## 1. Preliminaries


### 1.1 The Importance of Mental Causation



Mental causation--the mind's causal interaction with the
world, and in particular, its influence on behavior--is central
to our conception of ourselves as agents. Mind-world interaction
is taken for granted in everyday experience and in scientific
practice. The pain you feel when you sprain your ankle causes you to
open the freezer in search of an ice pack. An intention to go to the
cinema leads you to get into your car. Psychologists tell us that
mental images enable us to navigate our surroundings intelligently.
Economists explain fluctuations in financial markets by citing
traders' beliefs about the price of oil next month. In each
case, a mental occurrence appears to produce a series of complex and
coordinated bodily motions that subsequently have additional
downstream effects in the physical world. Instances of apparent mental
causation are so common that they often go unremarked, but they are
central to the commonsense picture we have of ourselves. It's
not surprising, then, that questions about the nature and possibility
of mental causation arise in a variety of philosophical contexts.



Ontology: Suppose you accept the "Eleatic Principle" that
power is the mark of being: to exist is to have causal powers
(Armstrong 1978, pp. 45-6; Oddie 1982). It's plausible to
think that if the mental has any causal powers at all, it can affect
the physical world. Without such powers, the mental faces ontological
embarrassment, even elimination.



Metaphysics: Mental causation is "at the heart of the mind-body
problem" (Shoemaker 2001, p. 74), often figuring explicitly in
how the problem is formulated (Mackie 1979; Campbell 1984; Crane
1999). To ask how mind and body are related just is, in part, to ask
how they could possibly affect one another.



Moral psychology: Agency of the sort required for
 free will
 and
 moral responsibility
 appears to require mental causation. If your behavior is not caused
by your mind's activities--its deliberations, decisions,
and the like--what sense would it make to hold you responsible
for what your body does? You would appear to be scarcely more than a
passive observer of your body's activities. We would then need
to abandon what
 Strawson
 (1962) calls our "reactive attitudes", the moral
attitudes and feelings (e.g., gratitude, resentment) so central to our
interpersonal lives.



Action theory: It is widely believed that psychological explanation
hinges on the possibility of mental causation. If your mind and its
states, such as your beliefs and desires, were causally isolated from
your bodily behavior, then what goes on in your mind could not explain
what you do (Davidson 1963; Mele 1992; for dissent, see
"noncausalists" such as Ginet 1990; Sehon 2005; Tanney
2013; and see the essays in D'Oro 2013). These observations
about agency suggest a more basic conceptual point: if minds did not
influence behavior, in what sense would anyone truly *act*?
Sounds would be made, but no one would mean anything by them. Bodies
would move, but no one would thereby do anything (Malcolm 1968; Horgan
2007).



Although each of the above points could be contested, collectively
they create pressure to address the problem of mental
causation--problem or *problems*: as will become clear,
there is more than one way in which puzzles about the mind's
causal efficacy can arise.


### 1.2 Is This an Empirical Issue?



At least since
 Hume,
 philosophers have assumed that causal questions are largely
empirical. We look to science to tell us, for example, the
moon's role in causing the tides, or smoking's
contribution to lung cancer: these are not considered philosophical
questions. It might seem equally obvious that the mind's causal
role in producing behavior is also a matter for science to settle. So
is it in fact the case that working scientists, and in particular,
psychologists, find it necessary to appeal to distinctively mental
phenomena to account for behavior? Is there evidence in neuroscience
that mental states and processes figure in the production of
actions?



Although most psychologists would without hesitation accept the causal
interaction of minds and bodies, a small but growing number of
empirical researchers have insisted that the evidence supports some
version of
 epiphenomenalism,
 the thesis that mental states, while caused by physical happenings,
exert no efficacy in return. Wegner, a psychologist, contends that
accumulated empirical evidence overwhelmingly supports
epiphenomenalism, at least with respect to conscious willing (Wegner
2002, 2004). He draws on influential work by Libet (1985, 2001, 2004)
and others to argue that conscious intending is itself a product of
nonconscious processes that do the real causal work, so that free will
is "an illusion". If Wegner and his colleagues are right,
these results could have ancillary implications for the physical
efficacy of mental states generally. (Note, that some dualists (e.g.
Lowe 2006; Gibb 2013) have appealed to the same work by Libet to
defend their own non-traditional models of psychophysical causation).




Because this research has received extensive treatment in recent work
on free will, we will not consider it further, but instead refer
interested readers to the sources cited above and to Mele 2014 for
critical discussion and references. Here we simply note that
traditional and contemporary attempts to assess the efficacy of mental
states have run up against philosophical difficulties as well,
difficulties that tend to overshadow the experimental evidence
accumulated thus far. In this sense, the efficacy of mind is quite
unlike that of, say, the moon or smoking. This will, we hope, become
clear in the discussion to follow.


## 2. The Problem of Interaction



Some historians (e.g., Matson 1966; King 2007) say the mind-body
problem is relatively recent, the most important source being
 Descartes's
 "real distinction" between mind and body. That said, you
can find topics closely related to mental causation in, for example,
Plato's *Phaedo* and Aristotle's *De Anima*,
and it might turn out that many features of the contemporary debate
are present in some form or other in pre-modern texts (Caston 1997).
Skirting such historical questions, we begin with Descartes, who, for
better or worse, set the agenda for modern discussions of mental
causation. The cluster of causal problems arising from the Cartesian
conception of mind is *The Problem of Interaction*.


### 2.1 What is the Mind-Body Nexus?



According to Descartes, minds and bodies are distinct kinds of thing,
or, in the technical terminology of the day, distinct kinds of
*substance*. Bodies, he held, are spatially extended
substances, incapable of feeling or thought; minds, in contrast, are
unextended, thinking, feeling substances: souls. (We use
"soul" with no theological implications to designate minds
considered in the Cartesian way as immaterial substances.) Despite
recognizing these deep differences, Descartes accepted the common
belief that mind and body causally interact: "Everyone feels
that he is a single person with both body and thought so related by
nature that the thought can move the body and feel the things which
happen to it" (in Cottingham et al. 1991, p. 228). But if minds
and bodies are so radically different, it is not easy to see how they
*could* interact. Descartes was well aware of the difficulty.
Princess Elisabeth of Bohemia puts it forcefully to him in a 1643
letter, pressing Descartes to tell her



> 
> how the human soul can determine the movement of the animal spirits in
> the body so as to perform voluntary acts--being as it is merely a
> conscious substance. For the determination of movement seems always to
> come about from the moving body's being propelled--to
> depend on the kind of impulse it gets from what sets it in motion, or
> again, on the nature and shape of this latter thing's surface.
> Now the first two conditions involve contact, and the third involves
> that the impelling thing has extension; but you utterly exclude
> extension from your notion of soul, and contact seems to me
> incompatible with a thing's being immaterial (in Anscombe and
> Geach 1954, pp. 274-5).
> 



Elisabeth is expressing the prevailing mechanistic view as to how
causation of bodies works: it must involve the cause's impelling
the body, where this requires contact between cause and effect. Since
a soul could never come into contact with a body--souls have no
spatial location--an immaterial soul could never impel, and so
could never causally interact with, a body.



Elisabeth's worries might seem quaint and outdated. Causal
relations countenanced by contemporary physics can take several forms,
not all of which are of the push-pull variety. Why shouldn't
soul-body interaction simply be included as another sort of
"non-mechanistic" causation (Richardson 1982)? But
Elisabeth's objection is in fact just one version of a more
general worry about soul-body interaction, a worry that rests on
the following thesis about causation:


* (CN)  Any causal
relation requires a *nexus*, some interface by means of which
cause and effect are connected.



Elisabeth presumes that when an effect is bodily motion, the required
nexus is spatial contact. But even if she is wrong about this (Garber
1983), (CN) nevertheless poses problems for the dualist: if contact is
not the mind-body nexus, what is?



One line of thought appeals to the transference theory of causality.
Here the idea is that identity--the persistence of something from
cause to effect--provides the needed link. If something in a soul
could become present in a body, this could bridge the immaterial and
material. Descartes himself appears to accept such a theory, declaring
in the Third Meditation that there could be nothing in an effect not
present in its total efficient cause (Descartes 1642/1996, p. 28). But
now the problem reasserts itself: if, as the substance dualist
insists, bodies and minds are radically different, they have no
properties in common. According to Descartes, a body's
properties are modes of extension, ways of being extended, while a
soul's properties are modes of something quite different,
thought or consciousness. If causation involved transference, a
Cartesian soul could not interact with a body (but see Hart 1988;
Hoffman and Rosenkrantz 1991).



Does a dualist need to accept (CN), however? The notion of a causal
nexus has come under criticism, often from philosophers working in the
Humean tradition (Blackburn 1990). More generally, (CN) and kindred
principles might be thought to rest on a conception of causality that
is now obsolete, finding no place in modern physics (for further
discussion, see
 the metaphysics of causation,
 SS2). But the next three versions of the problem can arise even
for those who reject the need for a causal nexus.


### 2.2 The Pairing Problem



A second version of the Problem of Interaction is the "Pairing
Problem" (Kim 1973, 2005; Sosa 1984; Foster 1991, ch. 6).
Imagine two exactly similar minds *M*1 and
*M*2 and the bodies *B*1 and
*B*2 to which they are "attached", that
is, the bodies with which they directly interact. In virtue of what is
*M*1 causally paired with *B*1,
and *M*2 with *B*2?



This is not the epistemological question of how we could know that
these are the pairings (although this is troublesome, too). The
question, rather, is metaphysical: in virtue of what are these the
pairings? If minds were, like bodies, located in space, causal pairing
could be achieved by the relative spatial locations of the substances
(Bailey et al. 2011). Particular minds might be inside or
"inhabit" particular bodies. But if minds are non-spatial
souls, relative spatial location is unavailable to fill the pairing
role. And since *M*1 and *M*2 are,
by hypothesis, exactly similar, we cannot appeal to the different
intrinsic properties that they might possess.



In reply, a dualist could appeal to "individualistic"
powers (Unger 2006, pp. 242-59; Foster 1991, pp. 167-8).
Powers are standardly thought of as powers to interact with some
*type* of object. A key has the power to open this lock, but
only by virtue of having the power to open any lock of this kind, the
power to open any intrinsically comparable lock. Individualistic
powers, in contrast, are powers possessed by an object to affect or be
affected by a *particular* object. Think of a key with the
power to open this lock, but without the power to open any
intrinsically indiscernible lock. Likewise, a soul could have the
power to interact with a particular body and no other. As the key
example suggests, however, it is by no means obvious that powers could
be individualistic in this sense (but see Audi 2011).


### 2.3 Conservation Laws



A third version of the Problem of Interaction appeals to conservation
laws. The leading idea is simple: Soul-body interaction would
have to change the amount of energy in the physical universe. When
souls act, new energy would appear in, say, the brain. When souls are
acted upon, some quantity of energy in the brain would vanish. But
either scenario would contravene established conservation laws, which
permit only the conversion and redistribution of energy (or
mass-energy) within the physical universe, not its addition or
subtraction.



This version of the problem has dogged dualism since the scientific
revolution (Lowe 1992; Papineau 2000), and a number of contemporary
philosophers present conservation as a major obstacle for dualists
(Fodor 1981; Dennett 1991, p. 35; Heil 2012, p. 26; Papineau 2000).
That said, turning the leading idea into a compelling argument has
proven difficult. First, the conservation laws do not dictate what
kinds of energy exist, only that they must operate conservatively.
Hence, if *sui generis* mental energy existed, as long as it
operated conservatively, this would be consistent with the
conservation laws. Appealing to this fact, Hart (1988) advances a
substance dualism and, combining it with a transference theory of
causation (SS2.1), argues that psychophysical causation consists
in the transfer of such psychic energy. (For arguments against the
existence of *sui generis* mental energy, see Papineau 2000).
Secondly, what is needed is a conservation law weak enough to have
been confirmed by physical science, but strong enough to preclude
soul-body interaction. Averill and Keating (1981) consider a
number of candidate "laws" and argue that none meets both
criteria. Thirdly, it's not clear in any case that a soul would
have to add energy to (or receive it from) the brain in order to
interact with it. Broad (1925, pp. 103-9) suggests a soul could
act merely by redistributing the brain's energy without changing
its quantity. Furthermore, Lowe (2000) and Gibb (2013) both advance
dualist models of psychophysical causation according to which the
mental does not affect the brain either by affecting the amount of
energy in it or by redistributing it. (For more recent discussion of
these and other complexities, see Montero 2006; Koksvik 2007; Gibb
2010.)


### 2.4 The Completeness of the Physical



A fourth version of the Problem of Interaction is related to the
third, but, because it is more prominent in the contemporary
literature, especially in some of the "property-based"
problems we examine below, we will develop this last version at
greater length. The first premise is:



*The Completeness of the Physical*: Every physical effect has a
sufficient physical cause.



When you trace the causal history of any physical effect--that
is, of anything physical that has a cause--you will never need to
appeal to anything non-physical. The physical universe contains within
itself the resources for a full causal explanation of any of its
(caused) elements, and in this sense is "complete". The
point applies, then, to whatever might occur to or within our bodies.
Any instance of bodily behavior has a sufficient physical cause, which
itself has a sufficient physical cause, and so on. In tracing the
causal history of what we do, we need never appeal to anything
non-physical.



This principle appears frequently in the mental causation literature
under a number of labels: most common are variations of
*Completeness of the Physical* (Crane 1995, 2001; Papineau
1993, 2000; O'Connor and Churchill 2010) or *Physical
Closure* (Crane 1992; Baker 1993; Melnyk 2003; Kim 2005).
We'll call it *Completeness* for short.



Labels aside, several versions of the premise appear in the
literature, and they can differ in strength. Note that the principle
as formulated says nothing about whether the non-physical can affect
the physical; a strengthened version prohibits this. (*Closure*
is sometimes reserved for this stronger principle: LePore and Loewer
1987; Kim 1998, p. 40; Marcus 2005; compare *Strong Causal
Closure* in Montero 2003.) An even more ambitious version blocks
the non-physical from being cause *or* effect; such is
suggested in Davidson's work (see SS5.1 and McLaughlin 1989,
who uses *Physical Comprehensiveness* for this thesis.) As for
weaker versions, *Completeness* could be limited to physical
effects within the human body without affecting its relevance to the
current topic. Note also that the principle is apparently committed to
deterministic physical causation; a weakened version permits
probabilistic causes. (For complications with such a weakening, see
Montero 2003, and for other challenges with formulating
*Completeness*, Lowe 2000; Gibb 2015.)



For simplicity, we stay with the principle as formulated at the
outset. Why think that it's true? Perhaps it is a conceptual
truth: for an effect to be physical is, at least in part, for it to
have a physical cause. This defense turns on the proper analysis of
the concept *physical*, itself the subject of a contentious
literature (see
 physicalism).
 Here we simply note that the principle does not seem analytic; it
appears to be a substantive, empirical claim about the causal
structure of the universe. (For more on the conceptual defense, see
Crane 1991; Papineau 1991, 1993, SS1.9; Lowe 1996, p. 56.)



It's natural, then, to look to science for a defense, and
especially physics (or physiology). Appeals to "current physical
theory" (Antony and Levine 1997, p. 100), "the development
of the sciences" (LePore and Loewer 1987, p. 630), and
"physics textbooks" (Melnyk 2003, p. 289) are common, but
what exactly in physical science supports the premise? An appeal to
the conservation laws (2.3) might be thought to generate one such
argument. Another argument is the no-gap argument. (See, for example
Melnyk 2003, pp. 288-90; Papineau 1993, pp. 31-32).
Physics has been hugely successful in identifying the causes of
various kinds of physical event. To do so physicists have only needed
to appeal to physical events. Not once have they had to appeal to
*sui generis* mental events. Without doubt the causal account
that physics provides of physical events contains gaps. But the
crucial point is that is highly unlikely that physics will ever need
to appeal to *sui* *generis* mental causes to fill these
gaps--or so proponents of the no-gap argument claim. A similar
no-gap argument can be presented at the level of neurophysiology.
(See, for example, Melnyk 2003, p.187).



We will look at challenges to *Completeness* in a moment, but
note for now that the premise by itself does not preclude the efficacy
of souls. Even if every physical effect has a sufficient physical
cause, some physical effects might have non-physical causes as well.
This latest version of the Problem of Interaction thus requires a
second premise:



*No Overdetermination*: There is no systematic
overdetermination of physical effects.



This principle enjoys wide support in the literature. It is said that
postulating systematic overdetermination in this context is
"absurd" (Kim 1993a, p. 281), one of the
"nonstarters" in the mental causation debate (Kim 1998, p.
65). But why? Perhaps it just looks like bad engineering (Schiffer
1987, p. 148). Or maybe the problem is that it would involve an
"intolerable coincidence" (Melnyk 2003, p. 291): every
time you act, there are two independent causal processes--one
from your brain, another from your soul--converging on the same
effect.



With the two premises now in place, the Problem of Interaction in our
final version is straightforward. Assume for *reductio* that
our souls routinely cause behavior. By *Completeness*, such
effects also have sufficient physical causes, so behavior is
systematically overdetermined. But this contradicts *No
Overdetermination*. The dualist's options would then seem to
be severely limited. One is to embrace epiphenomenalism, a doctrine on
which the mental, while caused by the physical, exerts no
"downward" causal influence in return. A more radical
option, parallelism, depicts bodies and souls as running in tandem,
with no causal influence in either direction.



The two premises can, however, be challenged. Start with
*Completeness*. Baker (1993), not herself a Cartesian dualist,
argues that if the principle threatens to undermine our ordinary (and
scientific) explanatory practices--many of which cite the
mental--it's *Completeness* that has to go.
Entrenched explanatory practices trump any abstract metaphysical
principles with which they might conflict (see also SSSS6.3,
7.5). Others argue that physical science, far from supporting the
principle, may in fact undermine it. Hendry (2006) finds indications
of "downward causation" in chemistry, while Stapp (2005)
culls evidence from contemporary physics suggesting that there are,
contrary to *Completeness*, causal gaps in the physical world,
gaps filled in by the mental (see also Sturgeon 1998; Davies 2006).
Emergentists in general deny the principle, either on scientific
grounds or by appeal to our conscious experiences of agency (see
 emergent properties,
 esp. SS4). And although the death of emergentism has been
declared more than once on empirical grounds (McLaughlin 1992;
Papineau 2000), the view continues to attract philosophers and
scientists. (See Wilson 2021 and the contributions to Clayton and
Davies 2006; Bedau and Humphreys 2008; Macdonald and Macdonald 2010;
Paoletti and Orilia 2017; and Gibb, Hendry and Lancaster 2019.)



*No Overdetermination* has been targeted as well. Mills (1996),
for example, defends mental-physical overdetermination as the
most plausible route for the dualist to take. Overdetermination is
plausible, the reasoning goes, if for any behavioral effect
*B*, both a non-physical (mental) cause *M* and physical
cause *P* satisfy the following counterfactual conditionals
(among others):


1. If *M* had occurred in the absence of *P*,
*B* would still have occurred.
2. If *P* had occurred in the absence of *M*,
*B* would still have occurred.



If the dualist can reasonably claim that (1) and (2) are true, this
will make a strong *prima facie* case for overdetermination.
Along different lines, Lowe (2003) presents a model of dualist
interaction on which, owing to systematic mind-body
dependencies, overdetermination is not the intolerable coincidence
worrying opponents of dualism. And more generally, the ban on
systematic overdetermination has come under increased scrutiny in the
context of the Exclusion Problem, to be discussed in SS6.


## 3. The Ascent to Properties



Cartesian dualism has fallen out of favor among philosophers and
cognitive scientists. There are, to be sure, non-Cartesian forms of
substance dualism that might have the resources to confront the
Problem of Interaction in its various guises (Hasker 1999; Lowe 2006).
But the dominant view today would appear to be that if the mind is a
substance at all, it is a physical substance--the brain, for
instance. This sort of "substance monism" is in fact a
consequence of the more general token identity theory: every concrete
mental particular (token) is physical. We will assume token identity
in what follows: minds, mental events, and any other mental
"objects" are physical (see
 the mind/brain identity theory).



What becomes of the Problem of Interaction on such a view? It would
seem to dissolve. While causation between brain and body is complex,
even to the point of being empirically inscrutable, it does not pose
the same problems as soul-body interaction. There are no special
philosophical problems with brain-body interaction, nor is there
anything especially odd or worrisome about an event in your brain
causing, say, your arm to go up. Any philosophical questions here
belong to
 the metaphysics of causation
 generally and have no special application to mental causation.



Nevertheless, philosophical worries about mental causation persist.
Theoretical and commonsensical considerations leading us to think the
mind or mental events cause behavior should also make us think that
they do so *as* mental, i.e., in virtue of their mental
 properties.
 Properties figure in causal relations (Kim 1973; Mackie 1974, ch. 10;
Armstrong 1989, pp. 28-9; Ehring 1997). Drop a square
paperweight into soft clay and it will produce an impression. The
shape of the impression can be traced to the shape of the paperweight,
the depth of the impression to the mass of the paperweight. Here shape
and mass are "causally relevant" or "causally
efficacious" properties. In particular, they are relevant to
certain properties of the impression. By contrast, other properties of
the paperweight, such as its color or value, appear to be irrelevant
to producing this kind of impression. Or consider a soprano who sings
a high note, thereby shattering a glass. The sound, we can suppose,
has a meaning--a semantic property--but it is the
sound's acoustic properties that are operative in producing the
shattering; the semantic properties play no causal role, at least not
with respect to this effect (Dretske 1989).



By themselves, these observations pose no special problem for the
philosopher of mind. While the notion of a causally relevant property
calls for analysis (Horgan 1989; Dardis 1993; Braun 1995), there is no
reason at the outset for a token-identity theorist to be especially
concerned about the efficacy of mental properties. Gus smiles because
of the way his food tastes, that phenomenal property; Lilian walks to
school along a particular route because of what she believes, that
representational property. Assuming the mind is something physical,
why should a mind's causing behavior in virtue of its mental
properties be any more puzzling than a paperweight's causing a
square impression in virtue of its shape?



Recent philosophical work on mental properties has revealed that
matters are not so simple, however. Mental properties are alleged to
have, not just one, but up to four features that make their efficacy
philosophically puzzling, no less problematic than mind-body
interaction is for the Cartesian dualist. These features will be
discussed in the following sections. Each feature makes it appear as
though mental properties, or some important family of them, are
irrelevant to the production of behavior. The threat is a form of
epiphenomenalism: even if minds and mental events are causes, they are
not causes *as* (or *qua*) mental.


## 4. Problem I: Property Dualism



This "new epiphenomenalism" (Campbell 1984, ch. 7)
immediately confronts a particularly strong version of
 property dualism,
 one insisting that mental properties are *sui generis*,
perhaps dependent on, but in no way reducible to the dispositional and
structural properties recognized by the physical sciences. Some
property dualists accord this status only to a certain class of mental
property, namely
 qualia,
 the "what it's like" features of conscious
experience. Other property dualists, including some
 emergentists,
 are willing to extend the thesis to all mental properties.



Suppose that this robust form of property dualism is true. Can mental
substances or events cause what they do *qua* mental, in virtue
of their mental properties? The arguments against soul-body
interaction, now couched in terms of properties, could enter again
here. For example, if you were worried about the mind-body nexus
for souls (SS2.1), it seems you should also wonder how
non-physical properties can find any traction in the physical world.
Similarly, *Completeness* (SS2.4) seems to lose none of its
attractiveness when formulated explicitly in terms of properties. You
could add to the principle a clause stipulating that a
"sufficient physical cause" is one that's sufficient
in virtue of its physical properties (see also SS5.4). Bring in
*No Overdetermination*, and the efficacy of mental properties
is again threatened. The arguments here and the responses to it are
structurally similar to those in SS2, so we will not pursue
further this version of the property-based problem. (Property dualism
also faces the Exclusion Problem, to be discussed in SS6.)


## 5. Problem II: Anomalous Monism



Another version of the property-based problem of mental causation can
be traced to
 Davidson's
 influential paper, "Mental Events" (Davidson 1970). There
Davidson defends an account of the mind-body relation he calls
 "anomalous monism",
 a view that at first appears to save mental causation, but in the end
might deny efficacy to mental properties.


### 5.1 The Argument for Anomalous Monism



At the core of anomalous monism are three principles:




*Principle of Causal Interaction*: Some mental events interact
causally with physical events.



*Principle of the Nomological Character of Causality*: Events
related as cause and effect fall under strict laws.



*Anomalism of the Mental*: There are no strict laws on the
basis of which mental events can be predicted and explained.




According to Davidson, the apparent tension among these principles
gives rise to the mind-body problem. Most of us unquestioningly
assent to the first principle. The second is more controversial,
although Davidson provides little argument for it. Here we just note
that it's not as strong as it seems, for "strict" is
not synonymous with "deterministic". A strict law is
exceptionless, but could be either deterministic or probabilistic.



The third principle is the most contested of the three. It rules out
strict laws in psychology; in particular--and most importantly
for present concerns--it rules out strict psychophysical laws,
that is, laws connecting the mental and physical. According to
Davidson, application conditions for mental predicates feature a
rationality constraint absent from the application conditions for
physical predicates. In ascribing beliefs to others, for instance, we
employ a principle of charity that counsels us to make these believers
as rational as possible. But this normative constraint has, as
Davidson puts it, "no echo" in the physical realm. In this
regard, mental and physical predicates are misaligned in a way that
precludes strict psychophysical laws.



Now the second two principles seem to rule out the first. If causation
requires strict laws, and there are no strict psychophysical laws, how
can the mental be causally efficacious? But Davidson notes there is a
way to save the first principle: as long as every mental event is
physical, the first principle is compatible with the other two. In
this way, the three principles entail *event monism*. At the
same time, Davidson's view entails *type dualism*, for
the anomalism of the mental (the third principle) precludes identities
between mental and physical types. Most philosophers find it natural
to say that types are properties, so Davidson is sometimes described
as a property dualist, a convenient label for the time being (but see
SS5.5).


### 5.2 The Charge of Epiphenomenalism



Davidson's property dualism, and the principle that lies behind
it, have led to a serious charge: anomalous monism robs mental
properties of any causal significance.



Suppose Gus decides to illuminate the room and subsequently flips a
switch, thereby turning on the light. In this case we have a cause
that, if Davidson is right, could be given both a mental and a
physical description, and an effect that has a physical description.
If this means that the cause has a mental property (in virtue of which
it satisfies a mental description) and a physical property (in virtue
of which it satisfies a physical description), we are faced with a
further question. Granting that the event with the mental property
*is* the event with a physical property, why should we think
that the mental property had anything at all to do with the
event's physical effect? Davidson's second two principles
appear to block such relevance. If all causal relations are subsumed
under strict laws, and if there are no strict psychophysical laws,
then any instance of mind-body causation is subsumed only by
physical laws. But then it looks as though only a mental event's
physical properties are relevant to what it causes. The mental
properties (or mental types) are causally irrelevant (see, e.g.,
Stoutland 1980; Honderich 1982; Sosa 1984; a review of this literature
is in McLaughlin 1989).


### 5.3 Counterfactual Dependence



LePore and Loewer (1987) look to counterfactuals to answer this charge
(see also Horgan 1989; LePore and Loewer 1989; Block 1990; Loewer
2007). The central idea is that anomalous monism permits physical
effects to depend counterfactually on mental properties. And such
dependence secures an important kind of causal relevance for the
mental, the sort that LePore and Loewer call "bringing
about". On their view, *a*'s being *F*
brings about *b*'s being *G* when the following
conditions are met:


1. *a* causes *b*.
2. *a* is *F* and *b* is *G*.
3. If *a* had not been *F*, *b* would not have
been *G*.
4. *a*'s being *F* and *b*'s being
*G* are logically and metaphysically independent.



Now suppose a mental event, such as a decision to turn on the light,
causes Gus to move his finger, thereby flipping the light switch. Here
the crucial counterfactual is: If the cause had not been a decision to
turn on the light, the effect would not have been a switch-flipping.
This is plausible, as are similar counterfactuals in a wide range of
cases. But are such counterfactuals compatible with anomalous monism?
LePore and Loewer say Yes: while Davidson prohibits *strict*
laws connecting mental and physical properties, he apparently leaves
room for non-strict laws. Such laws are enough to ground or
"support" counterfactuals. Consider, by analogy, the
properties of *being a match-striking* and *being a
match-lighting*. If there is a law connecting such properties, it
is evidently non-strict: striking causes lighting only *ceteris
paribus*. Nevertheless, we can assert with confidence, after a
given lighting, that if the match had not been struck, it
wouldn't have lit. Non-strict psychophysical laws would
similarly appear to ground counterfactuals connecting mental and
behavioral properties.



This counterfactual defense is attractive for a number of reasons. It
captures a sense in which mental properties make a difference to
behavior, but in a way that's apparently compatible with
anomalous monism. It respects our causal intuitions about a wide range
of cases. And it fits well with the more general
 counterfactual theory of causation,
 which many philosophers have found independently plausible. Moreover,
Davidson himself seems sympathetic to the defense (Davidson 1993; but
see SS5.5).



In spite of these advantages, a worry is that the pertinent
counterfactuals don't after all ensure causal relevance, and in
this sense don't vindicate anomalous monism. This objection can
take the form of direct counterexamples (Braun 1995; Garrett 1999),
but here we look at a broader concern.



When a counterfactual is true, there should be something in the world
that makes it true. Even granting that, if the cause had not had its
mental property, the effect would not have had its behavioral
property, in virtue of what is this true? This truthmaker, not the
counterfactual itself, is what matters in determining whether a
property is causally relevant. And the worry is that once we look at
the truthmakers in the mental case, the threat of epiphenomenalism
crops up again. Although the effect counterfactually depends on the
mental property, this is only because the mental property depends on a
physical property doing the real work. The mental property looks like
a freeloader (Kim 1998, pp. 70-3, 2007; compare Crane 2008 on a
similar issue).



LePore and Loewer discuss a version of this worry. Condition (3), an
objector might say, is too crude to test for causal relevance, for the
counterfactual holds only because removing *F* from *a*
also removes some other property *F\** of *a*, and
it's the absence of *F\** that's responsible for
*b*'s not being *G*. A better counterfactual test
evaluates the effect's status given that *a* is not
*F* *and* all of *a*'s other
properties--or at least all that are potential causal rivals to
*F*--are held fixed. If *b* is not *G* in
that case, only then can we credit *F* with causal relevance.
But mental properties fail this more refined test. Consider again
Gus's decision to turn on the light, and remove its mental
property, this time holding fixed its physical properties. It seems
clear that he would still flip the switch. After all, the physical
properties of the cause figure in an exceptionless law according to
Davidson. It looks as if the physical properties of your decision
"screen off" the mental property, making the latter
irrelevant.



LePore and Loewer concede that mental properties are screened off by
physical properties. But they argue that this more refined test is too
demanding, for it would also mean that the physical properties of a
mental cause are irrelevant. Note in particular that the
decision's mental properties screen off its physical properties:
if the cause had lacked its physical properties yet had still been a
decision to turn on the light, it would have caused Gus to flip the
switch (*ceteris paribus*: here a hedged law, which anomalous
monism permits, is in play). Screening off thus goes both ways, and
since few would want to deny causal relevance to the physical
properties, we should not let screening off impugn the significance of
mental properties either.



Antony (1991) replies that there is no symmetry here, at least not
given anomalous monism. While the decision's physical properties
screen off its mental properties, the reverse doesn't hold.
Suppose again that the cause had lacked its physical properties but
had still been a decision to turn on the light. On anomalous monism,
Antony argues, there's no saying what Gus's decision would
have caused, for mental properties, being anomalous, place no
constraints on the causal structure of the world. (See also Leiter and
Miller 1994.)



The freeloader problem arises in a variety of contexts in the mental
causation literature, not just in discussions of anomalous monism. It
will return under a number of guises in what follows.


### 5.4 Lawful Sufficiency



Fodor (1989) apparently agrees that counterfactuals capture a kind of
causal relevance, but he argues that LePore and Loewer have settled
for too little. On Fodor's view, mental properties can be
relevant to behavior in a stronger sense in which they are
*sufficient* for their effects and in this way "make a
difference". Fodor spells out sufficiency in terms of laws: a
property makes a difference if "it's a property in virtue
of the instantiation of which the occurrence of one event is
nomologically sufficient for the occurrence of another" (Fodor
1989, p. 65, note omitted).



Might such an account save anomalous monism from the charge of
epiphenomenalism? On the face of it, it cannot, for as we've
noted, mental properties on Davidson's view appear only in
hedged laws, laws that include an implicit *ceteris paribus*
rider. Consider a candidate psychological law:


* (L)  If an agent,
*a*, wants *x*, believes *x* is obtainable by
doing *y*, and judges *y* best, all things considered,
then *a* forms the intention to *y* and subsequently
*y*'s on the basis of this intention, *ceteris
paribus*.



The *ceteris paribus* clause here would seem to block the
mental properties in question from being causally sufficient for the
behavioral effect. But perhaps not: according to (L), the mental
properties are sufficient for the behavioral effect *when* the
*ceteris paribus* conditions are satisfied. And this sort of
causal sufficiency, Fodor argues, is all anyone could reasonably want
for mental properties.



But can Davidson help himself to such an account? Davidson appears to
think so (1993, p. 10), as does McLaughlin (1989), who also appeals to
hedged laws. Fodor, however, doubts his account is compatible with
anomalous monism; such doubts are developed by Antony (1991) and Kim
(1993b). The question turns largely on Davidson's reasons for
thinking the mental is anomalous, and on whether these reasons permit
him to appeal to hedged laws in the way the laws account requires.



Supposing anomalous monism is compatible with Fodor's account,
you might still wonder whether nomological sufficiency is enough for
causal relevance. An account of causal relevance in terms of laws is
natural given the tight connections between laws and properties (see
 laws of nature,
 SS3). But those sympathetic to Fodor's position might still
ask (as Fodor himself does) what the causal mechanism is in
mental-physical interactions. For example, it could turn out
that the reason psychophysical laws such as (L) hold is that mental
properties are themselves grounded in more basic, physical properties,
and that only the latter do genuine causal work: mental properties
again look like freeloaders (SS5.3), merely piggybacking on the
real bearers of causal powers (LePore and Loewer 1989; Block 1990;
Leiter and Miller 1994; Marras 2003).


### 5.5 The Ascent to Properties Reconsidered



Davidson replies to his critics in "Thinking Causes"
(Davidson 1993). In that paper he sometimes speaks favorably of
"causally efficacious" properties, and he helps himself to
both hedged laws and counterfactuals to secure the efficacy of mental
properties. But his considered position appears less conciliatory. He
clearly denies a crucial assumption of his critics, namely, that
causes do their causing in virtue of their properties. When an event
causes something, it doesn't do so *qua* this or that: it
just causes what it does, full stop. Were this so, none of the
property-based problems discussed here could get off the ground (Crane
1995; Campbell 1997; Gibb 2006).



Such a response seems to miss the point (Kim 1993b; McLaughlin 1993;
Sosa 1993). All parties in this dispute agree that mental events can
cause physical events. The difficulty is to understand how they could
do so in virtue of their mental (rather than their physical)
properties, how they could have physical effects *qua* mental.
The principle of the Nomological Character of Causation (SS5.1)
apparently requires that, when one event causes another, it does so in
solely virtue of its physical properties.



But Davidson is part of a
 nominalist
 tradition that rejects properties, at least as his critics conceive
of them. Davidson instead formulates anomalous monism in terms of
predicates and descriptions. An event is mental if it answers to a
mental predicate (that is, it can be picked out using a mental
description), physical if it answers to a physical predicate (it can
be referred to using a physical description). Davidson's critics
assume that if an event answers to both sorts of predicate, it
includes a mental property and a physical property. But Davidson
thinks about the mental-physical distinction as merely a
difference in description, not as the expression of an ontological
divide between kinds of property. For Davidson, then, it makes no more
sense to ask whether an event had a particular effect in virtue of
being mental or in virtue of being physical than it would to ask
whether its effect stemmed from its being described in English or in
German. (For further discussion, see Heil 2009.)


## 6. Problem III: Exclusion



While reflection on property dualism or anomalous monism can lead to
our next property-based problem, another route is by way of the
doctrine of
 non-reductive physicalism.
 Like the property dualist, the non-reductive physicalist holds that
mental properties are not physical. But unlike the property dualist,
the non-reductive physicalist insists on a strong dependence of the
mental on the physical: mental properties are "realized"
or "constituted" by physical properties. This strong tie
between the mental and physical is the subject of a large contemporary
literature, some of which we touch on below.


### 6.1 Functionalism and Multiple Realizability



Non-reductive physicalism in its current form grew out of
 functionalism,
 according to which mental properties are functional properties. To be
in pain, for example, is a matter of being in a state with a certain
causal profile, a state that's caused by tissue damage, and
causes certain overt responses (moans, attempts to repair the damage)
as well as other mental states (e.g., beliefs that one is in pain).
But, argue functionalists, it is most unlikely that we could identify
a single kind of physical state playing this role in every actual and
possible case of pain. Human beings differ in endless tiny
physiological ways: your neurological states, including states you go
into when you are in pain, probably differ subtly from another
person's. Human beings' neurological states, in turn,
differ from those of a cat or a dog, and perhaps dramatically from
states of an octopus. You might even imagine encountering aliens with
vastly different biologies, but to which you would unhesitatingly
ascribe pains.



Here we arrive at a core thesis of functionalism: states of mind are
 "multiply realizable".
 The property of being in pain can be realized in a wide variety of
physical (and perhaps non-physical) systems. A creature is in pain in
virtue of being in a state with the right sort of causal profile, some
sort of neurological state, say. But the property of being in pain
cannot be identified with this neurological state, because creatures
of other kinds can be in pain in virtue of being in vastly different
physical conditions. Functionalists often put this point by saying
that mental properties are "higher-level" properties,
properties possessed by objects by virtue of their possession of
appropriate "lower-level" properties, their realizers.


### 6.2 The Exclusion Problem



Now, however, we are again confronted with the threat of
epiphenomenalism. If mental properties are not physical, how
*could* they make a causal difference? Whenever any mental
(functional) property *M* is instantiated, it will be realized
by some particular physical property *P*. This physical
property is unproblematically relevant to producing various behavioral
effects. But then what causal work is left for *M* to do? It
seems to be causally idle, "excluded" by the work of
*P*.



This version of the problem of mental causation has appeared in
various guises. Much of the contemporary literature is inspired by
Malcolm 1968, especially as refined in Kim 1989, 1993c, 1998, 2005.
Whatever its precise formulation (cp. Shapiro and Sober 2007;
O'Connor and Churchill 2010; historical perspective is in
Patterson 2005), the Exclusion Problem has clear affinities with the
other problems we've looked at so far. Consider our claim that
the realizing property *P* must play a role in producing a
particular behavioral effect. This would seem to be justified either
by an appeal to *Completeness* (SS2.4) or to
Davidson's doctrine (SS5.1) that causal relations must fall
under strict (and so physical) laws. Moreover, the argument's
depiction of *P* and *M* as competing for causal
relevance--one must exclude the other--would seem to require
a principle such as *No Overdetermination* (SS2.4). And the
fundamental worry that *P* might exclude *M* looks
exactly like the freeloader problem that badgers mainstream attempts
to save anomalous monism (SSSS5.3-4).



In spite of these similarities, the Exclusion Problem is in one
important respect unique: unlike the problems we've looked at so
far, exclusion worries generalize to a wide range of phenomena outside
of the mental. Any properties, mental or otherwise, that are multiply
realizable in physical systems are threatened with causal irrelevance.
(For discussion of this and related issues, see Kim 1998, pp.
77-87; Noordhof 1999; Bontly 2001; Gillett and Rives 2001; Block
2003; Walter 2008.)



Some philosophers (e.g., Fodor 1989; Baker 1993; Shapiro 2010) take
this general nature of the problem to be an *encouraging* sign.
We happily accept biological, or meteorological, or geological
properties as causally significant despite their being distinct from
their physical realizers. Why then imagine that exclusion threatens
the efficacy of mental properties? Others turn this argument around,
insisting that the alleged efficacy of biological and other
"special science" properties is by no means sacrosanct
(Antony 1995). Causal powers we attribute to them must respect what
our best metaphysics tells us. And in any case, the central issue is
not so much *whether* mental properties (and the rest) are
causally relevant to the production of physical effects, but
*how* they *could* be (Kim 1998, pp. 61-2,
78-9; Antony and Levine 1997, p. 96; McLaughlin 2006). Even if
the Exclusion Problem, because it generalizes, does not tempt us to
embrace epiphenomenalism, it presses on us a responsibility to explain
how mental properties could play a causal role given that they appear
to be screened off by their physical realizers.



The Exclusion Problem is the subject of a large and still-growing
literature. In the next few sub-sections, we look at some of the main
lines of response, dividing them into three broad categories.


### 6.3 Autonomy Solutions



The Exclusion Problem presents us with a picture on which higher-level
mental properties compete with their lower-level physical realizers.
Physical properties are unproblematically relevant in the production
of behavior, and so mental properties must either find a way to do the
work that their realizers are already doing or face exclusion. But
some philosophers would insist that this picture is deeply misleading:
mental properties enjoy causal relevance in their own right and are
not threatened by exclusion from physical properties.



This "autonomy solution" (Jackson 1996, SS2) can take
a variety of forms. One version starts by observing that psychological
explanations--and more generally, explanations in the special
sciences--are in an important sense independent of physical
explanations. Psychological explanations typically abstract away from
details of lower-level implementation, appealing instead to their own
distinctive kinds and laws. Explanations in the special sciences can
thus proceed independently of those in the lower-level physical
sciences. If the structure of the causal order reflects these
explanatory practices, mental properties need not be threatened by
exclusion. Mental and physical causes can peacefully coexist.
(Variations on this theme appear in Dennett 1973; Baker 1993; Van
Gulick 1993; Garrett 1998; Hardcastle 1998; Marcus 2001; Menzies 2003;
Raymont 2003; Ross and Spurrett 2004; Woodward 2008; Zhong 2014; see
also SS7.5.)



This appeal to explanation can naturally lead to (though it does not
entail) another autonomy solution, the *dual explanandum*
strategy. The Exclusion Problem presents a mental (functional)
property *M* and its physical realizer *P* as competing
to be causally relevant to the same effect, namely a bit of behavior.
But *M* might not be threatened with exclusion if *M*
and *P* are causally relevant to different properties of the
effect. Return for a moment to the paperweight example from SS3.
The shape of the paperweight is relevant, not to the impression
*simpliciter*, but to the impression's *shape*. In
general, a causally relevant property is relevant to some particular
property of the effect (Horgan 1989). Perhaps, then, *M* and
*P* do not causally compete because they are parts of separate,
autonomous causal lines to different properties of the effect.



Consider one way this might work. Behavioral properties, just like
mental properties, appear to be multiply realizable. For example,
there is more than one way to hail a cab, many different physical
realizations of this kind of behavior. Now suppose a belief causes you
to hail a cab. In accordance with *Completeness*, some physical
property *P* of the belief is sufficient for your behavior. But
strictly speaking, *P* is relevant only to the particular
*way* in which you hailed the cab, the particular physical
realization of your hailing. What, then, is responsible for your
behavior's higher-level property of simply *being a
cab-hailing*? It's natural to suppose that it's a
higher-level property of your belief, namely, some mental property,
such as the belief's representational content. (For proposals
along these lines, see Yablo 1992; Thomasson 1998; Marras 1998; Crisp
and Warfield 2001; Gibbons 2006; Schlosser 2009; see also
SSSS7.3-4.)



A strength of autonomy solutions is that they secure a causal role for
mental properties without running afoul of *Completeness*, as
the physical realization of behavior is always matched with some
physical properties of its cause. But do autonomy solutions respect
*No Overdetermination*? Here matters are not as
straightforward. Autonomy solutions present us with two properties,
*P* and *M*, each sufficient for the behavioral effect.
It might seem as if the dual explanandum strategy avoids this
awkwardness, since *P* and *M* are relevant to different
properties of the effect. But even here, overdetermination threatens,
as the effect's behavioral property is produced twice: directly
by *M*, and indirectly by *P*, which produces the
behavioral property's physical realizer, which itself
necessitates the behavioral property.



Proponents of autonomy solutions might grant these points but claim
that such "overdetermination" is innocuous, far from the
"intolerable coincidence" threatening Cartesian dualist
accounts of mental causation (SS2.4), for the two causal lines
present are not independent. (The nature of overdetermination has
itself become the subject of a literature inspired, in part, by the
Exclusion Problem. See, e.g., Funkhouser 2002; Bennett 2003; Sider
2003; Walter 2008; Carey 2011; Bernstein 2016; Kroedel 2019.)


### 6.4 Inheritance Solutions



Autonomy solutions can make it appear that the causal powers of mental
properties "float free" of their physical realizers,
bringing to mind the doctrine of parallelism (for replies, see
Thomasson 1998; Marcus 2001, SS3.3). Some non-reductive
physicalists have accordingly looked to tie the causal powers of
mental properties more closely to those of their physical realizers.
The idea is that mental properties are so intimately related to their
realizers that the former "inherit" the causal powers of
the latter. The relation between levels is not one of rivalry, such
that the physical might exclude the mental, but one of cooperation.
Nor, moreover, does there seem to be any threat of overdetermination,
since the mental works *through* the physical. (Compare the
metaphor of "transparency" in Jackson 1996.)



On some versions of the inheritance solution, what the higher-level
mental property derives from its physical realizer is some weaker or
"lower-grade" form of causal relevance. For example,
Jackson and Pettit (1988, 1990) distinguish the robust "causal
efficacy" of physical properties from the weaker "causal
relevance" of higher-level properties. Causal relevance in this
sense is an explanatory notion: as one might put it, behavior is
*produced* at the physical level, but by being realized in the
physical, mental properties inherit an explanatory relevance they
wouldn't have otherwise. An advantage of such a view is that it
accords a derived form of relevance to mental properties, but in a way
that respects both the priority of physical causation embodied in
*Completeness* as well as the principle of *No
Overdetermination*. (For similar views, see Kim 1984; Levine 2001,
SS1.5; Segal 2009. Those who appeal to the counterfactual
dependence of behavior on the mental [SS5.3] might also fall into
this category. For an answer to the charge that counterfactual
dependence is "causation lite", see Loewer 2007, Menzies
2007.)



If such a weakening seems to amount to epiphenomenalism, you might
look for an inheritance solution on which mental properties are
efficacious in the same sense that their physical realizers are
(compare the "homogeneity assumption" in Crane 1995). How
can this be done without violating *No Overdetermination*?
Well, suppose that a mental property is, in spite of being distinct
from its physical realizer, *immanent in* this realizer;
*M*, that is, is somehow nothing over and above *P*. In
that case, any causal work done by *P* is, in a straightforward
way, inherited by *M*. Overdetermination is avoided because
*M*'s work is included in *P*'s.



The metaphysical details of such a picture matter. Otherwise,
"immanence", "nothing over and above", and the
like will turn into mere labels for that psychophysical relation, we
know not what, that solves the Exclusion Problem. Accordingly, several
promising lines of inquiry have been pursued. Mental and physical
properties are said to be related by, for example, the
determinable-determinate relation (Yablo 1992; critics include
Ehring 1996; Worley 1997; Funkhouser 2006), constitution (Pereboom
2002; critics include Ney 2007; Heil 2011), metaphysical necessitation
(Bennett 2003, 2008), physical explicability (Antony 1991), physical
implementation (Marras 2003), and grounding (Kroedel and Schulz
2016).



You might ask why any of these relations should secure the desired
solution. One thought is that if mental properties are immanent in
their physical realizers, the causal powers of a mental property are
included among those of its realizer. Consider again mental property
*M* and one of its realizers in a given instance, *P*.
Plausibly, *M*'s powers are included in
*P*'s. Both properties, for example, have the power to
cause a certain kind of behavior, but because of its greater
"specificity", *P* has in addition to this powers
that *M* lacks. Now in general we don't think that wholes
causally compete with, or are excluded by, their parts. When Gus steps
on Lilian's toe, his foot's causing Lilian discomfort
doesn't exclude Gus's causing her discomfort. Both Gus and
his foot coexist as causes, without competition and, we might add,
without overdetermination. A similar point could be made about
properties: if the causal powers of *M* are included in those
bestowed by *P*, then *P*'s causal relevance to
behavior, far from excluding *M*'s, *includes* it.
(Approaches along these lines have been developed in Antony 1999;
Shoemaker 2001, 2007; Wilson 1999, 2011; Clapp 2001; critical
discussions include Heil 1999, 2011; McLaughlin 2007; Kim 2010; Ney
2010; Audi 2012.)


### 6.5 Identity Solutions



Autonomy and inheritance solutions grant at least this much to the
Exclusion Problem: mental and physical properties are numerically
distinct, however intimately they are otherwise related. But a third
sort of strategy tries to undermine the argument at exactly this
point: any mental property just *is* its physical realizer. If
*M*=*P*, there's no question of one's
excluding the other, nor is there any mystery of how *M* can
work through *P*, for *M* and *P* are one and the
same.



This sort of psychophysical property identity would seem to be blocked
by the multiple realizability argument sketched earlier. But that
argument, in spite of its wide appeal, has come under attack from
several directions (see
 multiple realizability,
 SS2). For example, some (Kim 1992; Lewis 1994; Jackson 1995; Heil
2003) take the argument to show, not that mental properties are
distinct from their physical realizers, but that what we thought was
one kind of mental property is actually many. Pains realized by
different physical properties, in spite of having the same
*name* ("pain"), are different, though similar,
mental properties. There is no such property as pain
*simpliciter*, only pain-for-this-physical-structure and
pain-for-that-physical structure. Once such
"structure-specific" identities are allowed, we can say
that *M* (now just, say, pain-for-human beings) is identical
with *P*, *M*'s "realizer" in human
beings (replies include Fodor 1997; Block 1997; Marras 2003; Moore and
Campbell 2010).



This solution comes at a price: it forces us to abandon the belief
that pain is a single, natural kind. There is, however, a way to
preserve this doctrine while pursuing a strategy that's
otherwise similar to the one just sketched. The essential idea is that
"property" as we've used the term so far is
ambiguous. A property could be what characterizes an object (event),
or what unifies several objects as a "one across many".
Now suppose the characterizing properties are
 tropes:
 particularized properties, unique to each object. And suppose the
unifying properties are something else--call these
"types". If the mental "properties" that are
causally relevant to behavior are tropes, and the mental
"properties" mentioned in the multiple realizability
argument are types, there's no reason to think that this
argument rules out psychophysical property-identities in any way that
leads to exclusion worries. The *M*-trope and the
*P*-trope are one and the same trope falling under two types,
mental and physical. This proposal allows for a single type
*pain* shared by diverse creatures; it's just that this
type is not the same sort of entity (a trope) that's efficacious
in the production of behavior (Heil 1992; Robb 1997; Heil and Robb
2003; what appears to be a similar view is defended by Macdonald and
Macdonald 1986, 1995a; see also Whittle 2007.)



One worry about this proposal is that it appears to raise the
Exclusion Problem all over again, this time at the level of properties
(tropes). If a single property is both mental and physical,
*Completeness* and *No Overdetermination* force us to
say that it's efficacious only *qua* physical, not
*qua* mental. (For this and other criticisms, see Noordhof
1998; Raymont 2001; Gibb 2004; Macdonald and Macdonald 2006; Alward
2008; Maurin 2008; see Robb 2013 for some replies.)


### 6.6 Necessary Effects: A Deeper Problem for Functionalism?



Functionalism, along with any non-reductive theory of mind, faces the
problem just discussed. But even if exclusionary worries are finessed,
functionalism faces an additional and possibly more fundamental
problem.



As we noted earlier, functionalism characterizes states of mind
causally. To be in a given mental state is to be in a state with the
right sort of causal profile, a state bearing the right sorts of
relation to other states. Think of functional states as nodes in a
network of states, the identity of which depends on the relations they
bear to other nodes, and think of the realizers as occupants of these
nodes. All there is to a node is the potential causal relations it
bears to other nodes (not so for the occupants, which have intrinsic
properties). Suppose, then, that *F* and *G* are
functional properties--nodes in this network--and that all
there is to something's being *F* is its being a
*G*-causer. The resulting generalization, "*F*s
cause *G*s", is no doubt true, but it is vacuous,
equivalent to the generalization that *G*-causers cause
*G*s.



This appears to strip functional properties of their causal efficacy.
Why? One line of thought appeals to Hume's celebrated doctrine
that there can be no necessary connections between distinct
existences. A mental property and its would-be effect are distinct,
yet functionalism entails that they enjoy a necessary connection. On
the Humean doctrine, such a connection could not be causal. Another,
closely related, version of the problem requires that causal relations
be subsumed by empirical laws. But there are no such laws available
for functional properties if all of the relevant generalizations are
analytic and vacuous. (The foregoing argument in either version
threatens to generalize to all dispositional properties: see
  dispositions,
 SS6. For the problem aimed at functionalism in particular, see
Block 1990; Rupert 2006;
 functionalism,
 SS5.2.)



This argument echoes the *logical connection argument* advanced
in the 1950's and 60's against causal accounts of action
(e.g., Melden 1961, pp. 52-3). Given that reasons (desires,
intentions) are not logically distinct from the actions they
rationalize, reasons could not cause actions. In response, Davidson
(1963) noted that logical connections hold among predicates or
descriptions of events, not among events themselves. A cause could be
described in various ways, some of which will involve the effect:
consider "the cause of the fire caused the fire". This is
hardly informative, but it's not thereby false. And of course
the statement, far from precluding a causal relation, explicitly
asserts it. That said, if the claim is true, it should be possible to
identify the cause of the fire independently of reference to the
effect--as "the match's igniting", for
instance. In defense of his own causal theory of action, Davidson
argued that such a re-description of mental causes is always
available, at least in principle (see SS5.1).



But Davidson's saving move appears not to be available for the
functionalist, for in the case of functional states and properties, no
such independent descriptions are available, as the nature of a
functional property is exhausted by its place in the causal
network.



The functionalist has a number of options available, some of them
mirroring solutions to the Exclusion Problem (Rupert 2006 provides a
critical survey). For example, a functionalist could settle for a
weaker, explanatory role for functional properties, leaving causal
efficacy to the realizers of functional states (SS6.4; see, e.g.,
Segal 2009; compare Roth and Cummins 2014). Or a functionalist might
identify states of mind with their realizers (SS6.5); indeed, some
of the early functionalists were identity theorists (Lewis 1966, 1994;
Armstrong 1968/1993). This would permit the sort of re-description
that the more mainstream version of functionalism apparently blocks. A
third option is to look for non-vacuous, empirical generalizations
subsuming functional properties (Antony and Levine 1997). Yet a fourth
option rejects the Humean doctrine, permitting necessary connections
between a causally efficacious property and its effect. Such a
proposal would find a home in the more general "causal theory of
properties" defended by Shoemaker (1980, 1998) and others.


## 7. Problem IV: Externalism



Our final version of the property-based problem is restricted to
intentional mental properties, that is, properties in virtue of which
some mental states--propositional attitudes, perceptual
experiences, mental images, and so on--are about something,
properties in virtue of which mental states have representational
content. We assume here that
 externalism
 is true, so that the contents of representational states of mind
depend, not merely on intrinsic features of those states, but on
relations, in particular, on the causal, social, and historical
relations agents bear to their surroundings. In the simplest case,
Lilian is thinking about *water* (H2O) because she
stands in the right sorts of causal relation to water. The key move
here is to reject the idea that meaningful objects or states owe their
meaning to their intrinsic make-up alone.


### 7.1 How Could Content Make a Causal Difference?



The causally problematic feature for externalism is this contextual or
relational component of representational mental states. Suppose that
our mental representations are physical structures in the brain. Now
suppose with the externalist that the content of these representations
is determined, not just by our intrinsic features, but by context as
well. Lilian (or Lilian's brain) represents a tree in the quad
by going into state *T*. But *T* represents a tree in
the quad, not by virtue of *T*'s (or, for that matter,
Lilian's) intrinsic makeup, but by virtue of *T*'s
(and by extension Lilian's) standing in the right kind of
relation to the tree. The very same kind of state in a different
context (in the brain of someone in different circumstances) might
represent something very different--or nothing at all.



Now if the content of Lilian's thought that there is a tree in
the quad is "broad", if the significance of her thought
depends on factors outside Lilian's body, then it is indeed hard
to see how this content could figure in a causal account of her
actions, including Lilian's expressing her belief that there is
a tree in the quad by uttering the sentence, "There is a tree in
the quad". This is bad news for any attempt to explain why we do
what we do by reference to the contents of our thoughts.



Consider an analogy (Dretske 1998). Gus inserts a quarter into a
vending machine. The coin has a range of intrinsic qualities common to
quarters, but its being a quarter does not depend solely on these
intrinsic qualities: a quarter's intrinsic qualities would be
shared by a decent counterfeit. The coin's being a quarter
depends on its having the right sort of history: it was produced in a
United States mint. This is something the vending machine cares
nothing about. The machine reacts only to the coin's intrinsic
features. You might put this by saying that the coin affects the
machine, not *qua* quarter, but only *qua* possessor of
a particular kind of intrinsic makeup. (Vending machines are built to
take advantage of the contingent fact that objects with this intrinsic
makeup are almost always quarters.)



The worry is that we apparently operate, in important respects, as
vending machines do. We respond to incoming stimuli solely in virtue
of our intrinsic makeup and the intrinsic character of the stimuli.
But if our thoughts possess their content in virtue of our standing in
complicated environmental-social-historical relations to
our surroundings, it is hard to see how such contents could make a
causal difference in our psychological economy, how they could figure
in the production of behavior. Thoughts *have* contents, but
these contents could have no direct influence on the operation of
mental mechanisms (Stich 1978; Kim 1982; Fodor 1980, 1987, ch. 2,
1991; Jackson and Pettit 1988).


### 7.2 Intrinsic Causal Surrogates



One general line of response notes that whenever we explain a bit of
behavior by appeal to extrinsic content, there is a local, intrinsic
property available as a "causal surrogate" to produce the
behavior (Crane and Mellor 1990). Such a surrogate may be
neurophysiological or, as on
 computationalist
 views, a complex of "formal" or "syntactic"
properties of internal representations. Now by itself, this point
seems just to highlight the problem: if intrinsic surrogates are
always needed, all the more reason to reject the efficacy of content.
Some have indeed drawn such a lesson, concluding either that content
has no role to play in an explanatory psychology (Stich 1978, 1983),
or perhaps that psychological explanations appealing to content were
never causal to begin with (Owens 1993; see also the noncausalists
cited in SS1.1).



But this might be too hasty. Far from precluding the causal efficacy
of content, the surrogates might in fact play a role in ensuring it.
Note that while Lilian's intrinsic properties don't
guarantee the contents of her beliefs, her intrinsic properties are,
in her environment, reliably correlated with these contents--so
reliably, in fact, that content, in spite of being extrinsic, enters
into the *counterfactuals* or *laws* often thought to
ground causal efficacy. It seems clear, after all, that if Lilian had
not believed there was water in front of her, she would not have
extended her hand. This counterfactual could be secured by the fact
that Lilian's believing "There's water in front of
me" covaries with some internal state of her brain, but the
counterfactual, for all that, is still true. A similar point could be
made using (hedged) laws connecting content with behavior. The terrain
here in any case is similar to that explored earlier in
SSSS5.3-4, though the extrinsic nature of content
introduces its own complexities. (On the counterfactuals, see Mele
1992, ch. 2; Yablo 1997; on the laws, see Braun 1991; Fodor 1995.)



There's a more direct way that the intrinsic surrogates might
secure the efficacy of content: perhaps the surrogate properties
*are* content, or rather a kind of content. Distinguish narrow
from broad content. Think of narrow content as the content of a
representational state of mind minus its "broad"
components. Consider Lilian (or Lilian's brain) and an
intrinsically indiscernible brain in a vat wired to a supercomputer.
Grant that Lilian and the envatted brain entertain intrinsically
indiscernible thoughts with utterly different representational
contents. Now imagine that we could abstract a common element from the
contents of Lilian's and the brain's intrinsically
indiscernible thoughts. This element is their narrow content. Because
narrow content is something all intrinsic duplicates must have in
common, the hope is that such content could be the very intrinsic
properties that produce behavior.



The notion of narrow content might raise suspicion, however. Return to
the vending machine. The quarter Gus inserts in the machine has a
particular value owing to relations it bears to outside goings-on: it
was minted in the Denver mint. A counterfeit placed in the machine
could have the very same intrinsic makeup as the quarter, but it would
lack the quarter's value. It looks as though it is the
quarter's intrinsic makeup, not its value, that matters to the
operation of the machine. Now imagine someone arguing that a quarter
and an intrinsically indiscernible slug do in fact share a kind of
value: *narrow* value. Because narrow value accompanies an
object's intrinsic qualities, we need not regard narrow value as
epiphenomenal. But what could narrow value be? Whatever it is, could
it in any way resemble value ordinarily conceived--*broad
value*? Narrow value looks like a phony category posited *ad
hoc* to accommodate an otherwise embarrassing difficulty.
Nevertheless, some philosophers remain optimistic about the prospects
of a viable internalist account of content, one that would allow fully
fledged thoughts to have a role in the production of behavior. (For
references and further discussion, see
 narrow mental content.)


### 7.3 Reasons as Structuring Causes



Another, much different, attempt to preserve a causal role for content
can be found in Dretske 1988, 1989, 1993. So far we've assumed
that a behavioral event is distinct from the mental event that causes
it. On Dretske's view, however, behavior is a *process*
that includes, as a component, its mental cause. When mental event
*a* causes bodily movement *b*, the behavior in this
case is not *b* itself, but the process of *a*'s
*causing b*. When Lilian raises her hand because she wants to
get the teacher's attention and she believes that raising her
hand will accomplish this end, her behavior is not her hand's
going up, but the process of this belief-desire pair's causing
her hand to go up.



Dretske grants that when mental event *a* initiates
("triggers") a process ending in bodily movement
*b*, *a* does so solely in virtue of its intrinsic
makeup. Nevertheless, *a*'s relational, intentional
properties have a causal role, for they can be relevant to *the
fact that a causes b*. Reasons are "structuring
causes" of behavior: it's because of what *a*
indicates that it was "recruited" during the learning
process as a cause of *b*. (Indication here is a matter of
reliable co-variation.) It's because, for example,
Lilian's belief indicates what it does--raising one's
hand (in these circumstances) is a way to get the teacher's
attention--that it was (together with the relevant desire)
recruited as a cause of her hand-raising. Relational, intentional
mental properties thus become causally relevant to behavior, because
they are relevant to structuring the very causal processes that, on
Dretske's view, constitute instances of behavior.



Dretske's proposed solution quickly produced a number of
responses (e.g., Smith 1990; Block 1990; Baker 1991; Horgan 1991; Kim
1991; Mele 1991). One question is whether relational, intentional
properties in fact play a causal role in the structuring (or
"wiring") of causal processes in the brain. Even during
the learning process, the states of Lilian's brain would seem to
be sensitive only to local, intrinsic features of one another,
features that screen off external goings-on. Dretske might be able to
avoid such screening-off by appealing to the counterfactual dependence
of behavior-structuring on these goings-on. His view would then stand
or fall with the success of counterfactual theories of causal
relevance (SS5.3). A second question is whether intentional
states, even if they were relevant in the way Dretske says they are,
deliver the kind of causal relevance we want. When Lilian raises her
hand, the structuring of the relevant processes in her brain has
already occurred. If intentional properties are relevant at all, then,
they are apparently relevant only to what happened in the past during
the learning process. But we normally regard mental properties as
causally relevant to what's going on here and now, the very time
when Lilian (or anyone) acts (but cf. Allen 1995; Dretske replies to
critics in his 1991, esp. pp. 210-7; for a more recent
discussion see Hofmann and Schulte 2014).


### 7.4 Broad Behavior



Dretske's proposal is a version of the dual explanandum strategy
(SS6.3). The idea is that physical and mental properties are
causally responsible for different effects. For Dretske, the
(triggering) physical properties are responsible for bodily motions,
while the (structuring) mental properties are responsible for
behavior.



Another version of this strategy begins with a point also made in
SS6.3, namely that to question a property's causal relevance
is really to question its relevance to some property of the effect.
The form of our central causal question, that is, is whether a mental
cause *qua* *F* causes a behavioral effect *qua*
*G*. Now when *F* is an intentional mental property,
what *G* is the object of our question? One possibility is that
it is a behavioral property that, like the mental property, is itself
"broad" (see, e.g., Enc 1995).



Consider a simple example: Suppose Lilian believes that a glass in
front of her contains water, and this belief (together with her
desires) causes her to reach for the glass. Her behavior is an
instance of *trying to get water*, and it's the
instantiation of this property (and not, say, the property of being a
certain kind of bodily motion) that we're wondering about when
we ask whether the intentional property of her belief is causally
relevant. (If our interest lay solely in explaining a particular
bodily motion, we would rest content with a non-psychological, purely
physiological explanation.) But now the answer seems straightforward.
For what makes Lilian's behavior a trying for water is that
it's caused by a belief whose content concerns water. Once we
realize that the behavioral property of the effect is itself broad,
its connection to the intentional mental property seems clear.



This is not to say that the physical properties of Lilian's
belief do no work: it's just that they are responsible for a
different property of the effect, for instance, the property of being
a forward arm-movement. The intentional properties of her belief are
relevant to the effect *qua* (broad) behavior; the physical
properties are relevant to the effect *qua* (narrow) bodily
motion. And as we noted earlier (SS6.3), such a solution can be
employed in response to the Exclusion Problem as well. If a mental
property and its physical realizer are relevant to different
properties of the effect, they need not compete causally.



Because it promises to solve two outstanding problems of mental
causation, this approach is potentially quite powerful. (For
discussion, see Fodor 1991; Burge 1995.) One question to raise here,
however, is whether the fact that some behavior can be described
broadly makes the intentional mental property of its cause relevant.
The undeniable conceptual connections between mental and behavioral
descriptions might point to a kind of *explanatory* relevance,
but it's a further question whether causal connections grounding
these explanations involve broad properties. Those motivated by the
original epiphenomenalist arguments will worry that narrow, physical
properties are really doing all the work here: the apparent relevance
of the broad properties is an illusion created by the way we, in
describing and explaining behavior, conceptualize both cause and
effect (see Owens 1993). This point leads to a fourth, related
response to the problem.


### 7.5 The Appeal to Explanatory Practice



Some theorists would challenge the distinction--implicit in the
foregoing discussion--between explanation and causation. Our
concept of causality, they would insist, *is* bound up with the
concept of explanation: causally relevant properties are those that
figure in our best causal explanations (Segal and Sober 1991; Wilson
1992; Burge 1993; Raymont 2001; SS6.3). We find out what causal
relations amount to by starting with clear cases of causal
explanation. Given that we (and the cognitive scientists) routinely
explain physical events by citing mental causes (and mental events by
invoking physical causes), questioning whether *real* causal
relations answer to these explanations is to succumb to the kind of
metaphysical hubris that gives metaphysics a bad name.



This appeal to explanatory practice has the potential to answer in one
fell swoop all four of the property-based problems we've
considered.



Doubtless our understanding of the notions of causality and causal
relevance depends importantly on our grasp of causal explanations. But
there are at least two areas of concern about the explanatory strategy
(compare Kim 1998, pp. 60-7). First, you might wonder whether
the strategy addresses the right question. Earlier, we pointed out
that the central question of mental causation is not so much
*whether* mental properties are causally relevant but
*how* they could be, given some alleged feature of mental
properties (in the case at issue here, the feature is their being
relational properties). The explanatory strategy would at best seem to
be addressing only the "whether" question, not the
"how" question. Second, even when restricted to the
"whether" question, the strategy rests on a conflation of
what appears to be an epistemological notion (explanation) with
metaphysical notions (causation and causal relevance). A full
evaluation of the view would thus require a deeper look into how the
two are related.


## 8. Metaphysics and the Philosophy of Mind



We have been treating the problem of mental causation as though it
were a problem in applied metaphysics. Perhaps this approach is
wrong-headed. Perhaps the problem really falls under the purview of
the philosophy of science. What if we began with a look at actual
scientific practice (as suggested in SSSS6.3, 7.5) and
determined what exactly science requires for acceptable causal
explanation? An examination of established special sciences reveals
that the very features (multiple realizability, higher-level and
"broad" properties, for instance) metaphysically inclined
philosophers regard as posing apparently insuperable difficulties for
mental causation, are routinely invoked in causal explanations in
those sciences. This suggests that, rather than let *a priori*
conceptions of causation (or properties, or causal powers) lead us to
regard mental causation with suspicion, we should reason in the other
direction: revise our conception of causation to fit our actual
scientific beliefs and practices. If the metaphysicians were right
about causation, no science would be possible beyond basic physics
(biological properties, for instance, would lack causal efficacy).



This is one way to go. Another way is to take a step backward and ask
which features of our conception of the mental, features we commonly
take for granted, might be the source of our difficulties.
Eliminativists aside, all parties evidently agree that "realism
about the mental" requires that mental predicates figuring in
causal accounts of behavior designate distinctively mental properties.
If we aim to honor psychology (and the other special sciences), our
job is to show how these properties could be causally relevant to
physical goings-on. Suppose, in contrast, that you took the goal to
be, not the preservation of mental *properties*, but the
preservation of mental *truths*. In that case we would seek an
account of the mind that provides plausible truthmakers for
psychological and psycho-physical claims, including claims concerning
mental causation.



One possibility is that truthmakers for psychological truths include
irreducibly mental properties. This is not the only possibility,
however. Another is that psychological assertions are made true by
physical states and properties, states and properties answering to
predicates belonging to physics and chemistry. A view of this kind
(which is close to Davidson's as spelled out in SS5 and to
the identity solutions discussed in SS6.5) would endeavor to
resolve the problem of mental causation, not by tinkering with the
causal concept, but by rejecting the idea that mental and physical
properties are distinct kinds of property. All parties agree that
mental predicates and descriptions differ from physical predicates and
descriptions. Application conditions for mental terms and physical
terms diverge in ways that preclude definitional reduction of the one
to the other. Perhaps it is a mistake, however, to move from this
linguistic fact to a substantive ontological thesis: mental and
physical predicates designate properties belonging to distinct
families of properties.



Whether anything like this could be made to work is an open question.
To the extent that you regard the current state of play as
unsatisfying, however, it is perhaps a question worth pursuing.