explain
why bodily movements aren't always overdetermined, since they don't admit they
have mental causes in the first place.
But
there is a significant dialectical difference between the two cases. There is nothing pre-theoretically
objectionable about the denial of a strong manifestability premise: nothing obvious will go wrong with our
overall view of the world just because we allow the mere metaphysical
possibility of mental differences without physical manifestations. By contrast, it clearly flies in the face of
any number of normal assumptions to deny that mental events have physical
effects. If my conscious thirst isn't
what causes me go to the fridge for a beer, and my conscious map-reading isn't
what causes me to choose one route rather than another in a strange city, and
so on, then we are going to have to think again about most of our assumptions
about the way the human world works.
Given this, we can well ask why the epiphenomenalist wants to adopt the
curious view that conscious mental states are causally inefficacious,
especially given the availability of physicalist alternatives which avoid
it. The only plausible answer, I take
it, is to do with consciousness:
epiphenomenalists are persuaded that any physicalist account of the
mental will leave out the essential conscious features of the mental, and so
are persuaded to postulate a distinct, non-physical realm of mental events,
even at the cost of denying that the mental affects the physical. I shall return to this issue in chapter 4
below, where I shall argue that there is nothing in consciousness that is left
out by physicalism, and therefore that the epiphenomenalist denial of the
causal efficacy of the mental is ill-motivated.
Before proceding, let me quickly deal with one complication. This relates to broad mental states. We saw earlier how broad mental states
complicated the manifestability argument.
Similar complexities arise in connection with the causal argument.
Thus note how I illustrated the causal argument by focusing on the
bodily effects of mental states, like arms rising, and then inferred that the
mental causes of these bodily effects must be identical with their
"neuronal causes". However,
this specifically neuronal conclusion sits ill with the possibility of broad
mental states. For broad mental states
can't be identical with internal brain states, given that they depend on
matters outside the skin. Carl and Lrac
differ in their respective desires for H2O and XYZ, even though they are
internally physically identical. In line
with this, it seems wrong to say that their different desires cause their
bodily movements: bodily movements are
surely caused by matters inside the skin, not by features that stretch outside.
Still, the fact that broad mental causes can't be the same as brain
states doesn't mean they can't be equated with any physical states, in
particular with certain physical features of their
possessors-and-relevantly-related-systems.
And it is not hard to see how the causal argument might be made to
deliver this weaker conclusion. All we
need is a causal efficacy premise to the effect that broad mental states cause
some "broad" physical consequences, even if they don't cause the
bodily movements that result from neuronal causes alone. And there seems no difficulty about this
version of the efficacy premise. For
example, Carl's desire may cause a glass of H2O to move, where Lrac's desire
will cause a glass of XYZ to move. And
then, with the efficacy premise so restored, we can use the causal argument to
argue that Carl and Lrac's desires must be equated with those physical feaures
of themselves-and-their-surrounding-environments which are responsible for
these broad effects.
1.8
Generous Causation and Alternatives to Type Identity
I argued in the last section that the mental
causes of physical effects must be the same as the physical causes of those
effects. Exactly how we construe this
equivalence, however, depends on what view we take of the ontological status of
causes in general. Some philosophers,
most prominently Donald Davidson (1967), think that causation is a relation
betwen events construed as "bare particulars" shorn of any general
attributes. However, there are good
arguments for being dissatisfied with this anaemic view of causation, and for
preferring to view causal relata as facts rather than as Davidsonian bare
particulars17. Accordingly, I shall
assume the factual view of causal relata in what follows.
However, if you view causation as a relation between facts in this way,
then it may seem as if the causal argument is in danger of proving too
much. In particular, it may seem in
danger of proving that mental properties must be type identical with physical
properties, notwithstanding the intrinsic implausibility of this type identity
claim. For, if causes are facts, then the causal argument's conclusion, that
mental causes must be identical with physical causes, will require that mental
factsÑsuch as that I am in pain, sayÑare identical to certain physical factsÑI
have a certain brain feature, sayÑand these two facts cannot be identical
unless the properties they involveÑbeing in pain, having that brain featureÑare
themselves identical.
Well, this type identity would indeed follow from the causal argument if
we take a very strict view of causation, and insist that the only thing that
can cause a physical effect is another strictly physical fact. For then the principle of mental efficacy,
according to which mental facts cause physical effects, can only be satisfied
if mental facts are themselves instantiations of strictly physical
properties. However, suppose we
understand causation in a more generous sense, and allow that an instance of a
strongly supervening property causes the effects of those facts on which it
supervenes. Then the principle of mental
efficacy will only require that mental properties are type identical to
physical properties or that they strongly supervene on physical
properties. For as long as the latter
possibility is realized, then it will still be true, in the generous sense,
that mental facts cause the physical effects of the physical facts on which
they supervene.
As
an illustration of this possibility, consider the functionalist view that
mental states are causal intermediaries between perceptual inputs and
behavioural outputs. The orthodox
version of this view does not identify pain, say, with whichever first-order
property mediates causally between damage detectors and avoidance behaviour in
any given species. For this would have
the "chauvinist" implication that species with different internal
workings could not share the experience of pain. Rather the standard functionalist view is
that pain is a second-order property, the property-of-having-some-property
which mediates causally between damage detection and avoidance behaviour, which
second-order property can therefore be present across beings with different
innards.
Now, on this functionalist view, pains can't cause bodily movements in
the strict sense which requires identity with strictly physical facts. For, if pains are instantiations of
second-order properties, they cannot be identical with any first-order physical
facts. Still, such functionally
understood pains can still be "realized" by physical properties, in
that they can be present purely because some first-order physical fact which
mediates between damage and avoidance is presentÑand in that case a pain will
indeed cause bodily movements, in the generous sense in which superveners cause
what their subveners cause. For if a
pain is so realized by a physical fact, then it will supervene on this physical
fact, even though not identical with it, in that any metaphysically possible
being with this physical property will be in pain, since it will possess the
property-of-having-some-property which mediates causally between damage and
avoidance.
Let me clarify my direction of argument here. I am not at the moment concerned to uphold
functionalism, nor, consequently, am I particularly concerned to argue that
functionalist mental definitions are satisfied by physical states in
humans18. Rather I have introduced
functionalism merely as an illustration of how facts that are not themselves
physical facts can nevertheless cause physical effects, at least in the generous
sense of causation.
More generally, if we understand the causal argument in terms of the
generous sense of causation, then the conclusion will be mental facts must in
some way strongly supervene on physical facts (otherwise mental facts couldn't
cause physical facts even in the generous sense, given the completeness of
physics). Functionalism offers one
illustration of how this might be so, even when type identities are not
available. But my conclusion is not that
functionalism must be true, only that the mental must somehow strongly supevene
on the physical.
For a further example of a theory of this form, consider Donald
Davidson's view of the mental.
(Davidson, 1980, passim. Though
Davidson's view of the mental is standardly presented in harness with the
Davidsonian view of causation mentioned above, it is helpful to separate out
these two aspects of Davidson's thinking.)
Davidson holds, in effect, that to be in a given mental state M is to be
in some state which causes behaviour which would warrant the attribution of M
to you. This is a different theory from
functionalism, since it makes essential appeal to the non-scientific canons of
interpretation which Davidson takes to govern our attributions of mental states
to others. But, just like functionalism,
it allows room for the idea that the mental may be realized by the physical,
and consequently strongly supervene on it.
For if it is physical state P which causes the behaviour which warrants
the attribution of mental state M to person X, then X will be M purely in
virtue of being in P, and correspondingly any possible creature with P will
have M, since it will have some state which causes behaviour which would
warrant the attribution of M.
So
the Davidsonian view, like functionalism, will also satsify the requirement
that mental facts should cause physical facts, at least in the generous
sense. Still, as with functionalism, I
mention this, not as an argument for the Davidsonian view in particular, but
simply as another illustration of how the requirement of supervenience on physical
states allows the causation of physical effects, even in the absence of type
identity.
I
have no clear views about the full range of ways in which mental properties
might supervene on physical ones.
Functionalism and Davidsonianism are two such options, but there may
well be others. However, there is no
need to decide this issue here. It will
be enough if I have shown that some such view of the mental is demanded by the
causal argument.
Of
course, there remains the option of embracing epiphenomenalism, and denying
that the mental is efficacious, even in a generous sense. As I observed earlier, the normal motivation
for this unpalatable view is to do with consciousness, and the conviction that
conscious states at least must be ontologically quite distinct from any
physical states. The question this
raises, and to which I said I shall return in chapter 4, is whether this
anti-physicalist convinction about consciousness rests on solid enough grounds
to justify the radical step of denying that our thoughts and feelings affect
our actions. But for the moment I am
content merely to point out that the minimum price for rejecting physicalism is
epiphenomenalism. Anti-physicalists need
to deny some premise in the causal argument, and the line of least resistance
is to deny the principle of mental efficacy.
1.9
The Completeness of Physics Defended
There is an alternative, if less obvious,
way to resist the causal argumentÑnamely, by denying the completeness of
physics. This assumption may seem
initially plausible. But, as I allowed
earlier, it is by no means entirely unproblematic.
The central difficulty facing defenders of this assumption is an obvious
dilemma about what they mean by "physics". Either "physics" means the theory
currently taught in university departments of physics and presupposed by
articles in physics journals, or it means some ideal future theory that will
succeed current theory.
The trouble with the first horn of this dilemma is that, if the past
form of physical theorizing is anything to go by, current physics is no doubt
inadequate in certain respects, and in particular in failing to identify all
the antecedents for certain physical effects.
So current physics is not complete.
The trouble with the other horn, by contrast, is that we don't yet know
what physical categories will be assumed by the ideal future physics. So we scarcely seem to be in any position to
maintain that those categories will suffice for complete explanations of all
physical effects.
However, I think there is a version of the second horn of this dilemma
which will serve the purposes of the arguments of this chapter. Suppose we simply define "physics"
as the science of whatever categories are needed to give full explanations for
all physical effects. I accept, as
above, that this science will be different from current physical theory, and
thus that we don't yet know what it is.
But, even so, there is no difficulty about how we know that it is
complete, for we have simply defined it so as to be complete.
The obvious worry about this definitional strategy is that it seems to
remove any significant content from the thesis of completeness, and thereby to
make it doubtful that the thesis could have any substantial conclusions. There are two dimensions to this worry. First, the definitional strategy
characterizes physics as the science of whatever is needed to explain
"physical" effects. But what
are "physical effects", if we haven't yet specified what counts as "physics"? Second, even if we had some independent hold
on "physical effects", the proposed strategy would still make the
completeness thesis an empty analyticity, for it simply defines "physical"
categories as all those needed to explain physical effects, from which
completeness immediately follows.
Let me deal with these two worries in turn. To deal with the first worry, I propose that
we simply postulate some pre-theoretically given class of paradigmatic physical
effects, such as stones falling, the matter in our arms moving, and so on. If we take this class to be independently
given, then we can effectively characterize the rest of physics as all the
categories that need to be brought in to explain those paradigmatic physical
effects.
But this still leaves us with the second worry, that even we help
ourselves to a pre-theoretical class of paradigmatic physical effects, we are
still defining physics in such a way as to make the completeness of physics a
matter of definition. I still need to
explain how substantial conclusions about the truth of physicalism could
possibly follow from such a definition19.
My
answer is that no substantial conclusions follow from the completeness of
physics per se. But they do follow from
the joint assumption that (a) physics is complete and (b) that it does not make
any use of psychological categories.
Let me explain. In itself, the
above definition of physics leaves it open that psychological categories may
turn out to be needed as an essential part of physics. Maybe psychokinesis is true, and there are
physical effects that can't be accounted for without making essential mention
of distant volitions. Less exotically,
maybe some bits of behaviour can't in fact fully be accounted for purely in
terms of muscular activation, neuronal activity, and so, without bringing in
extra mention of prior mental states.
Now, if psychological categories do turn out to be needed to give full
explanations for physical effects in this way, then the issue of whether psychology
supervenes on the physical, as I have defined it, becomes trivial. Psychology will indeed supervene on the
physical, but only because it is included in the physical, not because
psychological variation requires variation in something else.
On
the other hand, if psychology is not part of the physical, as I have defined
it, then the arguments of this chapter will go through as before. That is, if psychological categories are not
in fact ever essential to explaining physical effects, then physics, in the
sense of whatever is needed to explain physical effects, will be both complete
and exclusive of psychology, and the arguments of this chapter will show that
psychological states are non-trivially supervenient on physical states.
It
seems to me highly unlikely that the psychological will turn out to be part of
the physical. Current physics, I take
it, aims to develop a complete theory of paradigm physical effects in terms of
the categories of energy, field and spacetime structure. I am quite prepared to believe that this this
aim cannot be achieved, and that the categories of current physics will need
supplementation before we can get a genuinely complete theory. What I do not believe is that they will need
supplementation by psychological categories.
I
am here making an empirical claim. The
history of science yields a great deal of empirical evidence about the kind of
causes that are responsible for the motion of stones and other kinds of
matter. This evidence does not, perhaps,
allow us to formulate a definitive list of all the necessary categories. But it does, it seems to me, provide
sufficient grounds for concluding that mental categories are not among them.20
To
help see what is at issue here, it is illuminating to consider Descartes' views
on the matter. Descartes did think that
there were physical effects that could not be explained without bringing in
mental antecedents. Descartes believed
that the total amount of motion, in the sense of mass times speed, is
conserved, according to regular laws, in all material interactions, and
therefore that the speeds of all material bodies are determined by earlier such
speeds. However, unlike us, Descartes
did not believe in the conservation of momentum, considered as a directional
quantity, and so did not think that the direction of motion of material bodies
was necessarily determined by prior physical factors. And it was this gap that Descartes exploited
to explain how the mental, although ontologically quite distinct from the
physical, can nevertheless affect the physical:
the mental interacts with the physical in the pineal gland, and
influences the direction of motion of certain particles (though not their
speed, since this is always fixed by prior physical states).
To
hold that the psychological is part of the physical is to believe a version of
what Descartes believed, namely, that there is a gap in the determination of
certain physical effects, which can only be filled by mental occurrences. And this is what seems highly unlikely to
me. It is one thing to hold that the
current categories of energy, field and spacetime structure leave a gap in the
determination of certain physical effects.
It is another to hold that this gap cannot be filled without bringing in
the mental. If that were true, after
all, then the obvious moral would be that physicists needn't build expensive
particle accelerators to generate theoretically anomalous physical
phenomena; instead they could find
plenty of currently inexplicable physical phenomena simply by looking inside
people's heads.21 I think we have good
empirical reason to reject this possibility as absurd.
Extra References
Papineau, D. 1996: "Theory-Dependent
Terms" Philosophy of Science 63
[Because of this new reference, remove Stich
(1991) from the Bibliography.]
Steward, H. 1996. "Comments on
Philosophical Naturalism" Philosophy and Phenomenological Research 57
Witmer, G. 1998. "What is Wrong with
the Manifestability Argument for Supervenience" Australasian Journal of
Philosophy 76
1
For arguments in favour of physicalism, see Lewis (1966), Davidson
(1970), Peacocke (1979, ch III.3), McGinn (1982, p 29), Smith and Jones (1986,
pp 57-59), McFetridge (1990, p 86), Lycan (1987, pp 2-3). Reasoned opposition to physicalism is offered
in Crane and Mellor (1990), Crane (1991).
Most of these contributions will be referred to further in what follows.
2 It
is sometimes suggested that this kind of shift, from a "local" to a
more "global" supervenience, makes room for ad hoc defences of
supervenience, and so dilutes physicalism beyond interest. To answer this charge, we should require that
wider systems should be admitted as subvening bases only if there are
independent grounds, apart from a desire to save physicalism, for regarding the
putatively supervening properties as relational.
3
For brevity I shall often focus in this chapter on the relation between
the psychological and the physical. But
the analysis will be of general significance, as the structure of my arguments
will indicate.
4 I
think that there is good reason to think that theoretical concepts in general,
and psychological concepts in particular, are vague, in that there will often
be no fact of the matter about how to apply them to cases where the theory that
defines them breaks down. For more on
this see Papineau (1996).
5
See Putnam (1975), Burge (1979, 1982) and Evans (1982).
6
Some readers might baulk at my use of "explanation" here, on
the grounds that a full physical specification of the antecedents of some
large-scale physical outcome won't necessarily be illuminating for us humans,
in the way that an explanation using chemical or biological or psychological
terminology might be (cf Putnam, 1978, p 42).
No matter. My argument only
requires that the physical antecedents fix or cause the physical outcome, not
that they illuminate it. David Owens
(1992) is even more particular, and would baulk at this last use of
"cause", on the grounds that causes aren't causes unless they
illuminiate. Again no matter. My arguments need only whatever is left in
the notion of cause after we take away the anthropocentric factor of
illumination.
7
Cf Crane (1991).
8
As it happens, when I do return to the completeness of physics in 1.9, I
suggest that this thesis itself can most usefully be understood as an analytic
truth, rather than as an empirical claim.
However, when we do read it in this way, the burden of my argument for
physicalism is then taken up by some closely related empirical assumptions.
9
This argument is found in McGinn (1982, p 29) and further discussed by
McFetridge (1990, p 86). In Papineau
(1990) I tried to run the argument with a weaker version of premise (2),
requiring only that mental differences have some different consequences, not
necessarily physical ones. But when I
presented this version of the argument at the Analysis 50 Conference in
10
Why doesn't the argument work in reverse, and also show that all physical
differences must depend on mental differences?
The essential reason is that the mental is not complete. Even if we accept, as is not entirely
implausible, the "mental manifestability of the physical" ("if
two systems are phyiscally different, there must be contexts in which this will
produce differential mental effects"), we cannot conclude that these
differential mental effects must always depend on prior mental differences, for
lack of the premise that mental effects are always fixed by mental antecedents.
11
This is the version of the argument articulated by McGinn. He does, however, observe that it may not
apply to all mental states.
12
Some readers may be wondering whether this equation of physicalism with
strong supervenience has not simply taken us a long way round back to the
earlier equation of physicalism with type identity. For haven't I just argued that the virtue of
strong supervenience is precisely that it ensures that special properties are
ontologically inseparable from physical properties, by contrast with weak
supervenience, which only requires that special properties are correlated with
physical properties by the actual laws of nature, but need not be ontologically
intertwined with them? Well, the virtue
of strong supervenience is indeed that it ensures an ontological dependence of
special properties on physical properties, and not just a correlation. But the point of formulating physicalism in
terms of supervenience, rather than type identity, is precisely that it is possible
to have such ontological dependence even when type identities are not
available. I shall return to this in
section 1.8 below.
13
For further discussion of the failings of the manifestability argument,
and for other criticisms of the original English version of this chapter, see
Steward (1996) and Witmer (1998).
14
David Owens (1992) is an exception.
But, as I said in footnote 6, I could grant Owens his stronger notion of
cause, and simply phrase my arguments in terms of a weaker one.
15
Are bodily movements, like arms raising, mouths moving, and so on,
properly counted as physical effects?
Strictly, no. "Arm" and
"mouth" are biological terms, not physical ones, and it is doubtful
that they can be reduced to physical notions.
So for full accuracy we ought to take the physical effects of mental
causes to be the motion of bits of matter, which happen to be in arms, mouths,
and so on. However, it will smooth the
exposition if I can be less than strict on this point.
16
This form of argument for token congruence is to be found in Peacocke
(1979, ch. III.3). It has obvious
affinities with the discussion in Davidson (1970).
17
For a defence of this view. see Mellor (1987). Another alternative to Davidson's view of
causation is to allow that causes are events, but insist that events are
instantiations of properties, rather than Davidsonian bare particulars (Kim,
1973). However, Mellor (op cit)argues
that "events" of this kind are simply a subspecies of facts.
18
David Lewis (1966) does argue from a version of functionalism to
mind-brain identity. Lewis's argument
shares one premise with my causal argument, namely, the completeness of
physics. But where my other premise is
only that each particular mental cause has some physical effect, Lewis makes
the stronger functionalist assumption that different mental types can
distinguished by their characteristic causal role in mediating between physical
causes and effects. (He then concludes,
from the completeness of physics, that such roles are always filled by physical
states.)
19
Crane (1991) argues on just these grounds that the version of my
argument for physicalism in (Papineau, 1990) collapses into triviality.
20
Let me guard against one possible source of confusion here. When I say that a compete physics excludes
psychology, and that psychological antecedents are therefore never needed to
explain physical effects, the emphasis here is on "needed". I am quite happy to allow that psychological
categories can be used to explain physical effects, as when I tell you that my
arm rose because I wanted to lift it. My
claim is only that in all such cases an alternative specification of a
sufficient antecedent, which does not mention psychological categories, will
also be available. I need the thesis
that psychological terms are not included in the minimal set which provides
sufficient conditions for all physical effects, not that they are not included
in any such set.
21
Cf. Lycan (1987, pp 2-3).