## 1. Neutral Monism


### 1.1 Neutrality



Both of the terms that make up the label "neutral monism"
are problematical. Because the questions to which the notion of
neutrality gives rise are unique to neutral monism, they need to be
addressed first. Most versions of neutral monism assume a plurality of
basic, neutral entities (these could be substances, events, tropes,
universals, etc., depending on the specific form of the view). What
does it mean for an entity to be neutral? Here are five proposals:


1. *The Neither View:* A basic entity is neutral just in case
it is intrinsically neither mental nor physical.
2. *The Actual Constituent View:* A basic entity is neutral
just in case it is a constituent of both physical and mental non-basic
entities.
3. *The Possible Constituent View:* A basic entity is neutral
just in case it can be a constituent of both physical and mental
non-basic entities.
4. *The Law View:* A basic entity is neutral just in case both
mental laws and physical laws are applicable to it.
5. *The Both View:* A basic entity is neutral just in case it
is intrinsically both mental and physical.



(1)-(5) are not always clearly distinguished; but even when they
are, two or more of these criteria may be used concurrently. This
invites confusion on the part of the neutral monists, as well as their
critics.



Questions arise about the relationships between (1)-(5). (1) and
(5) are similar in letting the question of an entity's
neutrality be settled by its intrinsic nature. (2)-(4) answer
this question by looking at the relationships of a given entity to
other things: to the nature of larger groups of entities of which the
given entity is/can be a member: are these groups mental or material?
Or by looking to the kinds of laws that govern the entity in question:
are the laws mental or physical? If one is inclined to think that
neutrality is a matter of intrinsic nature alone, (2)-(4) will
seem misguided. (2)-(4) still might have a place as epistemic
criteria--as ways of finding out whether a given entity is
neutral. The friends of (1) and (5) can agree that a given
entity's ability to actually or possibly belong to a certain
group, or to be governed by certain laws, is due solely to its
intrinsic nature. Their disagreement concerns only the question
whether this ability is conferred on that entity by its being both
mental and physical or neither mental nor physical. In either case
(2)-(4) can only be used to discover which things are the
neutral entities.



The traditional versions of neutral monism--those developed by
Ernst Mach, William James, and Bertrand Russell--accept (1) and
reject (2). Russell, for instance, describes neutral monism as the
view that "both mind and matter are composed of a neutral-stuff
which, in isolation, is neither mental nor material" (Russell
1921: 25). But some critics of neutral monism, notably Bostock (2012:
190, 195-6), interpret neutral monism as committed to (2). Based
on this interpretation of neutrality, Bostock arrives at the striking
conclusion that "Russell's version of neutral monism was
never properly 'neutral' or 'monistic'"
(Bostock 2012: 190).



It can be argued that none of the traditional neutral monists accept
(3). All of them would agree that neutral entities can be a
constituent of both physical and mental non-basic entities. But it is
doubtful whether any of them would agree that the fact that a basic
entity can be a constituent of both physical and mental non-basic
entities is sufficient to make it neutral. For it is conceivable that
a basic entity that fails to be neutral (according to (1), the
*Neither View*) should figure in the construction of both
physical and mental non-basic entities.



(4) is present in the thought of Mach and James. But it plays a
prominent part in Russell's thought. In the opening chapter of
his 1921 *The Analysis of Mind*, Russell remarks:



> 
> There are, it seems to me, *prima facie* different kinds of
> causal laws, one belonging to physics and the other to psychology. The
> law of gravitation, for example, is a physical law, while the law of
> association is a psychological law. Sensations are subject to both
> kinds of laws, and therefore are truly "neutral" in
> [Edwin] Holt's sense. But entities subject only to physical
> laws, or only to psychological laws, are not neutral, and may be
> called respectively purely material and purely mental. (Russell 1921:
> 25-26)
> 



His use of (1) in conjunction with (4), especially in Russell 1921, is
confusing, and it has led a number of his critics (Stace 1946; Bostock
2012) to argue that his neutral monism is best understood as a form of
dualism (at least in his early writings).



A more general issue with criteria (2)-(4) is that they make
neutral monism compatible with materialism, idealism, and/or dualism,
as traditionally understood. So long as the basic entities, *no
matter what their intrinsic nature may be*, satisfy the criteria
specified in (2), (3), or (4) they are, according to these criteria,
neutral. Gregory Landini, for instance, assumes that (3) captures
Russell's idea of neutrality and concludes that he is a
physicalist *and* a neutral monist (Landini 2011: ch.6).
According to Landini, Russell is a neutral monist because every basic
entity can be a constituent of both physical and mental non-basic
entities; and Russell is a physicalist because all basic entities of
his system are physical.



The traditional neutral monists reject (5), but many of their
formulations suggest otherwise. It is easy to find passages saying
that a given neutral entity is both physical and mental:



> 
> If we admit--as I think we should--that the patch of color
> may be both physical and psychical, the reason for distinguishing the
> sense-datum from the sensation disappears, and we may say that the
> patch of color and our sensation in seeing it are identical. (Russell
> 1921: 143)
> 



A sympathetic reading of passages such as these suggests that they are
best understood as an abbreviated way of saying that the given neutral
entity is a member of two groups of neutral entities, one of which
counts as physical, the other of which counts as mental. In fact, many
neutral monists hold that the terms "physical" and
"mental" apply only to groups of neutral entities, not
individual ones:



> 
> The stuff of which the world of our experience is composed is, in my
> belief, neither mind nor matter, but something more primitive than
> either. Both mind and matter seem to be composite, and the stuff of
> which they are compounded lies in a sense between the two, in a sense
> above them both, like a common ancestor. (Russell 1921: 10-11)
> 



But a number of contemporary discussions of neutral monism employ the
*Both View*. This view has, for instance, been attributed to
Thomas Nagel (Godfrey-Smith 2013: 1-2; Skrbina 2005:
 237).[1]
 The *Both View* has the advantage of capturing one intuitive
notion of neutrality: that which is both mental and material is biased
towards neither and hence neutral between them.



Despite this appeal, the *Both View* idea of neutrality
conflates (or at least invites conflation of) neutral monism with
other theories from which it should be distinguished. Indeed,
depending on how the claim that the basic entities are intrinsically
both mental and physical is understood, neutral monism becomes
difficult to distinguish from panpsychism, dual-aspect theory, and
even the identity theory--since "being a pain" and
"being a c-fiber stimulation", for example, pick out the
very same property, aspect, or state rather than two distinct and
irreducible ones (see sections 4.2, 4.3, and 4.6 below). In contrast,
the *Neither View* idea of neutrality allows us to understand
neutral monism as a distinct type of theory, and so is in this respect
preferable.



There are, however, a number of complications shared by (1)-(5).
For one thing, they all use the concepts "mental" and
"physical" to specify the notion of neutrality. And since
these terms are used in different ways, the notion of the neutral
varies accordingly. In the case of the term "mental", most
neutral monists understand it as picking out entities in terms of
features traditionally associated with mental phenomena, such as
intentionality, experientiality, subjectivity, qualitativity, unity,
purposiveness, and so on. But there are diverging views among neutral
monists about which of these features are unique to and/or possessed
by mental phenomena. Similarly, there is no consensus about whether
the term "physical" is restricted to those aspects of
reality that can be fully captured by the descriptions of physics or
extends beyond such descriptive resources (Stoljar 2001). And while
some use the terms "material" and "physical"
interchangeably, others see "material" as applying to
entities with the features traditionally ascribed to matter (such as
occupying space, crowding out other such entities, and enduring
through changes in features) whereas "physical" might pick
out entities lacking one or more of them. Most pressingly, if
"mental" and "physical" are defined as
complements of each other, thereby ruling out the possibility of
entities to which neither term applies, then there cannot *be*
any neutral entities (at least in the *neither sense*).



Another worry is that (1)-(4) don't say anything about the
nature of the neutral entities. (1) says only what they are not;
(2)-(4) specify certain roles that neutral entities can play,
without telling us anything about the nature of the role occupants.
(5) may fare better in this regard; but it arguably cannot distinguish
basic *neutral* entities from those of panpsychism, dual-aspect
theory, or the identity theory. Some versions of neutral monism
embrace this agnosticism about the nature of the neutral base. But
others have been more forthcoming, offering examples of neutral
entities with which we are familiar.



The three traditional neutral monists--Ernst Mach, William James,
and Bertrand Russell--all pointed to entities encountered in
experience as examples of neutral phenomena. Mach speaks of
"sensations", James of "pure experience", and
Russell of "sensations" and "percepts".
Predictably, this has fueled the suspicion that neutral monism is
nothing but rebranded phenomenalism. Others have sought the neutral
base in the realm of the abstract. Kenneth Sayre, for example, turns
to the mathematical theory of information and suggests that the
neutral base is to be understood as the "ontology of
informational states" (Sayre 1976: 16) that this theory
presupposes (see section 5.1 below). What is clear is that it is no
easy task to characterize basic entities in a way that is both
substantive and incontrovertibly neutral.


### 1.2 Monism



There is, then, considerable disagreement about how to interpret the
notion of neutrality. The notion of monism raises its own problems.
But as these are questions faced by all forms of monism, they are,
perhaps, not quite as pressing. As with the case of the notion of
neutrality employed, it is possible to distinguish different versions
of neutral monism based on the various ways they understand the notion
of monism (see Schaffer 2016). Neutral monism might, for example, be
interpreted as a form of existence monism--the view that only a
single concrete thing exists; but it might also interpreted as a
monism about the kinds of things there are--the view that
concrete things, be they simple or complex, are of the same neutral
kind. The evidence suggests that all past and present versions of
neutral monism have been pluralist on both these counts: they work
with a plurality of basic neutral entities; and, given their focus on
the mind-body problem, they recognize at least mental and physical
kinds of entities in addition to the neutral ones (though some
describe them as useful fictions). All entities belonging to such
non-neutral kinds must, however, count as derivative. In the final
analysis, all entities must be basic neutral entities or complexes of
such entities.



The talk about basic entities indicates another pluralist dimension:
the known versions of neutral monism work with a layered picture of
reality. Basic neutral entities form the bottom level of the
hierarchy; the non-basic, non-neutral entities at the higher levels
are composed of, or are in some sense derivative from, those lower
level neutral entities. So we are dealing with an ontology that
recognizes a plurality of levels or layers in reality.



And even within the level of the basic neutral entities we can
distinguish those versions of neutral monism that hold that all basic
entities belong to a single category, from those that distinguish two
or more categories of neutral entities. Russell's later version
of neutral monism seems to be monistic (in this particular sense): all
neutral entities are
 events.[2]
 But it is easy to envision a neutral monism that countenances neutral
substances and neutral properties among the basic neutral
 entities.[3]


### 1.3 Mind and Matter Revisited



Most extant versions of neutral monism seem to acknowledge that, in
addition to the basic neutral entities, there exist derivative,
non-basic, and non-neutral ones--mental and physical entities,
for example. This raises the question about the relationship between
these two kinds of entities. Every answer to this question faces two
challenges. On the one hand there is the threat of elimination of the
non-basic entities (see section 4.1); on the other hand there is the
threat that the non-basic, non-neutral entities turn into elements of
ultimate reality, thereby putting an end to neutral monism.



The question of the relationship between basic and non-basic entities
is a contested topic among the traditional neutral monists (Wishon
2021). James holds that "pure experience" constitutes an
interconnected continuum out of which different portions become
"mental" or "material" depending on their
context, their relations to other such portions, and the purposes they
serve and/or effects they result in (see 2.2 below). In contrast, Mach
and Russell arguably hold that the non-basic entities are composed
of--or perhaps merely practical groupings of--a multitude of
discrete neutral entities organized by physical and/or psychological
causal-functional relations.



Most contemporary neutral monists agree that the basic neutral
entities are related to the non-basic, non-neutral ones by way of
grounding relations. But many questions remain about the precise
nature of these grounding relations and of their relata. In any case,
neutral monists face the daunting challenge of explaining (or
explaining away) the features associated with both mental and physical
entities solely in terms of the basic neutral ones, their aspects, and
their relations.


## 2. Traditional Versions of Neutral Monism



Baruch Spinoza (1632-77) and David Hume (1711-76) are
often identified as the originators of neutral
 monism.[4]
 The case for including Spinoza runs as follows:



> 
> Spinoza's metaphysics of substance has been called *neutral
> monism;* it is a form of monism because it allows for only one
> substance, and it is neutral because he describes the one substance as
> both a body and a mind. (Rosenkrantz and Hoffman 2011: 287)
> 



To classify Spinoza as an existence monist may be permissible. But the
neutrality claim is based on the problematical *both view* of
neutrality. This suggests that it is best to classify Spinoza as a
double aspect theorist. Hume's case is more difficult to assess.
It has been said that the prevalent 20th century
interpretation of Hume has been neutral monist (see Flage 1982: 527).
H.H. Price's case for this view has been particularly
influential (see Price 1932: 105-6). On the other hand, the
neutral monist interpretation of Hume has been vigorously resisted
(Flage 1982; Backhaus 1991).



In light of these difficulties it is best to start the genealogy of
neutral monism with the *big three*: Ernst Mach, William James,
and Bertrand Russell. Of the three, only Russell uses the label
"neutral monism" (a term seemingly coined by his teacher
James
 Ward).[5]
 But there is widespread agreement that Mach, James, and Russell are
the three most important philosophers in this tradition. A striking
feature of traditional neutral monism--a reflection of the
empiricist leanings of its protagonists--is the close tie between
the neutral entities and experience. This connection has given rise to
one of the most enduring criticisms of neutral monism: that it
collapses into phenomenalism. It is therefore important to note that
this salient feature of traditional neutral monism need be no part of
it.


### 2.1 Ernst Mach



Ernst Mach (1838-1916) occupies a central position in the
history of neutral monism. He influenced William James and Bertrand
Russell and, through them, all of the writers on neutral monism in the
English-speaking world. His importance for the development of the view
in the German-speaking world is hard to overestimate. Among the
philosophers to build on Mach's ideas was Rudolf Carnap in his
*Aufbau* (1928). As a physicist who also did physiological and
psychological research, Mach strove to adopt an inclusive and
economical framework that would allow him to pursue all of these
inquiries in a unified and coherent fashion. In the simple components
of experience--colors, sounds, pressures, intensities, times,
spaces, motor sensations, etc.--he finds typical examples of a
much larger group of elements whose functional interrelations are
studied by the various sciences (see Banks 2014, ch. 1 on the resolute
realism of Mach's view). While a given element is,
intrinsically, neither mental nor physical, the various groups to
which it belongs may display functional relationships that are
characteristic of physics or of psychology. In this case the neutral
element forms part of the subject matter of physics and of psychology,
respectively. In the following quotation Mach uses a color and our
perception of it to illustrate this point. A single neutral
element--the color--gets to be both the physical color of a
physical object and our mental perception/sensation of it. The color
can be called physical, *qua* constituent of the one group, and
mental, *qua* constituent of the other group, but is the same
unchanging and intrinsically neutral element that figures in these two
different contexts:



> 
> Thus the great gulf between physical and psychological research
> persists only when we acquiesce in our habitual stereotyped
> conceptions. A color is a physical object as soon as we consider its
> dependence, for instance, upon its luminous source, upon other colors,
> upon temperatures, upon spaces, and so forth. When we consider,
> however, its dependence upon the retina...it is a psychological
> object, a sensation. Not the subject matter, but the direction of
> investigation, is different in the two domains. (Mach 1886:
> 17-18)
> 



In this way material objects and the ego are dissolved into
elements/sensations that are related in certain complex ways. Spelled
out for material objects, this reads:



> 
> thing, body, matter, are nothing apart from the combinations of the
> elements,--the colors, sounds, and so forth--nothing apart
> from their so-called attributes. (Mach 1886: 7)
> 



And the fate of the ego is similar:



> 
> The primary fact is not the ego, but the elements
> (sensations)...The elements constitute the I. *I* have the
> sensation green, signifies that the element green occurs in a given
> complex of other elements (sensations, memories). When *I*
> cease to have the sensation green, when *I* die, then the
> elements no longer occur in the ordinary, familiar association. That
> is all. Only an ideal mental-economical unity, not a real unity, has
> ceased to exist. (Mach 1886: 23-24)
> 



For Mach the world presents itself as "a viscous mass [of
elements], at certain places (as in the ego) more firmly coherent than
in others" (Mach 1886: 17). The neutral elements (only a minute
fraction of which are sensations) and their relations are the basic
reality. We draw boundaries around certain groups of elements that are
related to each other in interesting ways, because this serves our
biological, scientific, and/or practical purposes. We can continue to
talk about material things and selves; it is economical to do so. But,
strictly speaking, "both [object and ego] are provisional
fictions of the same kind" (Mach 1905: 9).



The primary source for Mach's views on neutral monism are a
number of essays and chapters contained in his books originally
published in 1883, 1886, 1894, and 1905. The size of these books grew
significantly as they went through numerous editions. Some of the
important papers on neutral monism are not contained in the available
English translations of these works.


### 2.2 William James



William James (1842-1910) uses the term "radical
empiricism" for the view he sets forth in James 1912--the
view that has become a paradigm of neutral monism. His critique of the
relational account of experience--according to which the self
directs an act onto an object--was the model upon which Russell
later shaped his analysis of experience. James presents this argument
as an attack on a particular conception of consciousness present in
various forms in the Neo-Kantian, early analytic, and phenomenological
traditions. Roughly, it is the notion of consciousness as a diaphanous
or transparent relation, medium, or container by means of which the
objects of consciousness are presented or represented to us. But the
consciousness that makes this kind of object presentation possible
eludes our grasp. This thin notion of consciousness is the one James
wants to eliminate:



> 
> I believe that 'consciousness,' when once it has
> evaporated to this estate of pure diaphaneity, is on the point of
> disappearing altogether. It is the name of a nonentity, and has no
> right to a place among first principles. Those who still cling to it
> are clinging to a mere echo, the faint rumor left behind by the
> disappearing 'soul' upon the air of philosophy. (James
> 1904b: 2)
> 



His radical proposal is to simply discard this shadowy something and
to make do with what remains, with what used to be the object of the
conscious act. He introduces the term "pure experience" to
stand for this datum. Prior to any further categorization, pure
experience is, according to James, neutral--neither mental nor
physical:



> 
> The instant field of the present is at all times what I call the
> 'pure' experience. It is only virtually or potentially
> either object or subject as yet. For the time being, it is plain,
> unqualified actuality, or existence, a simple that. (James 1904b: 23)
> 



Mind and matter, knower and known, thought and thing, representation
and represented are then interpreted as resulting from different
functional groupings/carvings of the continuous flux of pure
experience for different purposes (see James 1905: 64).



In perceptual knowledge perceiver and perceived fuse or merge (James
uses both terms): one bit of pure experience is the thing perceived as
well as the perceiving of this thing. The difference lies only in how
this single portion of pure experience is related to other portions of
pure experience:



> 
> The paper seen and the seeing of it are only two names for one
> indivisible fact which, properly named, is *the datum, the
> phenomenon, or the experience*. The paper is in the mind and the
> mind is around the paper, because paper and mind are only two names
> that are given later to the one experience, when, taken in a larger
> world of which it forms a part, its connections are traced in
> different directions. *To know immediately, then, or intuitively,
> is for mental content and object to be identical*. (James 1895:
> 110)
> 



Conceptual knowledge is more complex. In the simplest case of
conceptual representation we are dealing with



> 
> two pieces of *actual* experience belonging to the same
> subject, with definite tracts of conjunctive transitional experience
> between them. (James 1904a: 53)
> 



The first piece of pure experience is the thought--the episode of
thinking as well as the content that is thought--perhaps about
Harvard's Memorial Hall, as in James's famous example (see
James 1904a: 55ff). And, in the simplest case, the second piece of
pure experience is the thing--Memorial Hall--that was
thought of, as well as the perceiving of Memorial Hall. The thought
has, let's suppose, led us to Memorial Hall and now we stand in
front of it and see it. It is this function of *leading* (about
the details of which James has much to say) that constitutes the
thought's intentionality, that constitutes the fact that the
thought was a thought about a certain thing. Once the thought has led
one to the Hall,



> 
> the percept not only *verifies* the concept, proves its
> function of knowing that percept to be true, but the percept's
> existence as the terminus of the chain of intermediaries
> *creates* the function. Whatever terminates that chain was,
> because it now proves itself to be, what the concept 'had in
> mind'. (James 1904a: 60-1)
> 



This is James's way of reconstructing the idea of representation
in a way that does not invoke suspect mental powers of intrinsic
intentionality. He values this as a signal achievement of his radical
empiricism:



> 
> The towering importance for human life of this kind of knowing lies in
> the fact that an experience that knows another can figure as its
> *representative*, not in any quasi-miraculous
> 'epistemological' sense, but in the definite practical
> sense of being its *substitute* in various operations,
> sometimes physical and sometimes mental, which lead us to its
> associates and results. (James 1904a: 61)
> 



Not all conceptual experience results in knowledge. If the process of
leading does not get started, or fails to arrive at an experience of
termination, there is nothing that the first piece of experience gets
to know. In that case the representation is empty or false.



The essays in which James sets out his radical empiricism are among
the most influential and most readable documents of the neutral
monistic literature. It is probably fair to say that James converted
Russell to neutral monism. And his influence on American neutral
monists during the early decades of the twentieth century is massive
(see 2.4 below). The primary source for James's views on neutral
monism are the essays collected in his 1912.


### 2.3 Bertrand Russell



Following a series of critical engagements with neutral monism (see
especially Russell 1914a,b), Russell adopted it in 1918 and remained a
neutral monist for the rest of his long career: "I am conscious
of no major change in my opinions since the adoption of neutral
monism" is what he says in an interview from 1964 (Eames 1969:
108). But the question of whether Russell's neutral monism is
best seen as a single theory or as a sequence of related but
significantly different theories, as well as the question of which, if
any, of these different doctrines should count as versions of neutral
monism, have been much debated (see Wishon 2015; Pincock 2018; Bostock
2012; Stace 1946). Russell's 1919 and 1921 are generally
considered to represent the early versions of his neutral monism.
Russell's 1927a and 1927b contain the mature doctrine. In his
later works--Russell 1948 and 1956a--he no longer used the
term "neutral monism". But the doctrine seems largely
unchanged.



Here is a succinct first pass at describing the core of
Russell's neutral monism:



> 
> Russell argued that the traditional distinction between
> "mind" and "matter" is unfounded and that the
> subject matter of both physics and psychology concerns collections of
> causally ordered events in space-time. Some of these events, ones
> occurring in the brains of complex creatures like us, are the mental
> episodes that we are directly aware of in having conscious experience.
> These very same events can also be described in abstract structural
> terms by physics and neuroscience. Regarding the rest of the
> (extracranial) events in nature, Russell maintained that our knowledge
> of them is indirect and limited entirely to the abstract structural
> descriptions provided by the various physical sciences. (Wishon 2016:
> 64)
> 



Unsurprisingly, this brief summary leaves out many key details that
are important for understanding the character of Russell's
neutral monism.


#### 2.3.1 Logical Constructionism and Neutral Monism



When Russell looks at neutral monism, he sees a theory that embodies
the spirit of the "supreme maxim in scientific
philosophising" (Russell 1914c: 155) in an especially striking
way. This maxim is Occam's Razor: entities are not to be
multiplied without necessity. In one of Russell's formulations
it reads: "Wherever possible, substitute constructions out of
known entities for inferences to unknown entities" (Russell
1924: 326). Known entities are those that we know directly or
noninferentially; unknown entities are ones we accept only on the
basis of inferences from known entities. Russell's maxim
encourages us to discover complex structures of known entities that
can play the role the inferred entities were supposed to play. Upon
substituting these complex structures--the
constructions--for the inferred entities, everything continues to
work as before. The point of this complicated procedure is primarily
epistemic: we are now no longer burdened with the risky inference to
the inferred entities, and thus the risk of error is reduced. Strictly
speaking, Russell remains agnostic as to the existence of the original
entities targeted by the construction. They are neither identified
with nor eliminated by the corresponding construction. Still, the
constructions make it possible to arrive at a simplified account of
what there is that fits our empirical evidence and exhibits various
other theoretical virtues such as greater continuity, causal
uniformity, and so on. Thus, the resulting metaphysical theory
deserves our provisional assent over competing ones to a degree
proportional to such non-demonstrative grounds.



Neutral monism holds out the alluring promise that such constructions
can be found for all of the inferred entities of physics, psychology,
and common sense. All of these bodies of knowledge are preserved, but
at a hugely reduced epistemic cost. These are the considerations that
drive Russell to embrace neutral monism: it is a perfect fit with his
longstanding commitment to the program of logical construction. An
added benefit of substituting constructions of neutral entities for
physical and mental entities is an elegant solution to the mind-body
problem.



Logical construction is a process of discovery, not of manipulation:
to logically construct *x*s out of *y*s is to discover
that *y*s, when they manifest a certain type of structure, can
play the role of *x*s. Russell's constructions of a point
in space, an instant in time, a table, and of a belief may illustrate
the procedure. (i) A Russian doll is a good model of how Russell
(following Whitehead) proposes to construct a point in space. The
rough idea is to take "the class of all those objects which, as
one would naturally say, contain the point" (Russell 1914a: 117)
and to substitute this class of nested objects (the structure of
*y*s) for points in space (the *x*s). (ii) Experienced
instants in time (the *x*s) are constructed by discovering that
the experiences of a person, each of which is extended in time, can
overlap each other in ways that converge on an instant. It is



> 
> a group of events, all belonging to his experience, and having the
> following two properties: (1) any two of the events overlap; (2) no
> event outside the group overlaps with every member of the group.
> (Russell 1927b: 288)
> 



(iii) Rather than viewing a table as the cause of our table
sensations, Russell proposes (again just as a first approximation) to
view the table as "the set of all those particulars which would
naturally be called 'aspects' of the table from different
points of view" (Russell 1921: 98). (iv) The logical
construction that is to take the place of beliefs consists of the
following three components:



> 
> We have a proposition, consisting of interrelated images, and possibly
> partly of sensations; (b) we have the feeling of assent, which is
> presumably a complex sensation demanding analysis; (c) we have a
> relation, actually subsisting, between the assent and the proposition,
> such as is expressed by saying that the proposition in question is
> what is assented to. (Russell 1921: 251)
> 



As these examples show, there is no single method of construction, no
single type of structure, suitable for different projects of logical
construction. The nature of the *x*s (the targets of
construction) and of the *y*s (the construction materials)
guide the search for structures that will work in the case at
hand.


#### 2.3.2 Known Neutral Entities



The initial stock of known entities with which Russell proposes to
carry out this grand project of logical construction seems frightfully
small and strikingly non-neutral. It consists of his sensations and
images--these loom large in Russell 1919, 1921--and his
perceptions (percepts)--these play a prominent role in Russell
1927a, 1927b, 1948. Everything else--electrons, apples, galaxies,
experiences, and selves, etc.--is in need of logical
construction.



For an entity to be neutral is to "have neither the hardness and
indestructibility of matter, nor the reference to objects which is
supposed to characterize the mind" (Russell 1921: 36; cf. 124).
Russell never suspected sensations of being material (in this sense).
That sensations contain a mental element (in this sense)--that
they consist of a mental act of sensing directed at a non-mental
object--was, however, a pivotal part of his earlier view. But
then his views changed:



> 
> I formerly believed that my own inspection showed me the distinction
> between a noise [the object] and my hearing of a noise [the act of
> sensing], and I am now convinced that it shows me no such thing, and
> never did. (Russell 1918b: 255)
> 



All that Russell now finds upon introspecting a sensory episode is a
single item: a sensory quality "which may be called
indifferently a noise or hearing a noise" (Russell 1918b: 255).
Switching the example to colors, he writes that



> 
> the sensation we have when we see a patch of colour simply *is*
> that patch of colour...the patch of colour and our sensation in
> seeing it are identical. (Russell 1921: 142-3)
> 



And since noises and patches of color are not intentional--they
are not directed at anything, they are simply there--they are
intrinsically non-mental. They constitute "sensations"
only when related to other such entities in an organized system such
that they play the right *causal role* (Russell 1996; Wishon
2020, 2021). This establishes the neutrality of sensations. And
because images have the same intrinsic nature as sensations (cf.
Russell 1921: 117, 121, 154, 156, 287, 297), they too are
 neutral.[6]
 Percepts are composed of elements playing the roles of sensations and
images and are likewise intrinsically neutral.


#### 2.3.3 More Neutral Entities: Realism



Russell's focus on sensations, images, and percepts explains why
Russell's neutral monism has often been dismissed as a form of
phenomenalism. But in the preceding section we have seen how Russell
argued for the intrinsic nonmentality of these items. Moreover, this
criticism overlooks that Russell worked with a vastly expanded set of
construction materials. He allows for the enlargement of the realm of
the known by a series of cautious
 inferences.[7]
 In a first step he argues that "all our percepts are composed
of imperceptible parts" and have imperceptible structure
(Russell 1927a: 282, 386; Wishon 2020, 2021); second, there are
sensations, images, and percepts that are not our own; third, and most
importantly, we can infer the existence of vast numbers of entities
that "do not form part of any experience" (Russell 1921:
 25).[8]
 In this way Russell arrives at the view "that the world is very
full of events" (Russell 1927a: 258), only a vanishingly small
number of which are his own experiences.



Our own percepts, images, and
 sensations[9]
 occupy a privileged place in this system of entities--they are
"what is most indubitable in our knowledge of the world"
(Russell 1927b: 139). This knowledge is of a "more intimate
qualitative" (Russell 1927a: 389) kind; it reveals something of
the intrinsic character of these
 entities.[10]
 But our knowledge of the physical world [as described by physical
science] is (almost) entirely abstract: "we know certain logical
characteristics of its structure, but nothing of its intrinsic
character" (Russell 1927b:
 306-307).[11]
 In this sense, physics merely describes the "causal skeleton of
the world" and leaves us in the dark as to the intrinsic quality
of the events that make it up (Russell 1927a: 388, 391).



Russell takes all of these different entities to be neutral (in the
sense of the *Neither View*); and he takes all of them to be
 events[12],
 where an event is understood as "something occupying a small
finite amount of space-time" (Russell 1927b: 287). The
noise and the color patch considered above are typical examples of
events. A Russellian event is absolute, in C.D. Broad's sense:
"it should not *itself* be a state of invariance or of
change in the qualities or relationships of any thing" (Broad
1959: 739; see also Maxwell 1978: 385-6 on pure events).
According to Russell, physics teaches that events "just happen,
and do not happen 'to' matter or 'to' anything
else" (Russell 1927b: 289). This means that we must not think of
mental events (or any events) "as consisting of motions of bits
of matter" (Russell 1927b: 292), for "matter in
motion...is not an event in our sense" (Russell 1927b:
296).



Russell's neutral monist universe is a plenum of (absolute)
events--a vast assemblage of overlapping small occurrences which,
given their short lifespan, are constantly replaced by new ones. This
mass of ever-changing events manifests all sorts of complex dynamic
patterns. We have not yet discovered the laws that govern these events
and give rise to the formation of these complex dynamic patterns. But
we have physics, which describes the behavior of certain groups of
law-governed events under the name of "matter". And we
have psychology, which describes a set of quite different regularities
that obtain in very specific regions of this plenum of
events--viz. in those regions that are filled by the pieces of
matter that are called "brains". But all the building
blocks of the logical constructions that are matter, and all the
building blocks of the logical constructions that are mental episodes,
have the same nature--they are neutral events.


### 2.4 Some Other Early Neutral Monists



There are, of course, many other philosophers whom one might want to
include in the list of neutral monists. In the German-speaking world,
we find Richard Avenarius (1843-96), who was in contact with
Mach, and Joseph Petzoldt (1862-1929), who built on the work of
Avenarius. Avenarius's main reflections on neutral monism are
contained in his 1888/90, 1891, and 1894/95. Petzoldt's relevant
works are his 1900, 1904 and 1906. In the English-speaking world,
neutral monism thrived within the movement of American new realism.
Arguably, John Dewey (1859-1952) was the most eminent figure in
this group of neutral monists, but Ralph Barton Perry
(1876-1957)--a student and friend of James--is the one
most closely associated with the
 view.[13]
 Perry's 1912 is the main source for his views about neutral
monism. Edwin B. Holt (1873-1946) developed an ambitious neutral
monist program in his 1912 and 1914. Other notable early neutral
monists include Alois Riehl (1844-1924), Alexander Bogdanov
(1873-1928), Moritz Schlick (1882-1936), and (for a time)
Rudolf Carnap (1891-1970) and Alfred Ayer (1910-89), among
others.


## 3. The Case for Neutral Monism



The case for neutral monism is best made by enumerating its
virtues.


### 3.1 Parsimony



For starters, neutral monism shares the virtues of idealism and
materialism in pairing down the kinds of basic entities to which we
must be committed. Whereas dualism posits that there are basic
entities of two fundamentally different kinds--matter and
mind--neutral monism posits that the basic entities are all of a
single, neutral kind. On many such views, there are also non-basic
entities (such as minds and matter) grounded in the basic ones. But on
others, "mind" and "matter" are merely
convenient groupings of the basic entities and there are no non-basic
ones.



The "immense simplification" neutral monism affords is
what most attracted Russell to neutral monism (Russell 1959: 252). For
Mach and James, it also promises to free us from any commitment to the
existence of extra-empirical kinds of entities:



> 
> nothing is important except what can be observed or is a datum for us,
> and everything hypothetical, metaphysical and superfluous, is to be
> eliminated. (Mach 1886: 27-28)
> 



First on the list of "metaphysical entities" (Mach 1905:
13) that have to go are "the 'unfathomable' thing
and the equally 'unexplorable' ego" (Mach 1905: 8).
And James concurs: "Consciousness as it is ordinarily understood
does not exist, any more than does Matter" (James 1905: 63).


### 3.2 The Mind-Body Problem



Mach, James, and Russell agree that neutral monism solves the
mind-body problem. Russell's account of experience (of
perceptual consciousness) may serve as an illustration of the point.
Russell frequently emphasized the miracle or mystery involved in
traditional dualist accounts of perception (1927b: 147, 154; 1927a:
275, 400). At the end of a purely physical chain of causes there
mysteriously arises something of a completely different nature: an
experience (a sensation of red, say). This raises difficult questions
about how exactly these two seemingly different aspects of
reality--matter and mind--are related to one another and how
they could be causes and/or effects of each other.



Materialism holds that mind is wholly grounded in basic material
entities or features of reality and thus that mind-body causal
relations are ultimately a matter of physical causal relations. But
there are longstanding worries about whether properties of mental
phenomena such as qualitativity, intentionality, and/or subjectivity
can be wholly grounded in material entities or properties. Idealism,
in contrast, holds that matter is wholly grounded in basic mental
entities or features of reality and that mind-body causation is
ultimately a matter of mental causal relations. But critics question
whether material phenomena can be adequately accounted for wholly in
terms of mental phenomena.



Neutral monism can be an attractive alternative for those who see the
challenges faced by these traditional positions as
 insurmountable.[14]
 In some cases, such interest in neutral monism is, perhaps, best seen
as part of the general movement to explore all possible options in the
metaphysics of mind. As Jerry Fodor puts it, "the form of a
philosophical theory, often enough, is: *Let's try looking
over here*" (Fodor 1981: 31). But others see neutral monism
as having resources the traditional positions lack and perhaps can
make use of themselves.



Neutral monism addresses the mind-body problem by asserting that mind
and matter are not as different as they might first appear. On the
both view, this is because the basic neutral entities are
intrinsically both mental and material. But it is difficult to see how
such a view avoids collapsing into a form of panpsychism, dual-aspect
theory, idealism, or materialism. On the neither view, the apparent
"gap" between mind and matter is narrowed by the fact that
both are grounded in, or merely convenient groupings of, the same
basic entities which are themselves intrinsically neither mental nor
material. Accordingly, mind-body causal relations are ultimately just
a matter of causal relations among different groups of the basic
neutral
 entities.[15]



One of the chief attractions of neutral monism is its promise for
integrating qualitative entities or properties into the natural world
without positing experientiality among its most basic constituents. In
doing so, it has the potential to vindicate both naturalists who hold
that mental phenomena such as consciousness arise only in the context
of complex systems of elements and those who see no hope of grounding
the qualitative aspects of consciousness wholly in non-qualitative
material entities or properties. There is, of course, an open question
about whether neutral monism can deliver on such promise.



Many philosophers of mind who are in some ways sympathetic to neutral
monism also find Russell's views that physical theories
abstractly describe the structure of physical reality while leaving us
ignorant about its intrinsic nature especially attractive. In fact,
this aspect of Russell's neutral monism is the primary
inspiration for the assortment of "Russellian monist"
views that have been hotly debated in recent years (see 5.4
below).


### 3.3 Perceptual Evidence for Physics



*The Analysis of Matter* (Russell 1927a) is widely regarded as
the most important text of the neutral monist literature. Chapter one
of this book, entitled "The Nature of the Problem", states
it as follows:



> 
> the world of physics is, *prima facie*, so different from the
> world of perception that it is difficult to see how the one can afford
> evidence for the other; moreover, physics and physiology themselves
> seem to give grounds for supposing that perception cannot give very
> accurate information as to the external world, and thus weaken the
> props upon which they are built...We must therefore find an
> interpretation of physics which gives a due place to perceptions; if
> not, we have no right to appeal to the empirical evidence. (Russell
> 1927a: 6-7; see also Russell 1948: Part III, Ch. IV)
> 



The previous remarks addressed to the mind-body problem in general,
and to the problem of perceptual experience in particular, contain the
core of the solution Russell proposes. By "bridging the gulf
between physics (as commonly interpreted) and perception"
(Russell 1927a: 7), neutral monism promises to solve this
problem--a problem that Russell took to be difficult and largely
unnoticed (see Russell 1948: 175-176).


### 3.4 Knowledge of the External World



Mach and James understand neutral monism as an especially radical form
of achieving perceptual contact with the world. It might be understood
as a limiting case of naive realism--a case in which the
relation between the subject and its perceptual object becomes the
identity relation. In perception "subject and object
merge" (James 1905: 57). A single reality--a red patch,
say, when we see a tomato--is a constituent of two groups of
neutral entities: the group that is the perceiver, and the group that
is the tomato. The mind and its object become one. In James's
words:



> 
> A given undivided portion of experience, taken in one context of
> associates, play[s] the part of the knower, or a state of mind, or
> "consciousness"; while in a different context the same
> undivided bit of experience plays the part of a thing known, of an
> objective "content". In a word, in one group it figures as
> a thought, in another group as a thing. (James 1904b: 9-10)
> 



Some have seen this epistemic achievement as the most important reward
of neutral monism.



But Russell dissented. There are well known problems surrounding
accounts of perception that conceive it as a form of direct contact
with the world. Among them are the problems arising from the fact that
an object can present different and incompatible appearances to
different observers, or even the same observer in different
circumstances. Some neutral monists concluded that "nature is a
seething chaos of contradiction" (Holt 1914: 276). But Russell
chose to abandon the view that in perception "subject and object
merge". Instead he locates the red patch you see when looking at
a tomato in your brain, and distinguishes it from the tomato (from the
group of events that constitute the tomato). That is why he can assert
that what is happening in his brain is "exactly what naive
realism thinks it knows about what is happening in the outside
world" (Russell 1927b: 138). This solves the problem of
different and incompatible appearances: compatibility is restored by
moving the conflicting features away from the single physical object
and into the brains of the percipients. But in making this move
Russell abandons the naive realism of Mach and James and puts a
version of representative realism in its place. The problem of our
knowledge of the external world is thereby reintroduced in full
 force.[16]


### 3.5 Unity of Science



Mach operates on the assumption that all sciences form a whole (see
Mach 1886: 30). The great virtue of neutral monism is that it affords
a unified perspective from which scientific inquiry can be
undertaken:



> 
> what I aimed at was merely to attain a safe and clear philosophical
> standpoint, whence practicable paths, shrouded in no metaphysical
> clouds, might be seen leading not only into the field of physics but
> also into that of psycho-physiology. (Mach 1886: 47)
> 



The idea of neutral monism as a bridge between different sciences is
also present in Russell. He presents neutral monism as



> 
> an attempt to harmonize two different tendencies, one in psychology,
> the other in physics...[viz.]...the materialistic tendency
> of psychology and the anti-materialistic tendency of physics. (Russell
> 1921: 5-6)
> 


## 4. Objections to Neutral Monism


### 4.1 Reduction and the Threat of Elimination



Every version of reductionism must deal with the question of the
ontological status of the entities it reduces: are they retained or
eliminated? Both of these components are typically present in
traditional neutral
 monism.[17]
 Mach, for instance, alternates between claims such as that "the
supposed unities 'body' and 'ego' are only
makeshifts, designed for provisional survey and for certain practical
ends" and claims that they are composites constituted by
"a more strongly coherent group of elements [that is] less
strongly connected with other groups of this kind" (Mach 1886:
20-21). Similarly, alongside Russell's many logical
constructions of various mental phenomena are remarks such as
this:



> 
> There are a number of words which I think should disappear from the
> psychological vocabulary: among these I should include
> *knowledge*, *memory*, *perception*, and
> *sensation*. (Russell 1996: 295)
> 



What is clear is that neutral monism dispenses with mind *as
traditionally conceived by dualists and idealists* and matter
*as traditionally conceived by dualists and materialists*.
There is no need to suppose that there are "solid persistent
objects moving through space" or states with an inbuilt and
primitive "reference to objects" (Russell 1921: 124, 36).
But, on the other hand, it provides constructions of neutral entities
that are designed to play the roles of the entities they displace. The
success of this strategy will have to be assessed on a case by case
basis. There are relatively few objections to neutral monism that turn
on the details of proposed constructions or reductions. (But see C.D.
Broad's critique of Russell, 1925: 577-584). The most
challenging criticisms of neutral monism do not engage with technical
details, but aim squarely at the central idea of the
doctrine--the idea of neutrality.


### 4.2 The Mentalism Suspicion



The most frequent type of objection to the traditional versions of
neutral monism is that they are forms of mentalistic monism: Berkleyan
idealism, panpsychism, or phenomenalism. The core argument is simple:
sensations (Mach), pure experience (James) and sensations/percepts
(Russell) are paradigms of non-neutral, mental entities. Hence there
is nothing *neutral* about these neutral monisms. This type of
objection--the "mentalism suspicion"--has been
articulated by a diverse group of philosophers, among them: (Lenin
1909: 34; Hartshorne 1937: 221-2; Stace 1946; Ayer 1971; Feigl
1958: 426, 1975: 26-7; Maxwell 1976: 354; Popper & Eccles
1977: 199; Strawson 1994: 97; Chalmers 1996: 155; Tully 2003: 355,
369).



The *prima facie* plausibility of this objection is beyond
doubt. But Mach, James, and Russell were acutely aware of the problem
and took themselves to have responded in a satisfactory manner. They
reject the view that a sensation or an experience consists of a
subject directing a mental act--awareness, consciousness,
acquaintance--onto an object. Dispensing with the subject and the
act, they are left with what used to be the object--a red patch,
say. And they hold that there is nothing intrinsically mental about
red patches. A red patch becomes a red-sensation simply by being
appropriately related to other entities of the same kind. There is no
quick way to adjudicate this dispute; a careful assessment of each
case is required. Still, a comparison of neutral monism with its
closest mentalist alternatives can be a useful first step.



Panpsychism holds that every basic entity--usually understood as
a physical entity--is also a mental entity. The physical nature
(if present) and the mental nature of the basic entities are
fundamental, i.e., irreducible (to each other or to anything else). On
the face of it, panpsychism and neutral monism are strikingly
different. Neutral monism takes mental and physical phenomena to be
derivative, panpsychism does not; neutral monism holds that basic
reality is neutral, panpsychism does not. And neutral monism is
compatible with the view that most physical objects, and all their
parts, are absolutely nonmental, whereas panpsychism is not. This is
how things look on the *Neither View* of neutrality. On the
*Both View* of neutrality, things are less clear. The most
natural reading of the *Both View* will, however, not yield
panpsychism--the view that every basic physical entity has a
mind--but a dual aspect view--a view according to which
every basic entity has a physical and a mental aspect or side. In any
case, Mach, James, and Russell resisted panpsychist or dual-aspect
interpretations of their views, though there is evidence suggesting
that James adopted panpsychism sometime after 1904.



Phenomenalism has been defended as a doctrine about language, about
facts, and about things. Taken in this last sense, it attempts to
"reduce material objects to sensa, that is, to explain them as
consisting solely of sensa or as being primarily groups or patterns of
them" (Hirst 2006: 271). We might substitute the terms
"sense-data", "sensations",
"percepts", "experiences", etc. for
Hirst's term "sensa". As noted above, the
traditional neutral monists supposedly purged such terms of their
usual intrinsically mental dimension. Moreover, neutral monism is not
limited to those entities that are sensations, perceptions, and so
on--this is particularly clear in the case of Mach and Russell.
The existence of vast majority of neutral entities is inferred from
the minute set of elements that, due to their causal-functional roles,
happen to be sensations and perceptions. These inferred elements are
outside of all minds. These are the strong anti-phenomenalist (and
anti-idealist) positions. How successfully the various neutral monists
defend these claims is, of course, a difficult further question.


### 4.3 The Materialism Suspicion



In the past neutral monism has often been interpreted as a form of
mentalism. But a number of contemporary philosophers argue that it is
best understood as a form of physicalism.



We have noted how Landini's interpretation of the notion of
neutrality (according to (3)--the *Possible Constituent
View)* allows him to argue that neutral monism is compatible with
physicalism. As he sees it, Russell is committed to such a version of
(four-dimensionalist) physicalist neutral monism. Accordingly, Russell
is engaged in the project of "constructing both [minds and
matter] out of orderings of physical events that are their
stages" (Landini 2011: 280). After a detailed discussion,
Landini reaches the conclusion that Russell's basic transient
particulars (or events) "are without intrinsic phenomenal
character" (Landini 2011: 297). This makes it possible to regard
them as physical entities in good standing. Their neutrality consists
in the fact that they are the building blocks of both mental and
physical continuants (see Landini 2011: 292).



Similarly, Erik Banks (see section 5.6 below) presents his so-called
"realistic empiricism" as a direct descendant of
traditional neutral monism (Banks 2014, viii). But he also holds that
his neutral monism is "a kind of physicalism" (Banks 2014:
7, 142). Banks takes himself to follow Russell embracing an ontology
of events as manifestations of underlying powers or
energies--such as electromagnetism, gravitation, and nuclear
forces (Banks 149), as well as neural energies (Banks 2014: 142). But
event particulars such as these, Banks insists, "are *so*
physicalistic in nature that there does not seem to be any reason to
assume that these natural qualities in physics have anything at all in
common with our sensations, which are qualities of a very different
order... [involving] events in the human nervous system at a very
different scale of complexity and size" (Banks 2014: 156).



After a careful survey of the development of Russell's neutral
monism, Donovan Wishon observes that Russell's post-1940 version
of neutral monism "has a greater affinity to Russellian
Physicalism than any genuinely *neutral* monism" (Wishon
2015: 114-5). Among other things, Wishon draws our attention to
Russell's report that "I find myself in ontology
increasingly materialistic" (Russell 1946: 700). He also points
to Russell's remarks that "I should regard all events as
physical" and that "the distinction between what is mental
and what is physical does not lie in any intrinsic character of
either, but in the way in which we acquire knowledge of them"
(Russell 1958: 12). Hence, Wishon concludes, "mental events will
turn out to be a subclass of the physical events that make up
reality"--only their special epistemic accessibility
distinguishes them from the other physical events (2015: 112). But he
does not quite answer whether Russell, in this late period, took all
events to be physical due to their intrinsic natures or merely due to
their relations to other events.



These attempts to combine neutral monism with physicalism, or to
reinterpret neutral monism along physicalist lines pose a serious
challenge to neutral monism as usually understood. They deny the
central claim that the fundamental building blocks of the world are
neutral in the sense of being both nonmental and
 nonphysical.[18]
 But no less noteworthy is the degree to which these versions of
physicalism depart from more standard forms of physicalism. In
particular, they agree with neutral monism that physical theories do
not fully capture the nature of the world they describe. This suggests
that the difference between neutral monism and this sort of
physicalism may not be so deep after all (see Chalmers 2015).


### 4.4 The Problem of Experience



Even if materialist suspicions about it are misplaced, some critics
hold that neutral monism shares a common failing with materialism:
namely, that it cannot accommodate experience. The argument is driven
by two deep metaphysical convictions. First, experience cannot be
reduced to or constructed from the non-experiential; second, radical
emergence is unintelligible. Assuming that the neutral must be
non-experiential, it follows that the neutral monist world has no room
for experience. Galen Strawson has wielded this argument against all
forms of traditional materialism, and occasionally also against
neutral monism (Strawson 1994, 2016, 2020).



David Chalmers has raised a related worry about neutral monism,
especially in its panqualityist forms (see section 5.5 below): He
argues that there is no path that leads from the qualities we
experience (but which are not experiential themselves) to the
experience of those qualities. Having an experience is a matter of
having phenomenal qualities. Phenomenal qualities involve awareness of
qualities. But "no instantiations of qualities ever necessitate
awareness of qualities" (Chalmers 2015: 273). This
"quality/awareness gap" (Chalmers 2015: 273) shows that no
structure of qualities can add up to
 experience.[19]



The traditional neutral monists would all agree that the mere
instantiation of qualities (which are intrinsically neutral) does not
necessitate any awareness of qualities. After all, such awareness
results only when a number of such qualities are appropriately related
to each other. However, they depart from Chalmers in holding that
suitable relations among qualities are not only necessary for
awareness, they are also sufficient for it. That is, for a subject
(understood as an organized bundle of neutral events) to be aware of,
say, a red patch, just is for the red patch to bear the right kinds of
complex causal-functional relations to other neutral events in an
organized system of which it is a member (i.e. the bundle-self). Thus,
the traditional neutral monists would reject the view that no
*structure* of qualities can add up to experience.



Whether such a reply is satisfactory is a matter of ongoing debate.
Yet to the extent that this proposal succeeds, it does raise a further
question suggested by Strawson's argument above: is experience,
thus understood in a neutral monist setting, a feature that is
emergent in an objectionable way?


### 4.5 The Problem of Emergence



The basic idea of emergence has to do with the fact that complex
systems may display interesting novel properties--properties not
possessed by their parts. This idea has been regimented in various
ways. Following Chalmers, we can distinguish between
"weak" and "strong" emergence. Weakly emergent
phenomena are merely unexpected, given our knowledge of the domain
from which they arise. Strongly emergent phenomena are not just
unexpected; they cannot (not even in principle) be deduced from the
domain from which they arise (see Chalmers 2006: 244). There are
ongoing debates, however, about whether epistemic and/or logical
notions such as deducibility are reliable guides to the nature of
reality. Such issues can be set aside by instead using the notion of
"radical" emergence for cases in which complex systems
display novel features that are not wholly grounded in the nature,
features, or relations of their parts (Strawson 2006). Weak emergence
will, no doubt, be a wide-spread phenomenon in the neutral monist
world. It poses no problem. In contrast, neutral monism appears to
rule out radical emergence. Things are less clear in the case of
strong emergence. Russell's method of logical construction is
incompatible with strong emergence, but it is an open question whether
other neutral monist accounts are.



Landini adopts the contrary view that radical emergence plays a
crucial role in Russell's neutral monism. If the fundamental
transient physical particulars (or events) have no phenomenal
characters (or qualia), such qualities must be understood as radically
emergent features within the framework of Russell's
 account.[20]



> 
> Qualia never occur in transient particulars. In Russell's view,
> qualia emerge from the series of brain states...colors, pitches,
> smells, tastes and textures are emergent properties of series of brain
> states.... (Landini 2011: 302-305)
> 



But Russell's remarks about emergence (Russell 1927b:
293-96) allow different reading. When called upon to present
examples of events, Russell gives the following list:



> 
> seeing a flash of lightning...hearing a tyre burst, or smelling a
> rotten egg, or feeling the coldness of a frog...particular colors
> and sounds and so on are events. (Russell 1927b: 287-88)
> 



All of these events are percepts--the only kinds of events we can
know without inference. And, pace Landini, all of these events do have
qualitative characters. When Russell speaks of emergence, he has in
mind the relationship between physics' abstract mathematical
descriptions of events and their intrinsic qualitative characters. And
he maintains that latter are (strongly) emergent with respect to the
former because there is no inferential path leading from abstract
mathematical descriptions of an event to its intrinsic qualitative
features. For such abstract and mathematical considerations



> 
> cannot conceivably...prove that there are visual events, or
> auditory events, or events of any of the kind that we know by
> perception. (Russell 1927b: 295)
> 



But it is debatable whether such qualities radically emerge from
anything. In any case, this is not how the neutral monist (who follows
Russell's lead) starts out. Our percepts are our initial data,
and they are qualitative through and through. We simply do not have to
deduce the existence of quality from other known facts, such as those
involving complex brain states. Qualities are there, in the form of
our percepts, at the very foundation of Russell's theory. Such
other events as we may believe in, whose intrinsic qualities we do not
know, are all inferred from this qualitative bedrock. Even so, the
neutral monist insists, it doesn't follow from the fact that
something is irreducibly qualitative that it is intrinsically
mental--or mental at all.


### 4.6 The Dualism Suspicion



Another common objection to neutral monism is that it constitutes a
form of property dualism or dual aspect theory. The argument is
straightforward: On the neutral monist picture, physics describes
certain relations--namely, the physical ones--among the
basic entities without capturing their intrinsic qualities, or those
of the complexes of which they are parts. These latter features are
revealed to us only in the case of our sensations, percepts, and other
mental episodes. This suggests that the basic entities exhibit two
fundamentally different kinds of aspects or properties: extrinsic
physical relations and intrinsic mental qualities. But there's
nothing properly *neutral* about either kind of feature, and
neither is reducible to the other. At best, they are two radically
different aspects of an underlying reality which, in itself, is
neither mental nor physical.



The theory of dual or double aspects is usually traced back to Spinoza
(1677). The fundamental idea uniting the family of views under this
label is that there is an underlying reality that we can grasp as
mental or as physical, depending on the point of view from which we
apprehend it. Each one of us can know their own brain under each of
these aspects--via introspection and (scientific) observation.
But the claim of the theory is quite general: everything there is is
to be understood as consisting of an underlying reality that has these
two aspects.



Neutral monism and the dual-aspect theory share a central claim: there
is an underlying reality that is neither mental nor physical. But that
is where the agreement stops. Neutral monism has no room for the
central feature of the dual-aspect theory: the mental and physical
aspects, sides, or properties that characterize the underlying
entities of dual-aspect theory. The neutral monist accepts the
mental/physical distinction. But it resides at the level or groups of
neutral entities. Grouped one way, the neutral entities constituting
your brain are thoughts and feelings; grouped another way, they are
atoms and neurons and lobes. Whether a given group of interrelated
neutral entities counts as mental or physical depends on the
causal-functional role this group occupies. But the entities
themselves are free of intrinsically mental or physical
aspects/sides/properties. Therein consists their neutrality.



But if neutrality is understood according to the *Both View*,
the case for identifying neutral monism with dual-aspect theory is
strengthened. Much will turn on how the details of the *Both
View* are articulated. It must not, for example, be understood as
proposing the identification of mental and physical properties. For
the dual-aspect theory insists that the two aspects are fundamental
and irreducible to each other. Another question concerns the
relationship of the aspects and properties. If aspects are understood
as properties, dual-aspect theory may simply collapse into property
dualism (see, e.g., Van Gulick 2014). Dual-aspect theorists have
pushed back by insisting that aspects are not properties (see Skrbina
2014: 228-29). But this may, in turn, block the project of
reconciling the dual-aspect theory with neutral monism. For the
neutrality, on the *Both* as well as on the *Neither
View*, is understood in terms of mental and physical
properties.



There is a lively debate concerning the relationship between neutral
monism, property dualism, and dual-aspect theory (see, e.g., Velmans
2008; Skrbina 2014). The decision about these theories--whether
they are identical, distinct but compatible, or incompatible
rivals--is still out.


### 4.7 Error



The neutral monism of Mach and James is committed to a naively
realistic account of perception according to which our perceptual
experiences and the aspects of the world perceived are one and the
same. Nonveridical experiences--illusions, hallucinations,
dreams, etc.--are difficult to accommodate within such a picture.
For in cases of nonveridical experience--e.g. when a drunkard
"sees" a pink rat--we are inclined to say that the
world does not contain the relevant entities.



But traditional neutral monism provides an ontology rich enough to
address this problem. The pink patch experienced by the drunkard
exists: it is a portion of pure experience (James), or an element
(Mach). This much naive realism is true, even in the cases of
nonveridical experience. But the pink *rat* that the drunkard
takes himself to see does not exist--the pink patch he sees is
not a member of a group of neutral elements constituting a material
object such as a rat. There is nothing wrong with the drunkard's
visual experience; what is mistaken is his assumption about how what
he sees is connected with the rest of the world. Russell (though not a
naive realist himself) states this point most succinctly:
"There are in fact no illusions of the senses, but only mistakes
in interpreting sensational data as signs of things other than
themselves" (Russell 1948: 149-50). Mach puts the point as
follows:



> 
> When we consider elements like red, green, hot, cold and the rest,
> which are physical and mental in virtue of their dependence on both
> external and internal circumstances, and are in both respects
> immediately given and identical, the question as to illusion and
> reality loses its sense. Here we are simultaneously confronted by the
> elements of the real world and of the ego. The only possible further
> question of interest concerns their functional
> interdependence.... (Mach 1905: 7-8)
> 


### 5. New Directions for Neutral Monism



Neutral monism is not simply a historical curiosity. It has evolved
into a number of new forms and remains an important part of ongoing
discussions about the mind-body problem.


### 5.1 Information as Ultimate Reality



Anticipating the current revival in interest in neutral monism by a
few decades, Kenneth Sayre (1929-) published his main ideas on
neutral monism in the 1970s. Unlike the *big three*, Sayre
finds the neutral base of his system not in experience, but in the
form of pure information, where information is understood in the
strict information theoretical sense. His proposal must not be
mistaken for the uncontroversial claim that that mental and physical
processes can be *described* in information theoretic terms.
Sayre puts forward an ontological claim about the ultimate nature of
reality--that ultimate reality consists of informational
states:



> 
> If the project...is successful, it will have been shown not only
> that the concept of information provides a primitive for the analysis
> of both the physical and the mental, but also that states of
> information...existed previously to states of mind. Since
> information in this sense is prior to mentality, but also implicated
> in all mental states, it follows that information is prior also in the
> ontological sense...Success of the present project thus will show
> that an ontology of informational states is adequate for an
> explanation of the phenomena of mind, as distinct from an ontology of
> physical events. [And Sayre adds:] It is a reasonable conjecture that
> an ontology of information is similarly basic to the physical
> sciences.... (Sayre 1976: 16)
> 



One of the greatest challenges faced by the traditional versions of
neutral monism is to show how basic entities that are derived from
experience can be neutral, rather than mental. By choosing an ontology
of informational states as his "neutral stuff", Sayre
elegantly bypasses this problem. But while the neutrality of
informational states may be taken for granted, the question of the
relationship of this seemingly abstract "stuff" to
concrete world of physical and mental entities becomes all the more
 pressing.[21]



Sayre's main work on neutral monism is his 1976. In recent
decades, David Chalmers has similarly explored the idea of an
informational ontology (see Chalmers 1996). And the related ideas that
ultimate reality is purely structural (Ladyman and Ross 2007; Floridi
2008, 2009), is a computational process (Fredkin 2003; Lloyd 2006), or
is purely mathematical (Tegmark 2014) are the subject of a lively
discussion.


### 5.2 Mind & Matter: A Merely Conceptual Distinction



For want of a better label, John Heil presents his view on the
mind-body relation as a form of neutral monism. He resists being
labeled a materialist



> 
> because it carries with it the implication that there is an
> *asymmetry* in the identification of mental qualities with
> material qualities: the mental is supplanted by the material. (Heil
> 2013: 242)
> 



Nor does he see himself as an idealist. And he has systematic and deep
reasons for rejecting the property dualism that characterizes the
various versions of nonreductive materialism. The neutral monism that
he accepts is characterized as follows:



> 
> 
> Neutral monism includes the denial that there is a
> mental-material chasm to be bridged. The mental-material
> distinction is, as Spinoza and Donald Davidson contend, a distinction
> of conception only, not a *real* distinction, not a distinction
> in reality. (Heil 2013: 242)
> 
> 
> 



An example will help to illustrate the gist of this view. Take the
well-worn claim that pain is c-fiber
 stimulation.[22]
 According to Heil, this says that the predicate "c-fiber
stimulation" and the predicate "pain" apply to the
same things: all things that are truly described as being c-fiber
stimulations are also truly described as pains. But--and this is
a crucial claim that cannot be developed here--this does
*not* mean that there is a property of being a c-fiber
stimulation and a property of being a pain, and that these two
properties stand in some interesting relationship, such as identity,
reduction, realization, etc. What this does mean is that there is a
complex object that makes both of these claims true. The nature of the
parts of this object, as well as their intricate relationships to one
another, is the reality that is correctly described as a c-fiber
stimulation and as a pain.



This part of Heil's account appears to be consistent with the
spirit of traditional neutral monism. One is reminded of
Russell's pithy description of his project: "What I wish
to do in this essay is to restate the relations of mind and brain in
terms not implying the existence of either" (Russell 1956a:
145). This can easily be read as a denial of the existence of a chasm
between mental and physical properties. And Heil's complex
objects that serve as the truth-makers for mental and physical
descriptions may be seen as analogous to Russell's groups
(logical constructions) of events that can properly be described in
physical and mental terms.



But there is an additional part to the neutral monist
picture--one that (arguably) all of the mainline neutral monists
agree on--that Heil rejects. When he tells the "deep
story" (Heil's term) about the nature of the complex
objects (the truth-makers for claims about c-fibers and pains etc.),
he turns to fundamental physics:



> 
> I take it to be an empirical question--a question for science,
> for fundamental physics--what the substances are and what they
> are like, *how* they are. (Heil 2013: 201-2)
> 



So the basic constituents of the world--the substances and the
properties--that Heil describes are *physical*. Here the
neutral monist (who is committed to the *Neither View* of
neutrality) parts company with Heil. The deep story of the neutral
monist has it that the fundamental entities--be they events, bits
of information, substances, properties, etc.--are not physical
(and not mental, of course). That is to say, they are not the sort of
thing that fundamental physics (or psychology) reveals.


### 5.3 Complex Basic Entities



In his book *Mind and Cosmos* (2012) Thomas Nagel asserts that
"the weight of the evidence favors some form of neutral monism
over the traditional alternatives of materialism, idealism, and
dualism" (2012: 5). Neutral monism is understood as a view that
"accounts for the relation between mind and brain in terms of
something more basic about the natural order" (2012: 56). This
yields a picture of a "general monism according to which the
constituents of the universe have properties that explain not only its
physical but its mental character" (2012: 56). Borrowing a
concept from Tom Sorrell (whom Nagel cites approvingly), we can say
that these basic constituents of the universe are "transphysical
and transmental" (2012: 57). All that has been said up to this
point supports the view that Nagel endorses neutral monism.



But what Nagel says next seems to contradict this simple picture. He
writes:



> 
> Everything, living or not, is constituted from elements having a
> nature that is both physical and nonphysical--that is, capable of
> combining into mental wholes. So this reductive account can also be
> described as a form of panpsychism: all the elements of the physical
> world are also mental. (Nagel 2012: 57)
> 



Note that the description of the basic constituents has changed from
"transphysical and transmental" to "physical and
mental"--from the *Neither View* to the *Both
View*. And Nagel is very much aware that these are not the same
thing. In an earlier paper in which he considers neutral monism, he
writes:



> 
> this view would imply that the fundamental constituents of the world,
> out of which everything is composed, are neither physical nor mental
> but something more basic. This position is not equivalent to
> panpsychism. Panpsychism is, in effect, dualism all the way down. This
> is monism all the way down. (Nagel 2002: 231)
> 



Is there a way to overcome this apparent tension and to see these two
views as parts of a coherent whole? In a personal communication Prof.
Nagel offered the following explanation:



> 
> the fundamental elements would be neither merely physical nor merely
> mental, but something that was necessarily both physical and mental,
> (or protomental); but since this necessary connection can't hold
> directly between the physical and the mental as we conceive them, it
> would require that the real character of these fundamental
> constituents be something more basic that accounts for their being
> both physical and (proto)mental.
> 



The resulting picture is this. Described at the most fundamental
level, the constituents of the world have properties that are neither
mental nor physical. These neutral properties of every fundamental
entity give rise to physical and mental (or protomental) properties.
Thus each fundamental constituent is complex: it has mental (or
protomental) properties, it has physical properties, and it has these
two sets of properties as a necessary consequence of its having a
third set of properties--the neutral properties.



The foundational role played by the neutral properties (in the
*Neither Sense*) can be taken to suggest that the view is a
form of neutral monism. This interpretation can be further supported
by arguing that the neutral properties ground the other properties of
the basic entities, and that grounded properties make for "no
addition to being". However, it departs from traditional neutral
monism in holding that the basic constituents have *intrinsic*
physical and mental (or protomental) properties in addition to the
neutral
 ones.[23]
 Thus, it is perhaps best characterized as a form of the dual-aspect
theory or property dualism. But others might call it a panpsychism
since each fundamental entity has mental (or protomental)
features.


### 5.4 Russellian Monism, Panprotopsychism, and Panqualityism



In the search for a solution to the mind-body problem one may be
captivated by the thought that we know less about matter than we are
commonly led to believe. All we do (or can) know are the dispositional
properties of
 matter.[24]
 When combined with the insight that dispositions need categorical
grounds, one discovers that one's conception of matter is
radically incomplete. In addition to all the dispositional properties
that physics treats of, matter must have intrinsic categorical
properties. Since the nature of these intrinsic properties is unknown,
the following bold thought suggests itself. Perhaps these properties
play a dual role: in addition to grounding to dispositional properties
of matter, they also serve as the grounds of our conscious experience.
These are the kinds of considerations that may lead one to Russellian
monism, the view that



> 
> matter has intrinsic properties that both constitute consciousness and
> serve as categorical bases for the dispositional properties described
> in physics. (Alter and Nagasawa 2015: 1)
> 



The basic constituents of the world of Russellian monism are the
fundamental entities of physics. But their most fundamental properties
are not ones fully captured by physical descriptions, but rather the
intrinsic properties in virtue of which they fit such descriptions.
And these very same intrinsic properties, when arranged appropriately,
give rise to conscious experience.



There are many versions of Russellian Monism. Differences are due, in
part, to varying views about the relations between the intrinsic
properties, on the one side, and the mental and material properties,
on the other. But the central disagreement concerns the nature of the
intrinsic properties. They have been held to be physical (Stoljar
2001; Pereboom 2011, 2015; Montero 2015), mental (Bolender 2001;
Chalmers 2017; Schneider 2017), mental and physical (Strawson 2015,
2016, 2020; Goff 2017), or neutral (Coleman 2014, 2017a). Accordingly,
there are physicalist, idealist, panpsychist, and neutral monist
versions or Russellian monism. The precise nature of these
disagreements is difficult to pin down, given the number of different
notions of the mental and the material that are in play in this
discussion (see 1.1 above).



The best-known version of Russellian neutral monism is
panprotopsychism--a view that David Chalmers has explored over
many years (Chalmers 1996, 2015). In the standard form of this theory
the postulated intrinsic properties are characterized as being neither
experiential (they are *proto-*psychic) nor physical (they lack
the structural/dispositional nature of physical properties). That
makes them neutral (in the *Neither Sense*). But this purely
negative characterization of the intrinsic properties has struck many
as unsatisfying.



A variant of this theory--panqualityism--addresses this
problem in a most satisfactory manner, while creating new problems of
its own.



It takes redness, greenness, sweetness, roundness, etc.--the
primitive sensory qualities given to us in experience, and considered
just as such--to be the intrinsic properties of the fundamental
physical entities (cf. Chalmers 2015: 272). One's being aware of
redness--i.e., the property of phenomenal redness--is
mental, but redness itself is not. Nor is (this kind of) redness a
physical, structural property. So panqualityism has secured a neutral
base with which we are intimately familiar. Chalmers conjectures
that



> 
> versions of [panqualityism] were popular among the neutral monists of
> the early twentieth century, including William James (1904b), Ernst
> Mach (1886), and Bertrand Russell (1921). (Chalmers 2015: 271)
> 



But panqualityism also has a growing number of contemporary defenders.
Sam Coleman, for instance, has developed panqualityism in a number of
interesting new directions and restored its place within ongoing
debates about the metaphysics of mind (Coleman 2014, 2017a, 2017b,
2022).



Of course, one may wonder how an electron's being red can ground
its negative charge or share Wilfrid Sellars' (1963: 35) view
that it makes no sense to think that fundamental physical entities
might have primitive sensory qualities like redness. One may also
share Chalmers' worry that qualities, no matter how intricately
arranged, cannot ground the phenomenal properties of our experience
for "no instantiations of qualities ever necessitate awareness
of qualities" (Chalmers 2015: 273). Whether panqualityists can
offer satisfactory replies to such objections is a matter of ongoing
debate.



What the consideration of Russellian Monism shows is that (i) new
versions of neutral monism are currently being developed that
emphasize somewhat different considerations than those of traditional
neutral monism; (ii) but even the most promising and well-developed
versions of Russellian neutral monism face considerable
challenges.


### 5.5 Realistic Empiricism: Powerful Qualities



In his book *The Realistic Empiricism of Mach, James, and Russell.
Neutral Monism Reconceived* (2014), Erik Banks presents a critical
survey of the big three of neutral monism, as well as his own
development of the doctrine.



The neutral monist core of Banks's view amounts to this:



> 
> individual events are neutral, neither mental nor physical. Neutral
> events make up "physical" systems and extensions and
> "mental" sensations in minds through different functional
> relations. (Banks 2014: 203)
> 



This is very much in keeping with the traditional version of neutral
monism, especially Russell's event-based version. But Banks
embeds this core idea into a larger metaphysical framework. The
resulting theory is thoroughly original.



In a first step, Banks explains how this core idea fits into a larger
*a posteriori* physicalist picture. Physicalism, according to
Banks, is best thought of as the view that mental supervenes on the
physical. Standard physicalism focuses on the question of how mental
properties and relations depend physical properties and relations. But
Banks sides with the Russellian monist in holding that standard
physicalism does not specify the nature of the entities that exemplify
these properties and relations. So-called "enhanced
physicalism" goes beyond standard physicalism in specifying the
nature of the entities that bear or instantiate the relevant
properties and relations:



> 
> In enhanced physicalism...the instantiation of all physical
> properties are individualized event particulars in causal-functional
> relations to each other. (Banks 2014: 147)
> 



This is how the neutral events slot into the enhanced physicalist
picture that Banks favors.



In a second step, Banks provides us with an account of events. Events
have, and are individuated by, intrinsic characters or concrete
qualities. None of those qualities are mental; but experience
familiarizes us with some of them (see Banks 2014: 6). These qualities
are the ways certain powers manifest themselves in events (see Banks
2014: 6). Examples of such powers (or energies) include
electromagnetism, gravitation, and nuclear forces, and, most relevant
in the present context, neural energy--the internal energies in
neurons (see Banks 2014,149, 203). Manifesting itself qualitatively at
the level of the single neuron, this energy may yield an electrical
discharge event; but manifesting itself at the level of a complex
brain event--an event that is "somehow
'composed' of neurons firing in some kind of
cluster" (Banks 2014: 147)--this very same neural energy
may yield the event that is a sensation of blue. This closes the
apparent chasm between the experience of blue and the firing of a
bunch of neurons:



> 
> the quality blue and the individual electrical discharges are just
> different and mutually exclusive manifestations of the same natural
> powers which we mistakenly see as belonging to totally different
> categories of event. (Banks 2014: 164)
> 



This is an attractive picture, but it is difficult to see it as a
monism of neutral events. Wherein does the neutrality of these
events--an experience of blue and neural firing
events--consist? Moreover, it is not obvious that events (whether
neutral or not) play a fundamental ontological role. In their place we
find the powers/energies that give rise to events.



Banks addresses this second problem head on: Powers are identical with
their token manifestations, identical with the events that consist in
the individual qualities wherein these powers manifest themselves (see
Banks 2014:
 149).[25]
 There is no separate, more fundamental level of reality underlying
the events. The powers just *are* the events. The neural energy
is identical with the event that is the electrical discharging of a
group of neurons; this same neural energy is also identical with the
event that is a sensation of blue. But, and this is a crucial part of
Banks's view, the discharge events are not identical with the
blue sensation. Banks sees the question that this poses clearly:



> 
> how can it be the case that the powers are identical with each of
> their token manifestations and even identical qua powers across
> different token manifestation events, but that different token
> manifestation events are not identical to each other? (Banks 2014:
> 149)
> 



This is not an easy knot to unravel. Perhaps an appeal to the
controversial notion of "relative identity" might help.



Assuming that the fundamentality of events has been established, we
still face the question in which sense they qualify as neutral.
Consider



> 
> the event of seeing a blue patch and the event of having all the
> *configured* neurons fire in the region of the brain
> responsible for seeing the blue patch. (Banks 2014: 164)
> 



To the naive opinion that the first of these events is mental,
while the second is physical, Banks replies as follows:



> 
> The phenomenon of sensation simply falls into place as a certain type
> of physical event among others in nature. The separate category of
> mental phenomena simply ceases to exist, except as a provisional way
> of talking. (Banks 2014: 164)
> 



The monism expressed in this answer is beyond reasonable doubt; but
the neutrality is somewhat elusive.


## 6. Concluding Remarks



The traditional versions of neutral monism of Mach, James, and Russell
reflect the empiricist outlook of their authors. This leads them to
turn to experience when searching for their neutral entities. And this
leads many to see their doctrines as tainted by mentalism. Perhaps the
traditional neutral monists were wrong to forge this tight link
between the neutral entities and experience. Or perhaps critics are
too quick in dismissing their efforts to discover a neutral base in
this area. The rising interest in the history of analytic
philosophy--especially the gradual rediscovery of Russell as a
metaphysician and epistemologist--offers some hope for better
understanding of traditional neutral monism.



But the fortunes of neutral monism are no longer exclusively tied to
the fate of traditional neutral monism. The neutral monist strand
within the broader movement of Russellian monism is a promising new
development. And some contemporary attempts to revive neutral monism
are free of any ties to the traditional versions of the doctrine.
Thomas Nagel's recent defense of neutral monism comes to mind.
And the idea to make abstract entities--information, structure,
computation, mathematical reality--into the neutral basis of a
metaphysical system is being actively developed by philosophers and
scientists alike.



Until quite recently it seemed that neutral monism (in its traditional
form) was a brief and unimportant sideshow on the grand stage of
metaphysics. Notwithstanding the fact that the three main
protagonists--Mach, James, and Russell--were important
figures, whose ideas profoundly influenced many areas of contemporary
thought, their speculations about neutral monism seemed to have
vanished from the philosophical scene. But the current wave of
interest in the traditional versions of neutral monism, as well as the
lively development of alternative versions of neutral monism, indicate
that neutral monism is, once more, becoming a live option in the
ongoing efforts to explore the metaphysics of
 mind.[26]