## 1. Preliminaries


### 1.1 Terminology



Physicalism is sometimes known as 'materialism'. Indeed,
on one strand to contemporary usage, the terms
'physicalism' and 'materialism' are
interchangeable. But the two terms have very different histories. The
word 'materialism' appears in English towards the end of
the 17th century, but the word 'physicalism' was
introduced into philosophy only in the 1930s by
 Otto Neurath
 (1931) and
 Rudolf Carnap
 (1959/1932), both of whom were key members of the Vienna Circle, a
group of philosophers, scientists and mathematicians active in Vienna
prior to World War II. While it is not clear that Neurath and Carnap
understood physicalism in the same way, one thesis often attributed to
them (e.g. in Hempel 1949) is the linguistic thesis that every
statement is synonymous with (i.e. is equivalent in meaning with) some
physical statement. But materialism as traditionally construed is not
a linguistic thesis at all; rather it is a metaphysical thesis in the
sense that it tells us about the nature of the world. At least for the
positivists, therefore, there was a clear reason for distinguishing
physicalism (a linguistic thesis) from materialism (a metaphysical
thesis). Moreover, this reason was compounded by the fact that,
according to official positivist doctrine, metaphysics is nonsense.
Since the 1930s, however, the positivist philosophy that under-girded
this distinction has for the most part been rejected--for
example, physicalism is not a linguistic thesis for contemporary
philosophers--and this is one reason why the words
'materialism' and 'physicalism' are now often
interpreted as interchangeable. 



Some philosophers suggest that 'physicalism' is distinct
from 'materialism' for a reason quite unrelated to the one
emphasized by Neurath and Carnap. As the name suggests, materialists
historically held that everything was matter -- where matter was
conceived as "an inert, senseless substance, in which extension,
figure, and motion do actually subsist" (Berkeley, Principles of
Human Knowledge, par. 9). But physics itself has shown that not
everything is matter in this sense; for example, forces such as
gravity are physical but it is not clear that they are material in the
traditional sense (Lange 1865, Dijksterhuis 1961, Yolton 1983). So it
is tempting to use 'physicalism' to distance oneself from
what seems a historically important but no longer scientifically
relevant thesis of materialism, and related to this, to emphasize a
connection to physics and the physical sciences. However, while
physicalism is certainly unusual among metaphysical doctrines in being
associated with a commitment both to the sciences and to a particular
branch of science, namely physics, it is not clear that this is a good
reason for calling it 'physicalism' rather than
'materialism.' For one thing, many contemporary
physicalists do in fact use the word 'materialism' to
describe their doctrine (e.g. Smart 1963). Moreover, while
'physicalism' is no doubt related to 'physics'
it is also related to 'physical object' and this in turn
is very closely connected with 'material object', and via
that, with 'matter.' 



In this entry, I will adopt the policy of using both terms
interchangeably, though I will typically refer to the thesis we will
discuss as 'physicalism'. It is important to note, though,
that physicalism (aka materialism) is often associated with a number
of further doctrines distinct from the one we will focus on; for a
discussion of some of these, see the supplement on:



Physicalism and Associated Doctrines



### 1.2 Historical Issues



Setting aside what it is properly called, the thesis of physicalism is
often described as an extremely old, even ancient, thesis. The first
sentence of
 Friedrich Lange's
 *The History of Materialism*, which was the standard work on the
subject in the 19th century is: "Materialism is as old as philosophy,
but not older" (1925, 3). What Lange has in mind is the pre-Socratic
philosopher
 Democritus,
 who is usually thought of defending a kind of physicalism or
materialism when he said or allegedly said, "all is atoms and the
void." This view casts a long shadow over subsequent formulations of
physicalism. A position like that of Democritus was revived in the
early modern period just prior to Newton, by philosophers and
scientists such as
 Hobbes
 or
 Gassendi.
 In the eighteenth century, French philosophers like
 D'Holbach
 and La Mettrie thought of themselves as materialists (and would be
now classified as physicalists) somewhat in the same way since they
held that each human being is a complicated sort of machine. In the
nineteenth century, while
 Karl Marx's
 'dialectical materialism' is something different from the
metaphysical thesis we will focus on, he nevertheless developed his
social philosophy against the background of what we would call
physicalism; in fact, Marx's doctoral dissertation was a comparison of
Democritus and
 Epicurus. And in the twentieth century,
 analytic philosophers such as Smart and Lewis self-consciously
 defended their views in a way that acknowledges, as Lewis put it,
 "our intellectual ancestors" (1994, 293).



However, while there is certainly something in common here, the
underlying historical issues are extremely complicated, since they
involve subtle questions of scientific and philosophical change. We
won't discuss them in detail in this entry. It is worth emphasis,
though, that we should be careful in lumping different people in
different epochs together. The precise views they held are often
different from one another. As we noted above, and as Lange himself
emphasises, Newton did not think that, even in the physical world, all
is atoms and the void, since for him there are also forces such as
gravity. It follows that any post-Newtonian philosophers who think of
themselves as physicalists must have a different thesis in mind from
Democritus.


### 1.3 A Framework for Discussion



In approaching the topic of physicalism, one may distinguish what I
will call *the interpretation question* from *the truth
question*. The interpretation question asks: 


* What does it *mean* to say that everything is
physical?



The truth question asks: 


* Is it *true* to say that everything is physical?



There is obviously a sense in which the second question presupposes an
answer to the first -- you need to know what a statement means
before you can ask whether it's true -- and so we will
begin with the interpretation question. 



The interpretation question itself divides into two sub-questions,
which I will call *the condition question* and *the
completeness question*. The condition question asks:


* What does it mean to for something to be
*physical*?



The condition question holds fixed the issue of what it means for
everything to satisfy some condition, or to bear a relation to
something that satisfies that condition, and asks instead
what *is* the condition, *being physical*, that
everything satisfies or bears a relation to. Notice that a parallel
question could be asked of Thales: what is the condition, *being
water*, that according to Thales, everything satisfies?


The completeness question asks:


* What relation or relations must obtain between *everything*
and the physical if physicalism is to be true?



In other words, the completeness question holds fixed the issue of
what it means for something to be physical, and asks instead what
relation or relations obtain between everything and the physical if
physicalism is true; in what sense, in other words, is physicalism a
complete thesis, a thesis that applies to everything
whatsoever. Notice again that a parallel question could be asked of
Thales: assuming we know what condition you have to satisfy to be
water, what does it mean to say that everything satisfies that
condition? 



Once again there is a sense in which the second question here
presupposes an answer to the first -- you need to know what it is
for something to be physical in order to assess different proposals
about the relation everything bears to the physical. Nevertheless, it
is easier from a presentational point of view to discuss the
completeness question first, leaving our answer to the condition
question for the moment impressionistic, and that will be our
procedure. 


## 2. The Completeness Question



How should we approach the completeness question? In the history of
attempts to answer this question, people have tended to adopt one of
two strategies. One appeals to (what philosophers call) *modal*
notions, where a modal notion here means a notion connected to
possibility and necessity, to what might or must be the case. Another
appeals to *non-modal* notions, that is, notions distinct from
ideas about possibility and necessity. There is a wide variety of such
notions, though perhaps the most obvious one is *identity* in
the logical sense, according to which if x is identical to y, then
every property of x is a property of y. This notion has consequences
for possibility and necessity, but it is not itself modal. Other
non-modal notions include *realization*, or *grounding*,
which we will consider below.



In practice, most formulations of physicalism include *both*
modal and non-modal elements at some level. Proponents of modal
formulations often end up appealing to something non-modal in the
course of elaborating and defending their view. Non-modal versions are
usually interpreted so that they have the same modal consequences as
modal versions. Nevertheless, the issue of whether to answer the
completeness question by appealing to modal or non-modal ideas has
proved as controversial as any in the literature on what physicalism
is (for overviews, see Rabin 2020, Elpidorou 2018a, Tiehen 2018). In
what follows, we will first look at an influential modal answer to the
completeness question, which appeals to *supervenience*, and
then turn to several non-modal alternatives.


### 2.1 Supervenience and Necessity Physicalism



The idea of supervenience has its origins in meta-ethics but was
imported into philosophy of mind mainly by Davidson 1970; for a
survey, see
 supervenience.
 For our purposes, the general idea might be introduced via an example
due to David Lewis of a dot-matrix picture:



> 
> A dot-matrix picture has global properties -- it is symmetrical,
> it is cluttered, and whatnot -- and yet all there is to the
> picture is dots and non-dots at each point of the matrix. The global
> properties are nothing but patterns in the dots. They supervene: no
> two pictures could differ in their global properties without
> differing, somewhere, in whether there is or there isn't a dot
> (1986, p. 14).
> 



This gives us one way to think about the basic idea of physicalism.
The basic idea is that the physical features of the world are like the
dots in the picture, and the psychological or biological or social
features of the world are like the global properties of the picture.
Just as the global features of the picture supervene on the dots, so
too everything supervenes on the physical, if physicalism is true.



It is desirable to have a more explicit statement of physicalism, and
here too Lewis's example gives us direction. He says that, in
the case of the picture, supervenience means that "no two
pictures can be identical in the arrangement of dots but different in
their global properties". Similarly, one might say that, in the
case of physicalism, no two possible worlds can be identical in their
physical properties but differ, somewhere, in their mental, social or
biological properties. To weaken this slightly, we might say that if
physicalism is the case at *our* world - that is, is true of
our universe and everything in it - then no *other* world can
be physically identical to our world without being identical to it in
all respects. This suggests the following general account of what
physicalism is (in the following formulation and in subsequent ones,
we use "iff" to abbreviate "if and only
if"):



(1)
 Physicalism is true at a possible world *w* iff any world
which is a physical duplicate of *w* is a duplicate of
*w* *simpliciter*.


If physicalism is construed along the lines suggested in (1), we have
an answer to the completeness question. The completeness question
asks: what relation does everything bear to the physical if
physicalism is true? According to (1), the answer is that everything
must supervene on the physical; or, to put it more technically, there
is no possible world which is identical to our world in every physical
respect but which is not identical to it in a biological or social or
psychological respect. It will be useful to have a name for
physicalism so defined, so let us call it *supervenience
physicalism*. 



Supervenience offers one modal formulation of physicalism, but it is
worth taking note of a second modal formulation too. Suppose we say
that a property *G* is necessitated by a property *F*
just in case, in all possible worlds, if something is *F* then
it is *G*; in this sense, for example, being red necessitates
being colored, and being square necessitates having some extension in
space. This suggests a formulation of physicalism along the following
lines:



(2)
 Physicalism is true at a possible world *w* iff every
property instantiated at *w* is necessitated by a physical
property.


What is the relation between physicalism so defined, which we might
call *necessity physicalism*, and *supervenience
physicalism*? At least if necessitation is understood as a sort of
entailment, then these are not equivalent; for discussion of this
point, see
 supervenience.
 However, (1) and (2) are clearly similar, in particular they are
modal formulations of physicalism. In what follows we will concentrate
on supervenience physicalism, but what we will say will apply also to
necessity physicalism.



Supervenience physicalism was for many years the dominant version of
physicalism; perhaps because of this, many different problems have
been raised for it; some of these problems are discussed in the
supplement on:



Supervenience Physicalism: Further Issues




But the most influential objection to supervenience physicalism (and
to modal formulations generally) is what might be called *the
sufficiency problem*. This alleges that, while (1) articulates a
necessary condition for physicalism it does not provide a sufficient
condition. The underlying rationale is that, intuitively one thing can
supervene on another and yet be of a completely different nature. To
use Fine's famous (1994) example, consider the difference between
Socrates and his singleton set, the set that contains only Socrates as
a member. The facts about the set supervene on the facts about
Socrates; any world that is like ours in respect of the existence of
Socrates is like ours in respect of the existence of his singleton
set. And yet the set is quite different from Socrates. This in turn
raises the possibility that something might be of a completely
different nature from the physical and nevertheless supervene on
it.



One may bring out this objection further by considering positions in
philosophy which entail supervenience and yet deny physicalism. A good
example is *necessitation dualism*, which is an approach that
weaves together elements of both physicalism and its traditional
rival, dualism. On the one hand, the necessitation dualist wants to
say that mental facts and physical facts are metaphysically
distinct--just as a standard dualist does. On the other hand, the
necessitation dualist wants to agree with the physicalist that mental
facts are necessitated by, and supervene on, the physical facts. If
this sort of position is coherent, (1) does not articulate a
sufficient condition for physicalism. For if necessitation dualism is
true, any physical duplicate of the actual world is a duplicate
*simpliciter*. And yet, if dualism of any sort is true,
including necessitation dualism, physicalism is false.



How to respond to the sufficiency problem? Some respond by denying the
coherence of the position that causes the problem. Necessitation
dualism as we have just described it violates (what is known as)
Hume's dictum that there are no necessary connections between
distinct existences. According to necessitation dualism, mental and
physical properties are metaphysically distinct, and yet are
necessarily connected. However, Hume's dictum is itself a matter
of controversy (see Jackson 1993, Stalnaker 1996, Stoljar 2010, and
Wilson 2005, 2010). Another approach appeals to a priori physicalism
which will examine below (see Jackson 2006). But by far the most
common response has been to concede that the sufficiency problem shows
that supervenience formulation of physicalism is too weak (e.g. Kim
1998), and to look for an alternative. 


### 2.2 Identity Physicalism



Suppose then that (1) provides a necessary condition for physicalism
but not a sufficient condition; how might we strengthen it to make it
more plausible? The most obvious thing to do is to appeal to identity.
Indeed, in the history of attempts to answer the completeness
question, the appeal to identity predates the appeal to supervenience.
Nevertheless, this version of physicalism-- *identity
physicalism*  as we may call it--runs into serious
problems.

 *2.2.1. Token Physicalism*

In fact there are two different versions of identity physicalism,
*type physicalism* and *token physicalism*. Token
physicalism is the view that every particular thing in the world is a
physical particular. So the token physicalist says that physicalism
should be formulated in the following way:



(3)
 Physicalism is true at a possible world *w* iff for every
particular (object, event or process) *x* that exists at
*w*, there is some physical particular *y* such that
*x* = *y*


But (3) offers neither necessary nor sufficient conditions for
physicalism. To see that it is not sufficient, consider the variety of
dualism usually called *property dualism*. Property dualism
says that (a) every particular is a physical particular but (b) some
particulars (e.g. human beings) have psychological properties wholly
distinct from any physical properties. The contrast here is with
*substance dualism*. The substance dualist agrees with the
property dualist that some particulars have psychological properties
wholly distinct from any physical properties, but they will add that
such particulars are themselves non-physical. 



Token physicalism - physicalism according to (3) - is certainly
inconsistent with substance dualism. Substance dualism entails that
some particulars are non-physical, token physicalism denies it. But
token physicalism is compatible with property dualism; indeed property
dualism entails that token physicalism is true. On the other hand,
property dualism is usually understood as being inconsistent with
physicalism in any form. Hence token physicalism is not sufficient for
physicalism. 



The problem that property dualism presents for token physicalism is
noteworthy in several respects. For one thing, it is similar to the
problem necessitarian dualism presents for supervenience physicalism,
though necessitarian dualism can itself be developed either as a sort
of property dualism or as a sort of substance dualism. For another
thing, it brings out the important role properties rather than
particulars play in the contemporary discussion of physicalism. If one
ignores properties, the dispute between physicalism and dualism may
easily be understood as the dispute between token physicalism and
substance dualism; but once properties are factored in, things look
very different.



Not only does (3) not provide a sufficient condition for physicalism,
it does not provide a necessary condition either. Consider a social or
legal object such as the United States Court of Appeals for the
Seventh Circuit. According to (3), if physicalism is true, there must
be some physical object or particular for the court to be identical
with. But intuitively, there is no such physical object. Nevertheless,
physicalism might still be true. If so, token physicalism is not
necessary for physicalism. (Notice here that there is no parallel
problem for supervenience physicalism. It entails that the facts about
court supervene on physical facts but not that there is any physical
object that the court is identical with. For the classic presentation
of this point, see Haugeland 1983).

 *2.2.2. Type
Physicalism*

Turning now to type physicalism, this holds that every property (or at
least every property instantiated in the actual world) is identical
with some physical property. So the type physicalist supposes that
physicalism should be formulated in the following way:



(4)
 Physicalism is true at a possible world *w* iff every
property instantiated at *w* is identical to a physical
property.


Unlike both token physicalism and supervenience physicalism, type
physicalism *is* sufficient for physicalism: if every property
instantiated in the actual world is identical with some physical
property, then dualism will be false in any of the versions we have
considered



However, while (4) provides a sufficient condition for physicalism, it
does not provide a necessary condition. Consider again the United
States Court of Appeals for the Seventh Circuit. If type physicalism
is true, then every property that the court has (for example, having a
legal power over lower courts) must be identical with some physical
property. But on the face of it, that is unlikely. Nevertheless,
physicalism might still be true. If so, type physicalism is not
necessary for physicalism. (Notice again that there is no parallel
problem for supervenience physicalism. It entails that the property
having a legal power over lower courts supervenes on physical
properties, not that it is identical with a physical property.)



Another way to bring out the sense in which type physicalism is too
strong for physicalism is to focus on the possibility of *multiple
realizability*, as philosophers call it. This is roughly the idea
that physically very different creatures can nevertheless share
psychological properties; see
 multiple realizability.
 A major benefit of supervenience physicalism is that, as usually
understood, it is consistent with this possibility. But type
physicalism is often thought of as not consistent with it, at least if
we focus on instantiated psychological properties. If so, type
physicalism may be false while supervenience physicalism may be
true.



We have been assuming that supervenience physicalism is distinct from
type physicalism. But it is worth noting that, there are ways to
understand supervenience according to which this difference is less
obvious. The reason for this has to do with questions concerning the
logical (or Boolean) closure of the set of physical properties --
if *P*, *Q* and *R* are physical properties,
which of the various logical permutations of *P*, *Q*
and *R* are likewise physical properties? On some assumptions
concerning closure and supervenience, supervenience physicalism
(construed as a necessary truth) entails type physicalism; on other
assumptions, it doesn't. But the problem is that the assumptions
themselves are difficult to interpret and evaluate, and so the issue
remains a difficult one. It is not necessary for our purposes to
settle the question concerning closure here. (For further discussion
of these issues see Kim 1993, Bacon 1990, Van Cleve 1990, Stalnaker
1996.)


### 2.3 Realization Physicalism



Our discussion of the completeness question has so far yielded
negative results: supervenience physicalism is too weak, type
physicalism is too strong, and token physicalism is both too weak and
too strong. What seems to be required is an approach that, like the
modal formulations we started with, entails the supervenience of
everything on the physical, but unlike them avoids the sufficiency
problem. One prominent idea along these lines appeals to a relation
between properties distinct from both identity and supervenience,
usually called *realization*. As in the case of identity
physicalism, there are two different versions of *realization
physicalism* as we may call it; we will consider them in turn.



*2.3.1 Second-order Physicalism*



The first realization definition has been explored and defended in
most detail by Andrew Melnyk (see Melnyk 2003 and the references
therein). For Melnyk, a property *F* realizes a property
*G* if and only if (a) *G* is identical to a
second-order property, the property of having some property that has a
certain causal or theoretical role; and (b) *F* is the property
that plays the causal or theoretical role in question. We may call
this notion 'second-order realization' to distinguish it
from a different notion of realization to be considered in a moment.
This suggests: 



(5)
 Physicalism is true at a possible world *w* iff every
property instantiated at *w* is either a physical property or
is second-order realized in a physical property.


Suppose we call physicalism so defined *second-order realization
physicalism* or *second-order physicalism* for short; what
is the relation between it and supervenience physicalism?
Supervenience physicalism does not entail second-order physicalism
since the fact that a property *F* supervenes on a property
*G* does not entail that *F* is a second-order property.




Does second-order physicalism entail supervenience physicalism? The
usual assumption is that it does, but, as Melnyk himself notes at one
point (2003, p. 23), there is an issue here having to do with the
definition of a second-order property, the property of having some
property that has a certain causal or theoretical role. What are the
properties involved in spelling out these causal or theoretical roles?
If physicalism is true at all, it must be true of these properties as
much as any other properties. But then by second-order physicalism,
these properties themselves will be either physical or realized by
physical properties. If the first option is taken, the second-order
physicalist will stand revealed as holding a version of identity
physicalism (one level up, as it were), and thus will face the
multiple realization objection. If the second option is taken, the
second-order physicalist looks committed to an infinite regress, since
now we have further properties realized by physical properties and,
correlatively, further causal or theoretical roles. To avoid the
regress, the second-order physicalist might say that these properties
supervene on or are identical to physical properties. But now it hard
to see the difference between the realization physicalist and these
other doctrines. (For some further discussion of this issue, see
Elpidorou 2018a.)

 *2.3.2 Subset Physicalism*

The second realization definition of physicalism has been developed by
Wilson 1999, 2011 and Shoemaker 2007. On this view, a property
*F* realizes a property *G* if and only if (a)
*G* has some set of causal powers or features *S*; (b)
*F* has some set of causal powers or features *S\**; and
(c) *S* is a subset of *S\**. (We may call this notion
'subset realization' to distinguish it from the different
notion of realization just considered.) This suggests: 



(6)
 Physicalism is true at a possible world *w* iff every
property instantiated at *w* is either a physical property or
is subset realized in a physical property.


Suppose we call physicalism so defined *subset realization
physicalism* or *subset physicalism* for short; what is
the relation between it and supervenience physicalism? Supervenience
physicalism does not entail subset physicalism since the fact that a
property *G* supervenes on a property *F* does not
entail anything about their causal powers. For example, if causation
is a macro-phenomenon as some philosophers have held it to be, it may
be that *F* has no causal powers at all, while *G* does.




Does subset realization physicalism entail supervenience physicalism?
Well, there is a problem here too having to do with (what is sometimes
called) a *causal* theory of properties, that is, a theory
according to which the causal powers or features that a property
bestows on the things that have it are exhaustive of the nature of
that property. Suppose that a causal theory is false. Then, in
principle, one property might subset realize another and yet be quite
different from it in nature. And this in turn suggests that subset
physicalism does not by itself entail supervenience physicalism. Of
course, one might respond by asserting that the causal theory is true.
But to do that is controversial; indeed, even those philosophers who
hold both a subset model *and* a causal theory want to
separate out these two commitments (e.g. Shoemaker 2007; see also
Wilson 2011). Alternatively, one might respond by denying that
physicalism entails supervenience in the first place, by saying that
"lack of ... supervenience is compatible with physicalism"
(Wilson 2014, 255, see also Wilson 2011). But this too is
controversial; as we saw above, most philosophers assume that
supervenience is necessary for physicalism. Hence the status of the
subset approach remains controversial. 


### 2.4 Grounding Physicalism



An influential recent approach to the completeness question that is
different from any we have considered so far focuses on the idea of
grounding, something that has been extensively discussed recently in
the metaphysics literature (see, e.g., Fine 2001, Schaffer 2009, Rosen
2010, Wilson 2014, Bennett 2017 and the essays in Correia and
Schneider 2012). Intuitively, a property *F* is grounded in a
property *G* just in case *F* holds in virtue of
*G*, or the instantiation of *G* explains the
instantiation of *F*. This suggests: 



(7)
 Physicalism is true at a possible world *w* iff every
property instantiated at *w* is either a physical property or
is grounded in a physical property.


Suppose we call physicalism so defined *grounding physicalism*;
what is the relation between it and supervenience physicalism?
Supervenience physicalism does not entail grounding physicalism, since
the fact that a property *F* supervenes on a property
*G* does not entail that *F* is grounded by *G*.




Does grounding physicalism entail supervenience physicalism? Some
philosophers suppose it does (e.g. Rosen 2010) and so for them
grounding physicalism would entail supervenience physicalism. But
others suppose it does not (e.g. Schaffer 2009) which raises the
question of whether a thesis such as (7) by itself provides an account
of physicalism, or whether some compromise between it and (1) would
have to be reached.



Even if grounding physicalism entails supervenience physicalism, there
are further issues about it that have been raised. One problem
concerns abstract objects, i.e., entities apparently not located in
space and time, such as numbers, properties and relations, or
propositions. However, since this problem seems a general problem for
all kinds of physicalism, we will discuss it below.



A second problem for grounding physicalism is that the notion of
grounding is itself controversial in some quarters. Wilson (2014), for
example, points out that grounding per se is similar to supervenience
in that it leaves open many of the questions philosophers of mind are
interested in, viz., whether the mental exists, whether it is reduced
to the physical, and whether it is causally efficacious. She concludes
that grounding "cannot do the work" that its proponents
want it to do (2014, 542). One might respond that this depends on what
work grounding physicalism is supposed to do; indeed, it may be a
feature rather than a bug that grounding leaves these things open. But
whatever is the truth about this, there is no doubt that the precise
contours of the grounding relation are yet to be made out. Hence, the
proper assessment of grounding physicalism is at this point a bit
unclear (for some further discussion, see, e.g., Berker 2018 and
Schaffer 2016). 


### 2.5 Fundamentality Physicalism



The final answer to the completeness question we will consider focuses
on the idea of a fundamental property, which is a notion discussed
extensively in David Lewis's metaphysics, and the literature
that follows on from it. On Lewis's view, a fundamental or
perfectly natural property is a special kind of property, one that is,
as he says, "not at all disjunctive, or determinable, or
negative. They render their instances perfectly similar in some
respect. They are intrinsic; and all other intrinsic properties
supervene on them" (2009, 204). This suggests the following
formulation of physicalism:



(8)
 Physicalism is true at a possible world *w* iff every
fundamental property instantiated at *w* is a physical
property.


Suppose we call physicalism so defined *fundamentality
physicalism*; what is the relation between it and supervenience
physicalism? Supervenience physicalism does not entail fundamentality
physicalism, since the fact that a property *F* supervenes on a
property *G* does not entail that either property is
fundamental. Nevertheless, it is reasonable to think that
fundamentality physicalism entails supervenience physicalism,
especially in the light of Lewis's comment about supervenience
just quoted.



How plausible is (8) as an account of physicalism? One objection
concerns the notion of a fundamental property quite generally. For at
least some philosophers, Lewis's ideas about fundamentality are, as he
himself puts it, a throwback to medieval metaphysics (Lewis 1983).
Another objection is that physicalism on this view seems empirically
speculative, since it seems to entail that there is a fundamental
level in the world; if so, definitions in terms of grounding may do
better (Schaffer 2003). Whatever is the truth about these objections,
it is an interesting historical fact that Lewis defines physicalism
twice-over. He defines it as supervenience physicalism (as we saw
above) and also as fundamentality physicalism. There is no suggestion
in his work that these are in any sense in tension (for further
discussion, see Stoljar 2015). This underscores the point made
earlier, namely, that in practice all versions of physicalism, and
hence any answer to the completeness question, will include modal and
non-modal elements. 


## 3. Varieties of Physicalism



We have been considering various answers to the completeness question,
namely, what it means to say that everything is physical. At this
point, it is worth considering two issues associated with completeness
that have not so far been brought to the surface, and which suggest
different varieties of physicalism. The first is whether physicalism
involves a kind of reductionism; the second is whether physicalism
involves what philosophers call a priori entailment or deducibility.



### 3.1 Reductive and Non-reductive Physicalism



The main problem in assessing whether a physicalist must be a
reductionist is that there are various non-equivalent versions of
reductionism.



One idea is tied to the notion of conceptual or reductive analysis.
When philosophers attempt to provide an analysis of some concept or
notion, they often try to provide a reductive analysis of the notion
in question, i.e. to analyze it in other terms. Applied to the
philosophy of mind, this notion might be thought of entailing the idea
that every mental concept or predicate is analyzed in terms of a
physical concept or predicate. A formulation of this idea is (9):



(9)
 Reductionism is true iff for each mental predicate *F*,
there is a physical predicate *G* such that a sentence of the
form ' *x* is *F* iff *x* is
*G*' is analytically true.


While one occasionally finds in the literature the suggestion that
physicalists are committed to (9) in fact, no physicalist since before
Smart (1959) has (unqualifiedly) held anything like it. Adapting Ryle
(1949), Smart supposed that in addition to physical expressions there
is a class of expressions which are topic-neutral, i.e. expressions
which were neither mental nor physical but when conjoined with any
theory would greatly increase the expressive power of the theory.
Smart suggested that one might analyze mental expressions in
topic-neutral (but not physical) terms, which in effect means that a
physicalist could reject (9).



A different notion of reduction derives from the attempts of
philosophers of science to explain intertheoretic reduction. The
classic formulation of this notion was given by Ernest Nagel (1961).
Nagel said that one theory was reduced to another if you could
logically derive the first from the second together with what he
called bridge laws, i.e., laws connecting the predicates of the
reduced theory (the theory to be reduced) with the predicates of the
reducing theory (the theory to which one is reducing). Here is a
formulation of this idea, where the theories in question are
psychology and neuroscience:



(10)
 Reductionism is true iff for each mental predicate *F*
there is a neurobiological predicate *G* such that a sentence
of the form '*x* is *F* iff *x* is
*G*' expresses a bridge law.


Once again, however, there is no reason at all why physicalists need
to accept that reductionism is true in the sense of (10). We noted
earlier that the possibility of multiple realization renders type
physicalism implausible; something similar is true in this case. Many
different neurobiological processes (whether in our own species or a
different one) could underlie the same psychological process --
indeed, given science fiction, even non-neurobiological processes
might underlie the same psychological process. If so, (10) seems to be
false. (For a classic presentation of multiple realization and
reductionism, see Fodor 1974, but for an alternative view, see Kim
1993).



A third notion of reductionism is more metaphysical in focus than
either the conceptual or theoretical ideas reviewed so far. According
to this notion, reductionism means that the properties expressed by
the predicates of (say) a psychological theory are identical to the
properties expressed by the predicates of (say) a neurobiological
theory -- in other words, this version of reductionism is in
essence a version of type physicalism or the identity theory. As we
have already seen, however, since a physicalism need not be a type
physicalism, it need not be reductionism in this metaphysical sense
either



A final notion of reductionism that needs to be distinguished from the
previous three concerns whether mental statements follow *a
priori* from non-mental statements. Here is a statement of this
sort of idea,



(11)
 Reductionism is true iff for each mental predicate *F*
there is non-mental predicate *G* such that a sentence of the
form "if *x* is *F* then *x* is
*G*" is *a priori*.


What (11) says is that if reductionism is true, *a priori*
knowledge alone, plus knowledge of the physical or non-mental truths
will allow one to know the mental truths. This question is usually
debated in the context of another, viz., the question of *a
posteriori* and *a priori* physicalism. So we now turn to
that question.


### 3.2 *A Priori* and *A Posteriori* Physicalism



We have been assuming that supervenience is necessary for physicalism,
even if it is not sufficient. But what follows from supervenience? One
thing that is usually thought to follow is that the physical truths of
the world entail *all* the truths; hence physicalism is true at
all (12) is true



(12)
 The physical truths entail all the truths.


Now suppose that *S* is a statement which specifies the
physical nature of the actual world and *S\** is a statement
which specifies the total nature of the world. (It might be that
neither *S* nor *S\** are expressible in languages we can
understand, but let us set this aside.) Then physicalism tells us in
addition that that: 



(13)
 *S* entails *S\**


Another way to say this is to say that if supervenience physicalism is
true, the following conditional is necessarily true: 



(14)
 If *S* then *S\**


Indeed, this is a general feature of supervenience physicalism: if it
is true then there will always be a necessary truth of the form of
(14). 



Now, if (14) is necessary the question arises whether it is *a
priori*, i.e. knowable independent of empirical experience, or
whether it is *a posteriori*, i.e. knowable but not
independently of empirical experience. Traditionally, every statement
that was necessary was assumed to be *a priori*. However, since
Kripke's *Naming and Necessity* (1980), philosophers have
become used to the idea that there are truths which are both necessary
and *a posteriori*. Accordingly many recent philosophers have
defended a *posteriori physicalism*: the claim that statements
such as (14) are necessary and *a posteriori* (cf. Loar 1997).
Moreover, they have used this point to try to disarm many objections
to physicalism, including those concerning qualia and intentionality
that we will consider below. Moreover, as we have just noted, some
philosophers have suggested that the necessary *a posteriori*
provides the proper interpretation of non-reductive physicalism.



The appeal to the necessary *a posteriori* is on the surface an
attractive one, but it is also controversial. One problem arises from
the fact that Kripke's idea that there are necessary and *a
posteriori* truths can be interpreted in two rather different
ways. On the first interpretation -- I will call it the
derivation view -- while there are necessary *a
posteriori* truths, these truths can be derived *a priori*
from truths which are *a posteriori* and contingent. On the
second interpretation -- I will call it the non-derivation view
-- there are *non-*derived necessary *a posteriori*
truths, i.e. necessary truths which are not derived from any
contingent truths (or any *a priori* truths for that matter).
The problem is that when one combines the derivation view with the
claim that (14) is necessary and *a posteriori*, one encounters
a contradiction. If the derivation view is correct, then there is some
contingent and *a posteriori* statement *S#* that
logically entails (14). However, if *S#* logically entails (14)
then (since 'If *C*, then if *A* then
*B*' is equivalent to 'If *C* &
*A*, then *B*') we can infer that the following is
both necessary and *a priori*:



(15)
 If *S* & *S#* then *S\**.


One the other hand, if physicalism is true, and *S* summarizes
the total nature of the world then
*S#* *is already implicitly included in S*. In other
words, (15) is simply an expansion
of (14). But if (15) is just an expansion of (14), then if (15) is
*a priori*, (14) must also be *a priori*. But that means
our initial assumption is false: (14) is not a necessary *a
posteriori* truth after all (see Jackson 1998). 



How might an *a posteriori* physicalist respond to this
objection? The obvious response is to reject the derivation view of
the necessary *a posteriori* in favor of the non-derivation
view. But this is just to say that if one wants to defend *a
posteriori* physicalism, one will have to defend the
non-derivation view of the necessary *a posteriori*. However,
the non-derivation view is controversial, so it is not something that
we can hope to solve here. (For discussion, see Byrne 1999, Chalmers
1996, 1999, Jackson 1998, Loar 1997, Lewis 1994, Yablo 1999, and the
papers in Gendler and Hawthorne 2004)


## 4. The Condition Question



Earlier we distinguished two interpretative questions with respect to
physicalism, the completeness question and the condition question. So
far we have been concerned with the completeness question. I turn now
to the condition question, the question of what it is for something
(an object, an event, a process, a property) to be physical.



This issue has received less attention in the literature than the
questions we have been studying so far. But it is just as important.
Without any understanding of what the physical is, we can have no
serious understanding of what physicalism is. After all, what does it
mean to say that everything is physical, as opposed to chemical or
financial. (The point here is quite general: if Thales says that
everything is water, we don't understand what he says unless he
says something about what water is. The physicalist is in the same
boat.)


### 4.1 The Theory and Object Conceptions of the Physical



So what is the answer to the condition question? If we concentrate for
simplicity on the notion of a physical property, we can discern two
kinds of answers to this question (see Stoljar 2001, Stoljar 2010,
Tiehen 2018). The first ties the notion of a physical property to a
notion of a physical theory, for this reason we can call it the
theory-based conception of a physical property:



**The theory-based conception**:
  

A property is physical iff it is the sort of property that physical
theory tells us about.



According to the theory-based conception, for example, if physical
theory tells us about the property of having mass, then having mass is
a physical property. (The theory-based conception bears some relation
to the notion of physical1 discussed in Feigl 1967; more explicit
defense is found in Smart 1978, Lewis 1994, Braddon-Mitchell and
Jackson 1996, and Chalmers 1996.)



The second kind of answer ties the notion of a physical property to
the notion of a physical object, for this reason we can call it the
object-based conception of a physical property:



**The object-based conception**:
  

A property is physical iff it is the sort of property had by
paradigmatic physical objects and their constituents.



According to the object-based conception, for example if paradigmatic
physical objects such as rocks or planets are solid or are located in
space or have mass, then the property of being solid or located in
space or having mass is a physical property. (The best examples of
philosophers who operate with the object-conception of the physical
are Meehl and Sellars 1956 and Feigl 1967; more recent defense is to
be found in Jackson 1998.)



Do these conceptions characterize the same class of properties? There
are a number of different possibilities here, but one that has
received some attention in the literature is that physical theory only
tells us about the dispositional properties of physical objects, and
so does not tell us about the categorical properties, if any, that
they have -- a thesis of this sort has been defended by a number
of philosophers, among them Russell (1927), Armstrong (1968),
Blackburn (1992) and Chalmers (1996). Hence, if physical objects do
have categorical properties, those properties will not count as
physical by the standards of the theory conception. On the other hand,
there seems no reason not to count them as physical in some sense or
other. If that is right, however, then the possibility emerges that
the theory- and the object-conceptions characterize distinct classes
of properties. 



A further distinction that can be made here, and which cross-cuts the
theory/object distinction, is between a more narrow and a more broad
conception of a physical property (see Chalmers 2015). A narrow
conception counts as a physical property only those within a
restricted range. On the theory conception, for example, these might
the properties expressed by fundamental physics. A broader conception
counts as physical property any property which is either a property in
the narrow sense (a narrow physical property, as we might call it) or
else bears the right kind of relation to narrow physical properties,
for example, is definable in terms of them, or is grounded in them.
There are lots of complications here, partly because there are
different ways of restricting the narrow class, and different ways of
extending such a class. Here we will understand the theory and object
conceptions in a narrower rather than a broad way.


### 4.2 Circularity



Along with the concepts of space, time, causality, value, meaning,
truth and existence, the concept of the physical is one of the central
concepts of human thought. So it should not be surprising that any
attempt to come to grips with what a physical property is will be
controversial. The theory and object conceptions are no different:
each has provoked a number of different questions and criticisms. In
the remainder of our discussion of the condition question, I will
review some main ones.



To begin with, one might object that both conceptions are inadequate
because they are circular, i.e., both appeal to the notion of
something physical (a theory or an object) to characterize a physical
property. But how can you legitimately explain the notion of one sort
of physical thing by appealing to another? 



However, the response to this is that circularity is only a problem if
the conceptions are interpreted as providing a reductive analysis of
the notion of the physical. But there is no reason why they should be
interpreted in that way. After all, we have many concepts that we
understand without knowing how to analyze (cf. Lewis 1970). So there
seems no reason to suppose that either the theory or object conception
is providing anything else but a way of understanding the notion of
the physical.



The point here is an important one in the context of the condition
question. Earlier we said that the condition question was perfectly
legitimate because it is legitimate to ask what the condition of being
physical is that, according to physicalism, everything has. But this
legitimate question should not be interpreted as the demand for a
reductive analysis of the notion of the physical. Consider Thales
again: it is right to ask Thales what he means by 'water'
-- and in so doing demand an understanding of the notion of water
-- but it is wrong to demand of him a conceptual analysis of
water.


### 4.3 Hempel's dilemma



One might object that any formulation of physicalism which utilizes
the theory-based conception will be either trivial or false. Carl
Hempel (cf. Hempel 1969, see also Crane and Mellor 1990) provided a
classic formulation of this problem: if physicalism is defined via
reference to contemporary physics, then it is false -- after all,
who thinks that contemporary physics is complete? -- but if
physicalism is defined via reference to a future or ideal physics,
then it is trivial -- after all, who can predict what a future
physics contains? Perhaps, for example, it contains even mental items.
The conclusion of the dilemma is that one has no clear concept of a
physical property, or at least no concept that is clear enough to do
the job that philosophers of mind want the physical to play. 



One response to this objection is to take its first horn, and insist
that, at least in certain respects contemporary physics really is
complete or else that it is rational to believe that it is (cf. Smart
1978, Lewis 1994 and Melnyk 1997, 2003). There is an element of truth
in this. It may be rational to believe that contemporary science is
true, even if not that it is complete. Nevertheless, it also seems
mistaken to *define* physicalism with respect to the physics
that happens to be true in this world. The reason is that whether a
physical theory is true or not is a function of the contingent facts;
but whether a property is physical or not is not a function of the
contingent facts. For example, consider medieval impetus physics.
Medieval impetus physics is false (though of course it might not have
been) and thus it is irrational to suppose it true. Nevertheless, the
property of having impetus -- the central property that objects
have according to impetus physics -- is a physical property, and
a counterfactual world completely described by impetus physics would
be a world in which physicalism is true. But it is hard to see how any
of this could be right if physicalism were defined by reference to the
physics that we have now or by the physics that happens to be true in
our world. (For development of this point, and for a dilemma that is
similar to Hempel's but which casts the issue in modal rather
than temporal terms, see Stoljar 2010; for discussion, see Baltimore
2013, Fiorese 2016)



A different response to Hempel's dilemma is that what it shows,
if it shows anything, is that a particular proposal about how to
define a physical property -- namely, via reference to physics at
a particular stage of its development -- is mistaken. But from
this one can hardly conclude that we have no clear understanding of
the concept at all. As we have seen, we have many concepts that we
don't know how to analyze. So the mere fact -- if indeed it
is a fact -- that a certain style of analysis of the notion of
the physical fails does not mean that there is no notion of the
physical at all, still less that we don't understand the
notion.



One might object that, while these remarks are perfectly true, they
nevertheless don't speak to something that is right about
Hempel's dilemma, namely, that for the theory-conception to be
complete one needs to know what type of theory a physical theory is.
Perhaps one might appeal here to the fact that we have a number of
paradigms of what a physical theory is: common sense physical theory,
medieval impetus physics, Cartesian contact mechanics, Newtonian
physics, and modern quantum physics. While it seems unlikely that
there is any *one* factor that unifies this class of theories,
perhaps there is a cluster of factors -- a common or overlapping
set of theoretical constructs, for example, or a shared methodology.
If so, one might maintain that the notion of a physical theory is a
Wittgensteinian family resemblance concept. However, whether this is
enough to answer the question of what kind of theory a physical theory
is remains to be seen. (For further discussion of Hempel's
dilemma, see the papers in Elpidorou 2018a.) 


### 4.4 The panpsychism problem



Hempel's dilemma against the theory-conception is similar to an
objection that one often hears propounded against the
object-conception (cf. Jackson 1998). Consider a version of
panpsychism according to which all the physical objects of our
acquaintance are conscious beings just as we are. (For further
discussion and different versions of this view, see
 Panpsychism.)
 Would physicalism be true in that situation? It seems intuitively
not; however, if physicalism is defined via reference to the
object-conception of a physical property then it is hard to see why
not. After all, according to that conception, something is a physical
property just in case it is required by a complete account of
paradigmatic physical objects. But this makes no reference to the
*nature* of paradigmatic physical objects, and so allows the
possibility that physicalism is true in the imagined situation.



One thing to say in response to this objection is that the mere
possibility of panpsychism cannot really be what is at issue here. For
panpsychism per se is not inconsistent with physicalism (cf. Lewis
1983). After all, the fact that there are *some* conscious
beings is not contrary to physicalism -- why then should the
possibility that *everything* is a conscious being be contrary
to physicalism? If so, what is at issue in the objection is not
panpsychism so much as the possibility that the paradigms or exemplars
in terms of which one characterizes the notion of the physical might
turn out to be radically different from what we normally assume in a
quite specific sense -- they might turn out to be in some
essential or ultimate respect mental.



Once the problem is put like that, however, the panpsychism problem
looks similar to a problem that arises in general whenever one one
tries to understand or define a concept in terms of paradigmatic
objects which fall under it, viz., that these definitions have certain
sort of empirical presuppositions that might turn out to be false.
Suppose one tried to define the concept red in terms of similarity to
paradigmatic red things, such as blood. Pursuing this strategy commits
one to the idea that the belief that blood is red is a piece of common
knowledge shared among all those who are competent with the term. But
that seems wrong -- someone who thought that blood was green
would be mistaken about blood but not about red. Now this problem is a
difficult problem, however -- and this is the crucial point for
our purposes -- the problem is also a quite general problem; it
arises because of the paradigm style of definition. So to that extent,
the concept of the physical does not seem to be any worse off than the
concept of red, the panpsychism problem notwithstanding. (For
discussion of the general strategy see Lewis 1997)



Of course, one would reject this entire line of thought if one
rejected its starting point, viz., that panpsychism is consistent with
physicalism. Wilson (2006, 78-9), for example, suggests that
while physicalism is consistent with the view that some conscious
beings exist, it is not consistent with the view that some fundamental
conscious beings exist, and it is this last claim that is definitive
of panpsychism. But in fact even that is consistent with physicalism,
though admittedly of an unusual sort. To illustrate, imagine a world
in which the fundamental properties are both mental and physical. That
is certainly a far-fetched scenario but it doesn't seem to be
impossible. Would physicalism be true in such a world? It is hard to
see why not; at least it may be true at that world that any physical
duplicate of it is a duplicate *simpliciter*. Would panpsychism
likewise be true at such a world? Again, it is hard to see why not,
since the fundamental properties instantiated at such a world are
mental, though of course they are also physical.


### 4.5 The via negativa



One idea that often emerges in the context of Hempel's dilemma
and the panpsychism problem, but deserves separate treatment, is the
so-called Via Negativa (see e.g. Montero and Papineau 2005, Wilson
2006, Fiorese 2016). 



The simplest way to introduce the Via Negativa is to interpret it as a
definition of the notion a physical property something like this:
*F* is a physical property if and only if *F* is a
non-mental property. But there are many reasons to resist such a
definition. Take vitalism. Vitalism isn't true, but it might
have been true; there is no contradiction in it for example. So
imagine a world in which plants and animals instantiate the key
property associated with vitalism, viz., elan vital. It seems
reasonable to say that in that case plants and animals instantiate a
property that is non-physical, i.e. elan vital is not physical.
And yet one should not say on this account that plants and animals
instantiate a mental property, i.e., elan vital is not mental.
In short, elan vital is neither mental nor physical. But the
Via Negativa as stated cannot accommodate that fact.



One might try to meet this objection by revising the Via Negativa so
that what is intended is only a partial definition along these lines:
*F* is a physical property only if *F* is non-mental. Even
so problems remain. As we have seen elan vital causes a problem
because it is neither mental nor physical. But there might be
properties that are both mental and physical. Consider a version of
the identity theory according to which being in pain just is c-fibers
firing. If we suppose that such a theory is true, is the property of
being in pain then mental or physical? Both presumably; but this could
not be true on the Via Negativa construed as a definition of what a
physical property is, even a partial definition. For if a property is
mental and physical, then, given the Via Negativa, it will be both
mental and non-mental which (of course) it can't be! Now
obviously, there are good questions about whether an identity theory
along these lines is or could be true, but regardless of whether it is
true, it should not be ruled out simply because of a proposal about
how to define the words in which it is stated. 



Alternatively, one might try to meet the objection by adopting what
Wilson 2006 calls the 'no fundamental mentality'
constraint. On this interpretation, what proponents of the Via
Negativa have in mind is that *F* is a physical property only if
*F* is not fundamentally mental, where in turn to be 'not
fundamentally mental' is most naturally understood as entailing
that if *F* is a fundamental property then it is non-mental. This
version of the view avoids the problem about having c-fibers since
presumably that property is not fundamental. But once again problems
remain. Take the world we considered above at which the fundamental
properties are both mental and physical; in effect, what applies to
c-fibers firing (if the identity theory is true) applies to the
fundamental properties instantiated at this world. As I said, this
scenario is far-fetched, but it doesn't seem to be impossible,
and it is certainly not impossible simply as a matter of the
definition of the words. And yet it would be impossible for that
reason if the 'no fundamental mentality' version of the
Via Negativa were true.



Of course, to raise these problems for the Via Negativa is not to deny
that there is something right about it. For example, when we think of
properties that would falsify physicalism we do often think of
\*certain\* mental properties, e.g., the distinctive properties of
ectoplasm or ESP. However, this fact--that certain mental
properties would, if instantiated, falsify physicalism--can be
captured without defining the physical in general as the non-mental. A
better way would be to require, of any spelling out of the notion of
the physical, whether it be the object-based account or the
theory-based account, that it respect that fact that some
(uninstantiated) mental properties are non-physical.


### 4.6 Structuralist Approaches



Another idea about how to define the physical that has become
prominent in recent times is a structuralist approach to the
physical.



One way to introduce the structuralist approach is to see it as a
development of the theory conception formulated above. On the theory
view, a property is physical just in case it is tied in the right way
to a physical theory. One problem that arises for such a view, as we
have seen, is what 'physical theory' is supposed to mean
here. For the structuralist (at least in the context;
'structuralism' can mean many different things), a
physical theory is one that employs a restricted vocabulary. One
particularly frank version of this sort of view is suggested by
Russell 1927; on this view, the vocabulary in question is restricted
to logical or mathematical vocabulary. Contemporary philosophers adopt
a less restricted view according to which the vocabulary is either
logical or mathematical or causal or nomological (i.e. pertaining to
laws) or some combination of these (see Alter 2016, Chalmers 2020,
Goff 2017; for criticism see Stoljar 2020). In effect, proposals like
this provide a topic-neutral conception of the physical, and hence a
topic-neutral conception of physicalism.



An attractive feature of this approach is that it provides an answer
to Hempel's dilemma and similar problems. Physics may indeed
change over time, but according to the structuralist, any physical
theory must be restricted to this sort of vocabulary. Hence we can
appeal to the notion of a physical theory to formulate a version of
physicalism on which everything supervenes on or is realised by or is
grounded in physical properties that can be expressed in that limited
vocabulary. (Here structuralism about the physical draws support from
structural realism in philosophy of science, a position to which it is
in some ways quite similar; see, e.g.
 Structural Realism)



However, a problem for structuralism (developed in Stoljar 2020) is
that, while placing restrictions of this sort on the notion of a
physical theory helps with Hempel's dilemma, it also seems
overly stringent. Physical theories on the face of it tell you lots of
things about the physical world which are not topic-neutral, for
example, about mass, energy, electrons, protons and a myriad other
things. The idea that we can capture all of this using a language that
employs *only* logical/mathematical or causal/nomic vocabulary
may seem an overly ambitious one. 



While there are problems with structuralist approaches to the
physical, as with the Via Negativa, there is also something right
about it. We have noted one idea in philosophy of science that is
similar to the structuralist approach to the physical, namely,
structural realism. A different idea that philosophers of science have
emphasised a lot in recent literature is that physicists and other
scientists often construct mathematical models of the systems they are
interested in; moreover, they often focus on the mathematical
properties of these models themselves (see, e.g. Weisberg 2013).
Perhaps a structuralist approach to such mathematical models is
plausible. Nevertheless, structuralism about the physical is a thesis,
not about these mathematical models, but about the target systems that
these models correspond to, and it precisely this that causes the
problem.


### 4.7 Physicalism as an Attitude



In view of the difficulties posed by Hempel's dilemma and
related problems, some philosophers have explored the interesting idea
that to be a physicalist is not to hold some thesis or belief -
that is, to hold something that may be true or false - but is
rather to adopt a kind of attitude or stance. As Alyssa Ney (2008, p.
9, see also Van Fraassen 2002) develops this "attitudinal"
view, for example, "physicalism is an attitude one takes to form
one's ontology completely and solely according to what physics
says exists". 



Now, as with other ideas we have looked at, there is certainly
something right about the attitudinal view. As we will see below,
contemporary physicalists are often methodological naturalists, and
methodological naturalists may well hold the attitude Ney describes.
Nevertheless, there is a major problem for the view, viz., that on the
face of it holding this sort of attitude is neither necessary nor
sufficient for being a physicalist. 



To see it is not necessary, consider such ancient philosophers as
Democritus or Lucretius. These philosophers are physicalists, or at
least are usually classified that way, i.e., since they held the
doctrine traditionally called 'materialism'. But they did
not hold the attitude Ney describes, either implicitly or explicitly,
for physics (at least identified sociologically) did not exist in
their day at all.



In response, one might adjust the attitudinal view so that the
'physics' towards which one holds the relevant attitude is
not identified sociologically, but is instead understood as a certain
sort of theory considered in the abstract. But then further problems
arise. First, it is now difficult to see the difference between
holding the relevant attitude and simply believing a thesis. If one
resolves to be guided in one's ontology by the truth of a
particular theory, how is that different from just believing the
theory? Second, if one holds an attitude toward a particular theory,
Hempel's dilemma seems to arise again though in a slightly
different form. For which physical theory is meant? If one means
current physics, as in fact Ney suggests, then one might argue that
this is not an attitude that physicalists should reasonably hold,
since current physics is incomplete; and if one means ideal physics,
it is hard to see what the content or nature of the attitude is.



To see that the attitude is not sufficient, imagine a situation in
which physics postulates properties or objects which are like those
postulated by traditional dualists; as Ney puts it imagine "it
is the year 3000 AD and physicists have been forced to introduce
irreducible mental entities into their theory." (2008, p. 12).
In such a situation, a person might hold the attitude Ney describes,
and yet intuitively not be a physicalist. 



In response, Ney agrees that this is a possibility but points out,
first, it would still be reasonable to criticize the people who hold
the attitude - for example, on the grounds that those who hold a
different attitude might have arrived at correct ontology more quickly
- and, second, that it doesn't follow that the attitude
definitive of physicalism is identical to the attitude definitive of
dualism. (The ideas underlying this second point are (a) if one adopts
the attitudinal view about physicalism then one should in fairness
adopt it about dualism as well; and (b) that from the fact that two
attitudes coincide in a possible situation it does not follow that
they are identical.) However, while both these suggestions might be
true, it is hard to see them as responding to the basic point that
person who holds the attitude Ney describes in the imagined situation
is not correctly described as a physicalist. In principle, after all,
such a person may be criticized in many ways; moreover, the fact that
holding a particular attitude is not sufficient for being a
physicalist does not entail that doing so is necessary for being a
dualist.


## 5. The Case Against Physicalism



Having provided an answer to the interpretation question, I now turn
to the truth question: is physicalism (as we have interpreted it so
far) true? I will first discuss three reasons for supposing that
physicalism is not true. Then I will consider the case for
physicalism.


### 5.1 Qualia and Consciousness



The main argument against physicalism is usually thought to concern
the notion of qualia, the felt qualities of experience. The notion of
qualia raises puzzles of its own, puzzles having to do with its
connection to other notions such as consciousness, introspection,
epistemic access, acquaintance, the first-person perspective and so
on. However the idea that we will discuss here is the apparent
contradiction between the existence of qualia and physicalism.



Perhaps the clearest version of this argument is Jackson's
knowledge argument; see
 qualia: the knowledge argument.
 This argument asks us to imagine Mary, a famous neuroscientist
confined to a black and white room. Mary is forced to learn about the
world via black and white television and computers. However, despite
these hardships Mary learns (and therefore knows) all that physical
theory can teach her. Now, if physicalism were true, it is plausible
to suppose that Mary knows everything about the world. And yet --
and here is Jackson's point -- it seems she does not know
everything. For, upon being released into the world of color, it will
become obvious that, inside her room, she did not know what it is like
for both herself and others to see colors -- that is, she did not
know about the qualia instantiated by particular experiences of seeing
colors. Following Jackson (1986), we may summarize the argument as
follows:




P1. Mary (before her release) knows everything physical there is to
know about other people. 



P2. Mary (before her release) does not know everything there is to
know about other people (because she learns something about them on
being released).



Conclusion. There are truths about other people (and herself) that
escape the physicalist story.




Clearly this conclusion entails that physicalism is false: for if
there are truths which escape the physicalist story how can everything
supervene on the physical? So a physicalist must either reject a
premise or show that the premises don't entail the
conclusion.



There are many possible responses to this argument, but here I will
briefly mention only three. The first is *the ability
hypothesis* due to Lawrence Nemerow (1988) and developed and
defended by David Lewis (1994). The ability hypothesis follows Ryle
(1949) in drawing a sharp distinction between propositional knowledge
or knowledge-that (such as 'Mary knows that snow is
white') and knowledge-how (such as 'Mary knows how to ride
a bike'), and then suggests that all Mary gains is the latter.
On the other hand, P2 would only be true if Mary gained propositional
knowledge.



A second response appeals to the distinction between *a priori*
and *a posteriori* physicalism. As we saw above, the crucial
claim of *a posteriori* physicalism is that (13) -- i.e.
the claim that *S* entails *S\** -- is *a
posteriori*. Since (13) is *a posteriori*, you would need
certain experience to know it. But, it is argued, Mary has not had
(and cannot have) the relevant experience. Hence she does not know
(13). On the other hand, the mere fact that Mary has not had (and
cannot have) the experience to know (13) does not remove the
possibility that (13) is true. Hence *a posteriori* physicalism
can avoid the knowledge argument. (It is an interesting question which
premise of the knowledge argument is being attacked by this response.
The answer depends on whether (13) is physical or not: if (13) is
physical, then the response attacks P1. But if (13) is not physical,
the response is that the argument is invalid.).



A third response is to distinguish between various conceptions of the
physical. We saw above that potentially the class of properties
defined by the theory-conception of the physical was distinct from the
class of properties defined by the object-conception. But that
suggests that the first premise of the argument is open to
interpretation in either of two ways. On the other hand,
Jackson's thought experiment only seems to support the premise
if it is interpreted in the one way, since Mary learns by learning all
that physical *theory* can teach her. But leaves open the
possibility that one might appeal to the object-conception of the
physical to define a version of physicalism which evades the knowledge
argument.



One of the most lively areas of philosophy of mind concerns the issue
of which if any of these responses to the knowledge argument will be
successful. (See the papers in Ludlow, Nagasawa, and Stoljar 2004. See
also
 Qualia: The Knowledge Argument)
 The ability response raises questions about whether know-how is
genuinely non-propositional (cf. Lycan 1996, Loar 1997 and Stanley and
Williamson 2001), and about whether it gets the facts right to begin
with (Braddon Mitchell and Jackson 1996). As against *a
posteriori* physicalism, it has been argued both that it rests on
a mistaken approach to the necessary *a posteriori* (Chalmers
1996, 1999, Jackson 1998), and that the promise of the idea is
chimerical anyway (cf. Stoljar 2000). The third response raises
questions about the distinction between the object and the theory
conception of the physical and associated issues about dispositional
and categorical properties, and also about the relation between
physicalism on the one hand, and a related view, sometimes called
Russellian monism. See
 Russellian monism,
 as well as Chalmers 1996, Lockwood 1992, Stoljar 2000, 2001, Montero
2010, 2015) 


### 5.2 Meaning and Intentionality



Philosophers of mind often divide the problems of physicalism into
two: first, there are the problems of qualia, typified by the
knowledge argument; second, there are problems of intentionality. The
intentionality of mental states is their aboutness, their capacity to
represent the world as being a certain way. One does not simply think,
one thinks *of* (or *about*) Vienna; similarly, one does
not simply believe, one believes *that* snow is white. Just as
in the case of qualia, some of the puzzles of intentionality derive
from facts internal to the notion, and from the relation of this
notion to the others such as rationality, inference and language; see
 Intentionality.
 But others derive from the fact that it seems difficult to square the
fact that mental states have intentionality with physicalism. There
are a number of ways of developing this criticism but much recent work
has concentrated on a certain line of argument that Saul Kripke has
found in the work of Wittgenstein (1982; see also
 Private Language).



Kripke's argument is best approached by first considering what
is often called a dispositional theory of linguistic meaning.
According to the dispositional theory, a word means what it does
-- for example, the word 'red' means red --
because speakers of the word are disposed to apply to word to red
things. Now, for a number of reasons, this sort of theory has been
very popular among physicalists. First, the concept of a disposition
at issue here is clearly a concept that is compatible with
physicalism. After all, the mere fact that vases are fragile and sugar
cubes are soluble (both are classic examples of dispositional
properties) does not cause a problem for physicalism, so why should
the idea that human beings have similar dispositional properties?
Second, it seems possible to develop the dispositional theory of
linguistic meaning so that it might apply also to intentionality.
According to a dispositional theory of intentionality, a mental
concept would mean what it does because thinkers are disposed to
employ the concept in thought in a certain way. So a dispositional
theory seems to hold out the best promise of a theory of
intentionality that is compatible with physicalism.



Kripke's argument is designed to destroy that promise. (In fact,
Kripke's argument is designed to destroy considerably more than
this: the conclusion of his argument is a paradoxical one to the
effect that there can be no such a thing as a word's having a
meaning. However, we will concentrate on the aspects of the argument
that bear on physicalism.) In essence his argument is this. Imagine a
situation in which (a) the dispositional theory is true; (b) the word
'red' means red for a speaker S; and yet (c) the speaker
misapplies the word -- for example, S is looking at a white thing
through rose-tinted spectacles and calls it red. Now, in that
situation, it would seem that S is disposed to apply 'red'
to things which are (not merely red but)
either-red-or-white-but-seen-through-rose-tinted-spectacles. But then,
by the theory, the word 'red' means (not red but)
either-red-or-white-as-seen-through-rose-tinted-spectacles. But that
contradicts our initial claim (b), that 'red' means red.
In other words, the dispositional theory, when combined with a true
claim about the meaning of word, plus a truism about meaning --
that people can misapply meaningful words -- leads to a
contradiction and is therefore false.



How might a physicalist respond to Kripke's argument? As with
the knowledge argument, there are many responses but here I will
mention only two. The first response is to insist that Kripke's
argument neglects the distinction between *a priori* and *a
posteriori* physicalism. Kripke often does say that according to
the dispositionalist, one should be able to 'read off'
truths about meaning from truths a physicalist can accept. (For a
proposal like this, see Horwich 2000.) One problem with this proposal
is, as we have seen, that its background account of the necessary
*a posteriori* is controversial. As we saw, *a
posteriori* physicalists are committed to what we called the
non-derivation view about necessary *a posteriori* truths. But
the non-derivation view has come under attack in recent times.



The second response is to defend the dispositional theory against
Kripke's argument. One way to do this is to argue that the
argument only works against a very simple dispositionalism, and that a
more complicated version of such a theory would avoid these problems.
(For a proposal along these lines, see Fodor 1992 and the discussion
in Braddon-Mitchell and Jackson 1996). A different proposal is to
argue that Kripke's argument underestimates the complexity in
the notion of a disposition. The mere fact that in certain
circumstances someone would apply 'red' to white things
does not mean that they are disposed to apply red to white things
-- after all, the mere fact that in certain circumstances
something would burn does not mean that it is flammable in the
ordinary sense. (For a proposal along these lines see Hohwy 1998, and
Heil and Martin 1998)



As with the knowledge argument, the issues surrounding Kripke's
argument are very much wide open. But it is important to note that
most philosophers don't consider the issues of intentionality as
seriously as the issue of qualia when it comes to physicalism. In
different vocabularies, for example, both Block (1995) and Chalmers
(1996) distinguish between the intentional aspects of the mind or
consciousness, and the phenomenal aspects or qualia, and suggest that
it is really the latter that is the central issue. As Chalmers notes
(1996; p. 24), echoing Chomsky's famous distinction, the
intentionality issue is a *problem*, but the qualia issue is a
*mystery*.


### 5.3 Numbers and Abstracta



A third problem, which we mentioned briefly above, is *the problem
of abstracta* (Rabin 2020). This concerns the status within
physicalism of abstract objects, i.e., entities apparently not located
in space and time, such as numbers, properties and relations, or
propositions.



To see the problem, suppose that abstract objects, if they exist,
exist necessarily, i.e., in all possible worlds. If physicalism is
true, then the facts about such objects must either be physical facts,
or else bear a particular relation (grounding, realisation) to the
physical. But on the face of it, that is not so. Can one really say
that 5+7=12, for example, is realised in, or holds in virtue of, some
arrangement of atoms and void? Or can one say that it itself is a
physical fact or a fundamental physical fact? If not, physicalism is
false: the property of being such that 5+7=12 obtains the actual world
but is neither identical to, nor grounded in or realized by, any
physical property. (Sometimes the problem of abstracta is formulated
as concerning, not abstract objects such as numbers or properties, but
the grounding or realization facts themselves; see, e.g, Dasgupta
2015. We will set this aside here.)



There are a number of responses to this problem in the literature; for
an overview, see Rabin 2020, see also Dasgupta 2015 and Bennett 2017;
for more general discussion of physicalism and abstracta, see Montero
2017, Schneider 2017, and Witmer 2017.



One response points out that, while the problem of abstracta confronts
many different versions of physicalism, it does not arise for
supervenience physicalism. After all, since numbers exist in all
possible worlds, facts about them trivially supervene on the physical;
any world identical to the actual world in physical respects will be
identical to it in respect of whether 5+7=12, because any world at all
is identical to the actual world in that respect! But the difficulty
here is that supervenience physicalism seems, as we saw above, too
weak anyway. Indeed, one might think that the example of abstracta is
simply a different way to bring out that it is too weak.



Another option is to adopt a version of nominalism, and deny the
existence of abstracta entirely. The problem with this option is that
defending nominalism about mathematics is no easy matter, and in any
case nominalism and physicalism are normally thought of as distinct
commitments.



A third view, which seems more attractive than either of the two
mentioned so far, is to expand the notion of a physical property that
is in play in formulations of physicalism. For example, one might
treat the properties of abstract objects as *topic-neutral* in
something like the sense discussed in connection with Smart and
reductionism above (see section 3.1). Topic-neutral properties have
the interesting feature that, while they themselves are not physical,
but are capable of being instantiated in what is intuitively a
completely physical world, or indeed what is intuitively a completely
spiritual world or a world entirely made of water. If so, it becomes
possible to understand physicalism so that the reference to
'physical properties' within it is understood more
correctly as 'physical or topic-neutral properties'. 



A final response to the problem does not expand the notion of the
physical as much as it restricts the scope of physicalism to
properties of a certain sort. One suggestion along these lines has
been made by a number of writers (e.g. Rabin 2020, following Dasgupta
2015) in the context of grounding physicalism, though perhaps the
underlying idea can be extended to other varieties as well. They
suggest that grounding physicalism, which we formulated above as (7),
should be revised to take the following form:



(7\*)
 Physicalism is true at a possible world *w* iff every
*groundable* property instantiated at *w* is either a
physical property or is grounded in a physical property.


Here, a property is groundable just in case it is apt for being
grounded, i.e. it is the sort of property that can be either grounded
or not. Ordinary psychological properties are presumably in this
class. If grounding physicalism is true, they are grounded in physical
properties, whereas if it is false, they are not grounded in physical
properties; either way, they are groundable properties. But
mathematical properties and the properties of abstracta more generally
might in at least some instances *not* be groundable, they may
fail to be properties that are apt for being grounded. If so, we have
an alternative solution to the problem of abstracta: (7\*) permits that
some properties of abstracta are not grounded in the physical, so long
as they are ungroundable. 


### 5.4 Methodological Issues



The final argument I will consider against physicalism is of a more
methodological nature. It is sometimes suggested, not that physicalism
is false, but that the entire 'project of physicalism'
-- the project in philosophy of mind of debating whether
physicalism is true, and trying to establish or disprove its truth by
philosophical argument -- is misguided. This sort of argument has
been mounted by a number of writers, but perhaps its most vocal
advocate has been Noam Chomsky (2000; see also Searle 1992, 1999).




It is easiest to state Chomsky's criticism by beginning with two
points about methodological naturalism. In general it seems rational
to agree with the methodological naturalists that the best hope for a
theoretical understanding of the world is by pursuing the methods
which are typical of the sciences. It would then seem rational as a
special case that our best hope for a theoretical understanding of
consciousness or experience is by pursuing the methods of the sciences
-- by pursuing, as we might put it, the naturalistic project with
respect to consciousness. So Chomsky's first point is that it is
rational to pursue the naturalistic project with respect to
consciousness.



Chomsky's second point is that the physicalist project in
philosophy of mind is on the face of it rather different from the
naturalistic project. In the first place, the physicalist project is,
as we have noted, usually thought of a piece of metaphysics. But there
is nothing obviously metaphysical about the naturalistic project, it
simply raises questions about what we can hope to explain. In the
second place, the physicalist project is normally thought of as being
amenable to philosophical argument, whereas it is unclear where
philosophical argument (if this is different from scientific argument)
would enter the naturalistic project. In short, there doesn't
seem anything particularly 'philosophical' about the
naturalistic project -- it simply applies the methods of science
to consciousness. But the physicalist project is central to analytic
philosophy.



It is precisely at the place where the physicalist project departs
from the naturalistic project that Chomsky's criticism begins to
take shape. For insofar as it is different from the naturalistic
project, there are a number of ways in which the physicalist project
is questionable. First, it is hard to see what the project might be
-- it is true that throughout the history of philosophy and
science one encounters suggestions that one might find out about the
world in ways that are distinct from the ones used in the sciences,
but these suggestions have always been rather obscure. Second, it is
hard to see how this sort of project could recommend itself to
physicalists *themselves* -- such a project seems to be a
departure from methodological naturalism but most physicalists endorse
methodological naturalism as a matter of fact. On the other hand, if
the physicalist project does not depart from the naturalistic project,
then the usual ways of talking and thinking about that project are
highly misleading. For example, it is misleading to speak of it as a
piece of metaphysics as opposed to a piece of ordinary science.



In sum, Chomsky's criticism is best understood as a kind of
dilemma. The physicalist project is either identical to the
naturalistic project or it is not. If it is identical, then the
language and concepts that shape the project are potentially extremely
misleading; but if it is not identical, then there are a number of
ways in which it is illegitimate.



How is one to respond to this criticism? In my view, the strongest
answer to Chomsky accepts the first horn of his dilemma and suggests
that what philosophers of mind are really concerned with is the
naturalistic project. Now, of course, what concerns them is not so
much the details of the project -- that would not distinguish
them from working scientists. Rather they are concerned with what the
nature of the project and what its potential limits might be.



This theme might be developed in several ways, but one well-known
development of it has been suggested by Thomas Nagel (1983) and
Bernard Williams (1985). According to them, any form of scientific
inquiry will at least be objective, or will result in an objective
picture of the world. On the other hand, we have a number of arguments
-- the most prominent being the knowledge argument -- which
plausibly show that there is no place for experience or qualia in a
world that is described in purely objective terms. If Nagel and
Williams are right that any form of scientific inquiry will yield a
description of the world in objective terms, the knowledge argument is
nothing less than a negative argument to the effect that the
naturalistic project with respect to consciousness will not
succeed.



If what is at issue is the limits of the naturalist project, why is
the debate so often construed as a metaphysical debate rather than a
debate about the limits of inquiry? In answer to this question, we
need to sharply divorce the background metaphysical framework within
which the problems of philosophy of mind find their expression, and
the problems themselves. Physicalism is the background metaphysical
assumption against which the problems of philosophy of mind are posed
and discussed. Given that assumption, the question of the limits of
the naturalistic project *just is* the question of whether
there can be experience in a world that is totally physical.
Nevertheless, when properly understood, the problems that philosophers
of mind are interested in are not with the framework themselves, and
to that extent are not metaphysical. Thus, the common phrase
'metaphysics of mind' is misleading.


## 6. The Case for Physicalism



Having considered one side of the truth question, I will turn finally
to the other: what reason is there for believing that physicalism is
true?



The first thing to say when considering the truth of physicalism is
that we live in an overwhelmingly physicalist or materialist
intellectual culture. The result is that, as things currently stand,
the standards of argumentation required to persuade someone of the
truth of physicalism are much lower than the standards required to
persuade someone of its negation. (The point here is a perfectly
general one: if you already believe or want something to be true, you
are likely to accept fairly low standards of argumentation for its
truth.)



However, while it might be difficult to assess dispassionately the
arguments for or against physicalism, this is still something we
should endeavor to do. Here I will review two arguments that are
commonly thought to establish the truth of physicalism. What unites
the arguments is that each takes something from the physicalist
world-picture which we considered previously and tries to establish
the metaphysical this of physicalism.



The first argument is (what I will call) *The Argument from Causal
Closure*. The first premise of this argument is the thesis of the
Causal Closure of the Physical -- that is, the thesis that every
event which has a cause has a physical cause. The second premise is
that mental events cause physical events -- for example we
normally think that events such as wanting to raise your arm (a mental
event) cause events such as the raising of your arm (a physical
event). The third premise of the argument is a principle of causation
that is often called the exclusion principle (Kim 1993, Yablo 1992,
Bennett 2003). The correct formulation of the exclusion principle is a
matter of some controversy but a formulation that is both simple and
plausible is the following:



**Exclusion Principle**
  

If an event *e* causes event *e\**, then there is no
event *e#* such that *e#* is non-supervenient on
*e* and *e#* causes *e\**.



The conclusion of the argument is the mental events are supervenient
on physical events, or more briefly that physicalism is true. For of
course, if the thesis of Causal Closure is true then behavioral events
have physical causes, and if mental events also cause behavioral
events, then they must supervene on the physical if the exclusion
principle is true. 



The Argument from Causal Closure is perhaps the dominant argument for
physicalism in the literature today. But it is somewhat unclear
whether it is successful. (For some discussion see,
 Mental Causation).
 One response for the anti-physicalist is to reject the second premise
and to adopt a version of what is called epiphenomenalism, the view
that mental events are caused by, and yet do not cause, physical
events. The argument against this position is usually epistemological:
if pains don't cause pain behavior how can it be that your
telling me that you are in pain gives me any reason for supposing you
are? It might seem that epiphenomenalists are in trouble here, but as
a number of recent philosophers have argued, the issues here are very
far from being settled (Chalmers 1996, Hyslop 1999). The crucial point
is that the causal theory of evidence is open to serious
counterexamples so it is unclear that it can be used against
epiphenomenalism effectively.



A different sort of response is to reject the causal principles on
which the argument is based. As against the exclusion principle, for
example, it is often pointed out that certain events are
overdetermined. The classic example is the firing squad: both the
firing by soldier A and by soldier B caused the prisoner's death
but since these are distinct firings, the exclusion principle is
false. However, while this line of response is suggestive, it is in
fact rather limited. It is true that the case of the firing squad
represents an exception to the exclusion principle -- an
exception that the principle must be emended to accommodate. But is
difficult to believe that it represents an exception that can be
widespread. A more searching response is to reject the very idea of
causal closure on the grounds, perhaps, that (as Bertrand Russell
(1917) famously argued) causation plays no role in a mature portrayal
of the world. Once again, however, the promise of this response is
more imagined than real. While it is true that many sciences do not
explicitly use the notion of causation, it is extremely unlikely that
they do not imply that various causal claims are true.



The second argument for physicalism is (what I will call) *The
Argument from Methodological Naturalism*. The first premise of
this argument is that it is rational to be guided in one's
metaphysical commitments by the methods of natural science. Lying
behind this premise are the arguments of Quine and others that
metaphysics should not be approached in a way that is distinct from
the sciences but should rather be thought of as continuous with it.
The second premise of the argument is that, as a matter of fact, the
metaphysical picture of the world that one is led to by the methods of
natural science is physicalism. The conclusion is that it is rational
to believe physicalism, or, more briefly that physicalism is true.



The Argument from Methodological Naturalism has received somewhat less
attention in the literature than the Argument from Causal Closure. But
it seems just as persuasive -- in fact, rather more so. For how
might one respond? One possibility is to reject its first premise. But
this is not something that most people are attracted to (or at least
are attracted to explicitly.)



The other possibility is to reject its second premise. However, if
physicalism can be clearly stated -- admittedly, this is a big
'if' -- it is not terribly clear what this would
amount to or what the motivation for it would be. In the first place,
our earlier discussion shows that physicalism is not inconsistent with
explanatory autonomy of the various sciences, so that one should not
reject physicalism merely because one can't see how to reduce
those sciences to others. In the second place, while it is perfectly
true that there are examples of non-physicalist approaches to the
world -- vitalism in biology is perhaps the best example --
this is beside the point. The second premise of the Argument from
Methodological Naturalism does not deny that other views are possible,
it simply says that physicalism is the most likely view at the moment.
Finally, one might be inclined to appeal to arguments such as the
knowledge argument to show that physicalism is false, and hence that
methodological naturalism could not show that physicalism is false.
However, this suggestion represents a sort of confusion about the
knowledge argument. As we saw above, if successful the knowledge
argument suggests, not simply that physicalism is false but that any
approach to the world that is compatible with methodological
naturalism is false. But if that is so, it is mistaken to suppose that
the knowledge argument gives one any reason to endorse
anti-physicalism if that is supposed to be a position compatible with
methodological naturalism.