Neutral Monism

First published Thu Feb 3, 2005; substantive revision Tue Jan 31, 2023

Neutral monism is a monistic metaphysics. It holds that ultimatereality is all of one kind. To this extent neutral monism is inagreement with the more familiar versions of monism: idealism andmaterialism. What distinguishes neutral monism from its monisticrivals is the claim that the intrinsic nature of ultimate reality isneither mental nor material but rather, in some sense, neutral betweenthe two.

Neutral monism is compatible with the existence of many neutralentities or kinds. And neutral monism is compatible with the existenceof non-neutral entities or kinds—mental and material ones, forexample—provided that they are, in some sense, derivative ofultimate reality’s neutral intrinsic nature. Most versions ofneutral monism have been pluralist in both these respects. They wereconceived as solutions to the mind-body problem. Their goal was toclose the apparent chasm between mental and material entities byexhibiting both as grounded in more basic neutral entities.

Any version of neutral monism will therefore have to answer thefollowing three questions: (1) What are the neutral entities and whatis their nature?, (2) What is the relationship of these neutralentities to matter?, and (3) What is the relationship of these neutralentities to mind?

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 ofneutrality gives rise are unique to neutral monism, they need to beaddressed first. Most versions of neutral monism assume a plurality ofbasic, neutral entities (these could be substances, events, tropes,universals, etc., depending on the specific form of the view). Whatdoes it mean for an entity to be neutral? Here are five proposals:

The Neither View: A basic entity is neutral just in caseit is intrinsically neither mental nor physical.
The Actual Constituent View: A basic entity is neutraljust in case it is a constituent of both physical and mental non-basicentities.
The Possible Constituent View: A basic entity is neutraljust in case it can be a constituent of both physical and mentalnon-basic entities.
The Law View: A basic entity is neutral just in case bothmental laws and physical laws are applicable to it.
The Both View: A basic entity is neutral just in case itis intrinsically both mental and physical.

(1)–(5) are not always clearly distinguished; but even when theyare, two or more of these criteria may be used concurrently. Thisinvites confusion on the part of the neutral monists, as well as theircritics.

Questions arise about the relationships between (1)–(5). (1) and(5) are similar in letting the question of an entity’sneutrality be settled by its intrinsic nature. (2)–(4) answerthis question by looking at the relationships of a given entity toother things: to the nature of larger groups of entities of which thegiven 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 thatneutrality is a matter of intrinsic nature alone, (2)–(4) willseem misguided. (2)–(4) still might have a place as epistemiccriteria—as ways of finding out whether a given entity isneutral. The friends of (1) and (5) can agree that a givenentity’s ability to actually or possibly belong to a certaingroup, or to be governed by certain laws, is due solely to itsintrinsic nature. Their disagreement concerns only the questionwhether this ability is conferred on that entity by its being bothmental and physical or neither mental nor physical. In either case(2)–(4) can only be used to discover which things are theneutral entities.

The traditional versions of neutral monism—those developed byErnst Mach, William James, and Bertrand Russell—accept (1) andreject (2). Russell, for instance, describes neutral monism as theview that “both mind and matter are composed of a neutral-stuffwhich, in isolation, is neither mental nor material” (Russell1921: 25). But some critics of neutral monism, notably Bostock (2012:190, 195–6), interpret neutral monism as committed to (2). Basedon this interpretation of neutrality, Bostock arrives at the strikingconclusion that “Russell’s version of neutral monism wasnever 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 aconstituent of both physical and mental non-basic entities. But it isdoubtful whether any of them would agree that the fact that a basicentity can be a constituent of both physical and mental non-basicentities is sufficient to make it neutral. For it is conceivable thata basic entity that fails to be neutral (according to (1), theNeither View) should figure in the construction of bothphysical and mental non-basic entities.

(4) is present in the thought of Mach and James. But it plays aprominent part in Russell’s thought. In the opening chapter ofhis 1921The Analysis of Mind, Russell remarks:

There are, it seems to me,prima facie different kinds ofcausal laws, one belonging to physics and the other to psychology. Thelaw of gravitation, for example, is a physical law, while the law ofassociation is a psychological law. Sensations are subject to bothkinds of laws, and therefore are truly “neutral” in[Edwin] Holt’s sense. But entities subject only to physicallaws, or only to psychological laws, are not neutral, and may becalled respectively purely material and purely mental. (Russell 1921:25–26)

His use of (1) in conjunction with (4), especially in Russell 1921, isconfusing, and it has led a number of his critics (Stace 1946; Bostock2012) to argue that his neutral monism is best understood as a form ofdualism (at least in his early writings).

A more general issue with criteria (2)–(4) is that they makeneutral monism compatible with materialism, idealism, and/or dualism,as traditionally understood. So long as the basic entities,nomatter what their intrinsic nature may be, satisfy the criteriaspecified in (2), (3), or (4) they are, according to these criteria,neutral. Gregory Landini, for instance, assumes that (3) capturesRussell’s idea of neutrality and concludes that he is aphysicalistand a neutral monist (Landini 2011: ch.6).According to Landini, Russell is a neutral monist because every basicentity can be a constituent of both physical and mental non-basicentities; and Russell is a physicalist because all basic entities ofhis system are physical.

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

If we admit—as I think we should—that the patch of colormay be both physical and psychical, the reason for distinguishing thesense-datum from the sensation disappears, and we may say that thepatch of color and our sensation in seeing it are identical. (Russell1921: 143)

A sympathetic reading of passages such as these suggests that they arebest understood as an abbreviated way of saying that the given neutralentity is a member of two groups of neutral entities, one of whichcounts as physical, the other of which counts as mental. In fact, manyneutral monists hold that the terms “physical” and“mental” apply only to groups of neutral entities, notindividual ones:

The stuff of which the world of our experience is composed is, in mybelief, neither mind nor matter, but something more primitive thaneither. Both mind and matter seem to be composite, and the stuff ofwhich they are compounded lies in a sense between the two, in a senseabove them both, like a common ancestor. (Russell 1921: 10–11)

But a number of contemporary discussions of neutral monism employ theBoth View. This view has, for instance, been attributed toThomas Nagel (Godfrey-Smith 2013: 1–2; Skrbina 2005: 237).^[1] TheBoth View has the advantage of capturing one intuitivenotion of neutrality: that which is both mental and material is biasedtowards neither and hence neutral between them.

Despite this appeal, theBoth View idea of neutralityconflates (or at least invites conflation of) neutral monism withother theories from which it should be distinguished. Indeed,depending on how the claim that the basic entities are intrinsicallyboth mental and physical is understood, neutral monism becomesdifficult to distinguish from panpsychism, dual-aspect theory, andeven the identity theory—since “being a pain” and“being a c-fiber stimulation”, for example, pick out thevery same property, aspect, or state rather than two distinct andirreducible ones (see sections 4.2, 4.3, and 4.6 below). In contrast,theNeither View idea of neutrality allows us to understandneutral monism as a distinct type of theory, and so is in this respectpreferable.

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 sincethese terms are used in different ways, the notion of the neutralvaries accordingly. In the case of the term “mental”, mostneutral monists understand it as picking out entities in terms offeatures traditionally associated with mental phenomena, such asintentionality, experientiality, subjectivity, qualitativity, unity,purposiveness, and so on. But there are diverging views among neutralmonists about which of these features are unique to and/or possessedby mental phenomena. Similarly, there is no consensus about whetherthe term “physical” is restricted to those aspects ofreality that can be fully captured by the descriptions of physics orextends beyond such descriptive resources (Stoljar 2001). And whilesome use the terms “material” and “physical”interchangeably, others see “material” as applying toentities with the features traditionally ascribed to matter (such asoccupying space, crowding out other such entities, and enduringthrough changes in features) whereas “physical” might pickout entities lacking one or more of them. Most pressingly, if“mental” and “physical” are defined ascomplements of each other, thereby ruling out the possibility ofentities to which neither term applies, then there cannotbeany neutral entities (at least in theneither sense).

Another worry is that (1)–(4) don’t say anything about thenature 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 distinguishbasicneutral entities from those of panpsychism, dual-aspecttheory, or the identity theory. Some versions of neutral monismembrace this agnosticism about the nature of the neutral base. Butothers have been more forthcoming, offering examples of neutralentities with which we are familiar.

The three traditional neutral monists—Ernst Mach, William James,and Bertrand Russell—all pointed to entities encountered inexperience as examples of neutral phenomena. Mach speaks of“sensations”, James of “pure experience”, andRussell of “sensations” and “percepts”.Predictably, this has fueled the suspicion that neutral monism isnothing but rebranded phenomenalism. Others have sought the neutralbase in the realm of the abstract. Kenneth Sayre, for example, turnsto the mathematical theory of information and suggests that theneutral base is to be understood as the “ontology ofinformational states” (Sayre 1976: 16) that this theorypresupposes (see section 5.1 below). What is clear is that it is noeasy task to characterize basic entities in a way that is bothsubstantive and incontrovertibly neutral.

1.2 Monism

There is, then, considerable disagreement about how to interpret thenotion 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 ofneutrality employed, it is possible to distinguish different versionsof neutral monism based on the various ways they understand the notionof monism (see Schaffer 2016). Neutral monism might, for example, beinterpreted as a form of existence monism—the view that only asingle concrete thing exists; but it might also interpreted as amonism about the kinds of things there are—the view thatconcrete things, be they simple or complex, are of the same neutralkind. The evidence suggests that all past and present versions ofneutral monism have been pluralist on both these counts: they workwith a plurality of basic neutral entities; and, given their focus onthe mind-body problem, they recognize at least mental and physicalkinds of entities in addition to the neutral ones (though somedescribe them as useful fictions). All entities belonging to suchnon-neutral kinds must, however, count as derivative. In the finalanalysis, all entities must be basic neutral entities or complexes ofsuch entities.

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

And even within the level of the basic neutral entities we candistinguish those versions of neutral monism that hold that all basicentities belong to a single category, from those that distinguish twoor more categories of neutral entities. Russell’s later versionof neutral monism seems to be monistic (in this particular sense): allneutral entities are events.^[2] But it is easy to envision a neutral monism that countenances neutralsubstances 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, inaddition 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 betweenthese two kinds of entities. Every answer to this question faces twochallenges. On the one hand there is the threat of elimination of thenon-basic entities (see section 4.1); on the other hand there is thethreat that the non-basic, non-neutral entities turn into elements ofultimate reality, thereby putting an end to neutral monism.

The question of the relationship between basic and non-basic entitiesis a contested topic among the traditional neutral monists (Wishon2021). James holds that “pure experience” constitutes aninterconnected continuum out of which different portions become“mental” or “material” depending on theircontext, their relations to other such portions, and the purposes theyserve and/or effects they result in (see 2.2 below). In contrast, Machand Russell arguably hold that the non-basic entities are composedof—or perhaps merely practical groupings of—a multitude ofdiscrete neutral entities organized by physical and/or psychologicalcausal-functional relations.

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

2. Traditional Versions of Neutral Monism

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

Spinoza’s metaphysics of substance has been calledneutralmonism; it is a form of monism because it allows for only onesubstance, and it is neutral because he describes the one substance asboth a body and a mind. (Rosenkrantz and Hoffman 2011: 287)

To classify Spinoza as an existence monist may be permissible. But theneutrality claim is based on the problematicalboth view ofneutrality. This suggests that it is best to classify Spinoza as adouble aspect theorist. Hume’s case is more difficult to assess.It has been said that the prevalent 20^th centuryinterpretation of Hume has been neutral monist (see Flage 1982: 527).H.H. Price’s case for this view has been particularlyinfluential (see Price 1932: 105–6). On the other hand, theneutral monist interpretation of Hume has been vigorously resisted(Flage 1982; Backhaus 1991).

In light of these difficulties it is best to start the genealogy ofneutral monism with thebig three: Ernst Mach, William James,and Bertrand Russell. Of the three, only Russell uses the label“neutral monism” (a term seemingly coined by his teacherJames Ward).^[5] But there is widespread agreement that Mach, James, and Russell arethe three most important philosophers in this tradition. A strikingfeature of traditional neutral monism—a reflection of theempiricist leanings of its protagonists—is the close tie betweenthe neutral entities and experience. This connection has given rise toone of the most enduring criticisms of neutral monism: that itcollapses into phenomenalism. It is therefore important to note thatthis salient feature of traditional neutral monism need be no part ofit.

2.1 Ernst Mach

Ernst Mach (1838–1916) occupies a central position in thehistory of neutral monism. He influenced William James and BertrandRussell and, through them, all of the writers on neutral monism in theEnglish-speaking world. His importance for the development of the viewin the German-speaking world is hard to overestimate. Among thephilosophers to build on Mach’s ideas was Rudolf Carnap in hisAufbau (1928). As a physicist who also did physiological andpsychological research, Mach strove to adopt an inclusive andeconomical framework that would allow him to pursue all of theseinquiries in a unified and coherent fashion. In the simple componentsof experience—colors, sounds, pressures, intensities, times,spaces, motor sensations, etc.—he finds typical examples of amuch larger group of elements whose functional interrelations arestudied by the various sciences (see Banks 2014, ch. 1 on the resoluterealism of Mach’s view). While a given element is,intrinsically, neither mental nor physical, the various groups towhich it belongs may display functional relationships that arecharacteristic of physics or of psychology. In this case the neutralelement forms part of the subject matter of physics and of psychology,respectively. In the following quotation Mach uses a color and ourperception of it to illustrate this point. A single neutralelement—the color—gets to be both the physical color of aphysical object and our mental perception/sensation of it. The colorcan be called physical,qua constituent of the one group, andmental,qua constituent of the other group, but is the sameunchanging and intrinsically neutral element that figures in these twodifferent contexts:

Thus the great gulf between physical and psychological researchpersists only when we acquiesce in our habitual stereotypedconceptions. A color is a physical object as soon as we consider itsdependence, 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 psychologicalobject, a sensation. Not the subject matter, but the direction ofinvestigation, is different in the two domains. (Mach 1886:17–18)

In this way material objects and the ego are dissolved intoelements/sensations that are related in certain complex ways. Spelledout for material objects, this reads:

thing, body, matter, are nothing apart from the combinations of theelements,—the colors, sounds, and so forth—nothing apartfrom 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 thesensation green, signifies that the element green occurs in a givencomplex of other elements (sensations, memories). WhenIcease to have the sensation green, whenI die, then theelements no longer occur in the ordinary, familiar association. Thatis all. Only an ideal mental-economical unity, not a real unity, hasceased to exist. (Mach 1886: 23–24)

For Mach the world presents itself as “a viscous mass [ofelements], at certain places (as in the ego) more firmly coherent thanin others” (Mach 1886: 17). The neutral elements (only a minutefraction of which are sensations) and their relations are the basicreality. We draw boundaries around certain groups of elements that arerelated to each other in interesting ways, because this serves ourbiological, scientific, and/or practical purposes. We can continue totalk about material things and selves; it is economical to do so. But,strictly speaking, “both [object and ego] are provisionalfictions of the same kind” (Mach 1905: 9).

The primary source for Mach’s views on neutral monism are anumber of essays and chapters contained in his books originallypublished in 1883, 1886, 1894, and 1905. The size of these books grewsignificantly as they went through numerous editions. Some of theimportant papers on neutral monism are not contained in the availableEnglish translations of these works.

2.2 William James

William James (1842–1910) uses the term “radicalempiricism” for the view he sets forth in James 1912—theview that has become a paradigm of neutral monism. His critique of therelational account of experience—according to which the selfdirects an act onto an object—was the model upon which Russelllater shaped his analysis of experience. James presents this argumentas an attack on a particular conception of consciousness present invarious forms in the Neo-Kantian, early analytic, and phenomenologicaltraditions. Roughly, it is the notion of consciousness as a diaphanousor transparent relation, medium, or container by means of which theobjects of consciousness are presented or represented to us. But theconsciousness that makes this kind of object presentation possibleeludes our grasp. This thin notion of consciousness is the one Jameswants to eliminate:

I believe that ‘consciousness,’ when once it hasevaporated to this estate of pure diaphaneity, is on the point ofdisappearing altogether. It is the name of a nonentity, and has noright to a place among first principles. Those who still cling to itare clinging to a mere echo, the faint rumor left behind by thedisappearing ‘soul’ upon the air of philosophy. (James1904b: 2)

His radical proposal is to simply discard this shadowy something andto make do with what remains, with what used to be the object of theconscious act. He introduces the term “pure experience” tostand for this datum. Prior to any further categorization, pureexperience is, according to James, neutral—neither mental norphysical:

The instant field of the present is at all times what I call the‘pure’ experience. It is only virtually or potentiallyeither 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, representationand represented are then interpreted as resulting from differentfunctional groupings/carvings of the continuous flux of pureexperience for different purposes (see James 1905: 64).

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

The paper seen and the seeing of it are only two names for oneindivisible fact which, properly named, isthe datum, thephenomenon, or the experience. The paper is in the mind and themind is around the paper, because paper and mind are only two namesthat are given later to the one experience, when, taken in a largerworld of which it forms a part, its connections are traced indifferent 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 ofconceptual representation we are dealing with

two pieces ofactual experience belonging to the samesubject, with definite tracts of conjunctive transitional experiencebetween them. (James 1904a: 53)

The first piece of pure experience is the thought—the episode ofthinking as well as the content that is thought—perhaps aboutHarvard’s Memorial Hall, as in James’s famous example (seeJames 1904a: 55ff). And, in the simplest case, the second piece ofpure experience is the thing—Memorial Hall—that wasthought of, as well as the perceiving of Memorial Hall. The thoughthas, let’s suppose, led us to Memorial Hall and now we stand infront of it and see it. It is this function ofleading (aboutthe details of which James has much to say) that constitutes thethought’s intentionality, that constitutes the fact that thethought was a thought about a certain thing. Once the thought has ledone to the Hall,

the percept not onlyverifies the concept, proves itsfunction of knowing that percept to be true, but the percept’sexistence as the terminus of the chain of intermediariescreates the function. Whatever terminates that chain was,because it now proves itself to be, what the concept ‘had inmind’. (James 1904a: 60–1)

This is James’s way of reconstructing the idea of representationin a way that does not invoke suspect mental powers of intrinsicintentionality. He values this as a signal achievement of his radicalempiricism:

The towering importance for human life of this kind of knowing lies inthe fact that an experience that knows another can figure as itsrepresentative, not in any quasi-miraculous‘epistemological’ sense, but in the definite practicalsense of being itssubstitute in various operations,sometimes physical and sometimes mental, which lead us to itsassociates and results. (James 1904a: 61)

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

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

2.3 Bertrand Russell

Following a series of critical engagements with neutral monism (seeespecially Russell 1914a,b), Russell adopted it in 1918 and remained aneutral monist for the rest of his long career: “I am consciousof no major change in my opinions since the adoption of neutralmonism” is what he says in an interview from 1964 (Eames 1969:108). But the question of whether Russell’s neutral monism isbest seen as a single theory or as a sequence of related butsignificantly different theories, as well as the question of which, ifany, of these different doctrines should count as versions of neutralmonism, have been much debated (see Wishon 2015; Pincock 2018; Bostock2012; Stace 1946). Russell’s 1919 and 1921 are generallyconsidered to represent the early versions of his neutral monism.Russell’s 1927a and 1927b contain the mature doctrine. In hislater works—Russell 1948 and 1956a—he no longer used theterm “neutral monism”. But the doctrine seems largelyunchanged.

Here is a succinct first pass at describing the core ofRussell’s neutral monism:

Russell argued that the traditional distinction between“mind” and “matter” is unfounded and that thesubject matter of both physics and psychology concerns collections ofcausally ordered events in space-time. Some of these events, onesoccurring in the brains of complex creatures like us, are the mentalepisodes that we are directly aware of in having conscious experience.These very same events can also be described in abstract structuralterms by physics and neuroscience. Regarding the rest of the(extracranial) events in nature, Russell maintained that our knowledgeof them is indirect and limited entirely to the abstract structuraldescriptions provided by the various physical sciences. (Wishon 2016:64)

Unsurprisingly, this brief summary leaves out many key details thatare important for understanding the character of Russell’sneutral monism.

2.3.1 Logical Constructionism and Neutral Monism

When Russell looks at neutral monism, he sees a theory that embodiesthe spirit of the “supreme maxim in scientificphilosophising” (Russell 1914c: 155) in an especially strikingway. This maxim is Occam’s Razor: entities are not to bemultiplied without necessity. In one of Russell’s formulationsit reads: “Wherever possible, substitute constructions out ofknown entities for inferences to unknown entities” (Russell1924: 326). Known entities are those that we know directly ornoninferentially; unknown entities are ones we accept only on thebasis of inferences from known entities. Russell’s maximencourages us to discover complex structures of known entities thatcan play the role the inferred entities were supposed to play. Uponsubstituting these complex structures—theconstructions—for the inferred entities, everything continues towork as before. The point of this complicated procedure is primarilyepistemic: we are now no longer burdened with the risky inference tothe inferred entities, and thus the risk of error is reduced. Strictlyspeaking, Russell remains agnostic as to the existence of the originalentities targeted by the construction. They are neither identifiedwith nor eliminated by the corresponding construction. Still, theconstructions make it possible to arrive at a simplified account ofwhat there is that fits our empirical evidence and exhibits variousother theoretical virtues such as greater continuity, causaluniformity, and so on. Thus, the resulting metaphysical theorydeserves our provisional assent over competing ones to a degreeproportional to such non-demonstrative grounds.

Neutral monism holds out the alluring promise that such constructionscan be found for all of the inferred entities of physics, psychology,and common sense. All of these bodies of knowledge are preserved, butat a hugely reduced epistemic cost. These are the considerations thatdrive Russell to embrace neutral monism: it is a perfect fit with hislongstanding commitment to the program of logical construction. Anadded benefit of substituting constructions of neutral entities forphysical and mental entities is an elegant solution to the mind-bodyproblem.

Logical construction is a process of discovery, not of manipulation:to logically constructxs out ofys is to discoverthatys, when they manifest a certain type of structure, canplay the role ofxs. Russell’s constructions of a pointin space, an instant in time, a table, and of a belief may illustratethe procedure. (i) A Russian doll is a good model of how Russell(following Whitehead) proposes to construct a point in space. Therough idea is to take “the class of all those objects which, asone would naturally say, contain the point” (Russell 1914a: 117)and to substitute this class of nested objects (the structure ofys) for points in space (thexs). (ii) Experiencedinstants in time (thexs) are constructed by discovering thatthe experiences of a person, each of which is extended in time, canoverlap each other in ways that converge on an instant. It is

a group of events, all belonging to his experience, and having thefollowing two properties: (1) any two of the events overlap; (2) noevent outside the group overlaps with every member of the group.(Russell 1927b: 288)

(iii) Rather than viewing a table as the cause of our tablesensations, Russell proposes (again just as a first approximation) toview the table as “the set of all those particulars which wouldnaturally be called ‘aspects’ of the table from differentpoints of view” (Russell 1921: 98). (iv) The logicalconstruction that is to take the place of beliefs consists of thefollowing three components:

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

As these examples show, there is no single method of construction, nosingle type of structure, suitable for different projects of logicalconstruction. The nature of thexs (the targets ofconstruction) and of theys (the construction materials)guide the search for structures that will work in the case athand.

2.3.2 Known Neutral Entities

The initial stock of known entities with which Russell proposes tocarry out this grand project of logical construction seems frightfullysmall and strikingly non-neutral. It consists of his sensations andimages—these loom large in Russell 1919, 1921—and hisperceptions (percepts)—these play a prominent role in Russell1927a, 1927b, 1948. Everything else—electrons, apples, galaxies,experiences, and selves, etc.—is in need of logicalconstruction.

For an entity to be neutral is to “have neither the hardness andindestructibility of matter, nor the reference to objects which issupposed 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)—thatthey consist of a mental act of sensing directed at a non-mentalobject—was, however, a pivotal part of his earlier view. Butthen his views changed:

I formerly believed that my own inspection showed me the distinctionbetween a noise [the object] and my hearing of a noise [the act ofsensing], and I am now convinced that it shows me no such thing, andnever did. (Russell 1918b: 255)

All that Russell now finds upon introspecting a sensory episode is asingle item: a sensory quality “which may be calledindifferently 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 simplyisthat patch of colour…the patch of colour and our sensation inseeing it are identical. (Russell 1921: 142–3)

And since noises and patches of color are not intentional—theyare not directed at anything, they are simply there—they areintrinsically non-mental. They constitute “sensations”only when related to other such entities in an organized system suchthat they play the rightcausal role (Russell 1996; Wishon2020, 2021). This establishes the neutrality of sensations. Andbecause 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 andimages and are likewise intrinsically neutral.

2.3.3 More Neutral Entities: Realism

Russell’s focus on sensations, images, and percepts explains whyRussell’s neutral monism has often been dismissed as a form ofphenomenalism. But in the preceding section we have seen how Russellargued for the intrinsic nonmentality of these items. Moreover, thiscriticism overlooks that Russell worked with a vastly expanded set ofconstruction materials. He allows for the enlargement of the realm ofthe known by a series of cautious inferences.^[7] In a first step he argues that “all our percepts are composedof imperceptible parts” and have imperceptible structure(Russell 1927a: 282, 386; Wishon 2020, 2021); second, there aresensations, images, and percepts that are not our own; third, and mostimportantly, we can infer the existence of vast numbers of entitiesthat “do not form part of any experience” (Russell 1921: 25).^[8] In this way Russell arrives at the view “that the world is veryfull of events” (Russell 1927a: 258), only a vanishingly smallnumber 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 intimatequalitative” (Russell 1927a: 389) kind; it reveals something ofthe intrinsic character of these entities.^[10] But our knowledge of the physical world [as described by physicalscience] is (almost) entirely abstract: “we know certain logicalcharacteristics of its structure, but nothing of its intrinsiccharacter” (Russell 1927b: 306–307).^[11] In this sense, physics merely describes the “causal skeleton ofthe world” and leaves us in the dark as to the intrinsic qualityof the events that make it up (Russell 1927a: 388, 391).

Russell takes all of these different entities to be neutral (in thesense of theNeither View); and he takes all of them to be events^[12], where an event is understood as “something occupying a smallfinite amount of space–time” (Russell 1927b: 287). Thenoise and the color patch considered above are typical examples ofevents. A Russellian event is absolute, in C.D. Broad’s sense:“it should notitself be a state of invariance or ofchange in the qualities or relationships of any thing” (Broad1959: 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’ anythingelse” (Russell 1927b: 289). This means that we must not think ofmental events (or any events) “as consisting of motions of bitsof matter” (Russell 1927b: 292), for “matter inmotion…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. Thismass of ever-changing events manifests all sorts of complex dynamicpatterns. We have not yet discovered the laws that govern these eventsand give rise to the formation of these complex dynamic patterns. Butwe have physics, which describes the behavior of certain groups oflaw-governed events under the name of “matter”. And wehave psychology, which describes a set of quite different regularitiesthat obtain in very specific regions of this plenum ofevents—viz. in those regions that are filled by the pieces ofmatter that are called “brains”. But all the buildingblocks of the logical constructions that are matter, and all thebuilding 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 toinclude in the list of neutral monists. In the German-speaking world,we find Richard Avenarius (1843–96), who was in contact withMach, and Joseph Petzoldt (1862–1929), who built on the work ofAvenarius. Avenarius’s main reflections on neutral monism arecontained in his 1888/90, 1891, and 1894/95. Petzoldt’s relevantworks 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 inthis group of neutral monists, but Ralph Barton Perry(1876–1957)—a student and friend of James—is the onemost closely associated with the view.^[13] Perry’s 1912 is the main source for his views about neutralmonism. Edwin B. Holt (1873–1946) developed an ambitious neutralmonist program in his 1912 and 1914. Other notable early neutralmonists 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), amongothers.

3. The Case for Neutral Monism

The case for neutral monism is best made by enumerating itsvirtues.

3.1 Parsimony

For starters, neutral monism shares the virtues of idealism andmaterialism in pairing down the kinds of basic entities to which wemust be committed. Whereas dualism posits that there are basicentities of two fundamentally different kinds—matter andmind—neutral monism posits that the basic entities are all of asingle, neutral kind. On many such views, there are also non-basicentities (such as minds and matter) grounded in the basic ones. But onothers, “mind” and “matter” are merelyconvenient groupings of the basic entities and there are no non-basicones.

The “immense simplification” neutral monism affords iswhat most attracted Russell to neutral monism (Russell 1959: 252). ForMach and James, it also promises to free us from any commitment to theexistence 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 beeliminated. (Mach 1886: 27–28)

First on the list of “metaphysical entities” (Mach 1905:13) that have to go are “the ‘unfathomable’ thingand the equally ‘unexplorable’ ego” (Mach 1905: 8).And James concurs: “Consciousness as it is ordinarily understooddoes 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 themind-body problem. Russell’s account of experience (ofperceptual consciousness) may serve as an illustration of the point.Russell frequently emphasized the miracle or mystery involved intraditional dualist accounts of perception (1927b: 147, 154; 1927a:275, 400). At the end of a purely physical chain of causes theremysteriously arises something of a completely different nature: anexperience (a sensation of red, say). This raises difficult questionsabout how exactly these two seemingly different aspects ofreality—matter and mind—are related to one another and howthey could be causes and/or effects of each other.

Materialism holds that mind is wholly grounded in basic materialentities or features of reality and thus that mind-body causalrelations are ultimately a matter of physical causal relations. Butthere are longstanding worries about whether properties of mentalphenomena such as qualitativity, intentionality, and/or subjectivitycan be wholly grounded in material entities or properties. Idealism,in contrast, holds that matter is wholly grounded in basic mentalentities or features of reality and that mind-body causation isultimately a matter of mental causal relations. But critics questionwhether material phenomena can be adequately accounted for wholly interms of mental phenomena.

Neutral monism can be an attractive alternative for those who see thechallenges faced by these traditional positions as insurmountable.^[14] In some cases, such interest in neutral monism is, perhaps, best seenas part of the general movement to explore all possible options in themetaphysics of mind. As Jerry Fodor puts it, “the form of aphilosophical theory, often enough, is:Let’s try lookingover here” (Fodor 1981: 31). But others see neutral monismas having resources the traditional positions lack and perhaps canmake use of themselves.

Neutral monism addresses the mind-body problem by asserting that mindand matter are not as different as they might first appear. On theboth view, this is because the basic neutral entities areintrinsically both mental and material. But it is difficult to see howsuch a view avoids collapsing into a form of panpsychism, dual-aspecttheory, idealism, or materialism. On the neither view, the apparent“gap” between mind and matter is narrowed by the fact thatboth are grounded in, or merely convenient groupings of, the samebasic entities which are themselves intrinsically neither mental normaterial. Accordingly, mind-body causal relations are ultimately justa matter of causal relations among different groups of the basicneutral entities.^[15]

One of the chief attractions of neutral monism is its promise forintegrating qualitative entities or properties into the natural worldwithout positing experientiality among its most basic constituents. Indoing so, it has the potential to vindicate both naturalists who holdthat mental phenomena such as consciousness arise only in the contextof complex systems of elements and those who see no hope of groundingthe qualitative aspects of consciousness wholly in non-qualitativematerial entities or properties. There is, of course, an open questionabout whether neutral monism can deliver on such promise.

Many philosophers of mind who are in some ways sympathetic to neutralmonism also find Russell’s views that physical theoriesabstractly describe the structure of physical reality while leaving usignorant about its intrinsic nature especially attractive. In fact,this aspect of Russell’s neutral monism is the primaryinspiration for the assortment of “Russellian monist”views that have been hotly debated in recent years (see 5.4below).

3.3 Perceptual Evidence for Physics

The Analysis of Matter (Russell 1927a) is widely regarded asthe most important text of the neutral monist literature. Chapter oneof this book, entitled “The Nature of the Problem”, statesit as follows:

the world of physics is,prima facie, so different from theworld of perception that it is difficult to see how the one can affordevidence for the other; moreover, physics and physiology themselvesseem to give grounds for supposing that perception cannot give veryaccurate information as to the external world, and thus weaken theprops upon which they are built…We must therefore find aninterpretation of physics which gives a due place to perceptions; ifnot, we have no right to appeal to the empirical evidence. (Russell1927a: 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 thecore of the solution Russell proposes. By “bridging the gulfbetween physics (as commonly interpreted) and perception”(Russell 1927a: 7), neutral monism promises to solve thisproblem—a problem that Russell took to be difficult and largelyunnoticed (see Russell 1948: 175–176).

3.4 Knowledge of the External World

Mach and James understand neutral monism as an especially radical formof achieving perceptual contact with the world. It might be understoodas a limiting case of naïve realism—a case in which therelation between the subject and its perceptual object becomes theidentity relation. In perception “subject and objectmerge” (James 1905: 57). A single reality—a red patch,say, when we see a tomato—is a constituent of two groups ofneutral entities: the group that is the perceiver, and the group thatis the tomato. The mind and its object become one. In James’swords:

A given undivided portion of experience, taken in one context ofassociates, play[s] the part of the knower, or a state of mind, or“consciousness”; while in a different context the sameundivided bit of experience plays the part of a thing known, of anobjective “content”. In a word, in one group it figures asa thought, in another group as a thing. (James 1904b: 9–10)

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

But Russell dissented. There are well known problems surroundingaccounts of perception that conceive it as a form of direct contactwith the world. Among them are the problems arising from the fact thatan object can present different and incompatible appearances todifferent observers, or even the same observer in differentcircumstances. Some neutral monists concluded that “nature is aseething chaos of contradiction” (Holt 1914: 276). But Russellchose to abandon the view that in perception “subject and objectmerge”. Instead he locates the red patch you see when looking ata tomato in your brain, and distinguishes it from the tomato (from thegroup of events that constitute the tomato). That is why he can assertthat what is happening in his brain is “exactly what naïverealism thinks it knows about what is happening in the outsideworld” (Russell 1927b: 138). This solves the problem ofdifferent and incompatible appearances: compatibility is restored bymoving the conflicting features away from the single physical objectand into the brains of the percipients. But in making this moveRussell abandons the naïve realism of Mach and James and puts aversion of representative realism in its place. The problem of ourknowledge 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 (seeMach 1886: 30). The great virtue of neutral monism is that it affordsa unified perspective from which scientific inquiry can beundertaken:

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

The idea of neutral monism as a bridge between different sciences isalso 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 tendencyof psychology and the anti-materialistic tendency of physics. (Russell1921: 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 theontological status of the entities it reduces: are they retained oreliminated? Both of these components are typically present intraditional neutral monism.^[17] Mach, for instance, alternates between claims such as that “thesupposed unities ‘body’ and ‘ego’ are onlymakeshifts, designed for provisional survey and for certain practicalends” and claims that they are composites constituted by“a more strongly coherent group of elements [that is] lessstrongly connected with other groups of this kind” (Mach 1886:20–21). Similarly, alongside Russell’s many logicalconstructions of various mental phenomena are remarks such asthis:

There are a number of words which I think should disappear from thepsychological vocabulary: among these I should includeknowledge,memory,perception, andsensation. (Russell 1996: 295)

What is clear is that neutral monism dispenses with mindastraditionally conceived by dualists and idealists and matteras traditionally conceived by dualists and materialists.There is no need to suppose that there are “solid persistentobjects moving through space” or states with an inbuilt andprimitive “reference to objects” (Russell 1921: 124, 36).But, on the other hand, it provides constructions of neutral entitiesthat are designed to play the roles of the entities they displace. Thesuccess of this strategy will have to be assessed on a case by casebasis. There are relatively few objections to neutral monism that turnon the details of proposed constructions or reductions. (But see C.D.Broad’s critique of Russell, 1925: 577–584). The mostchallenging criticisms of neutral monism do not engage with technicaldetails, but aim squarely at the central idea of thedoctrine—the idea of neutrality.

4.2 The Mentalism Suspicion

The most frequent type of objection to the traditional versions ofneutral monism is that they are forms of mentalistic monism: Berkleyanidealism, 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 thereis nothingneutral about these neutral monisms. This type ofobjection—the “mentalism suspicion”—has beenarticulated by a diverse group of philosophers, among them: (Lenin1909: 34; Hartshorne 1937: 221–2; Stace 1946; Ayer 1971; Feigl1958: 426, 1975: 26–7; Maxwell 1976: 354; Popper & Eccles1977: 199; Strawson 1994: 97; Chalmers 1996: 155; Tully 2003: 355,369).

Theprima facie plausibility of this objection is beyonddoubt. But Mach, James, and Russell were acutely aware of the problemand took themselves to have responded in a satisfactory manner. Theyreject the view that a sensation or an experience consists of asubject directing a mental act—awareness, consciousness,acquaintance—onto an object. Dispensing with the subject and theact, they are left with what used to be the object—a red patch,say. And they hold that there is nothing intrinsically mental aboutred patches. A red patch becomes a red-sensation simply by beingappropriately related to other entities of the same kind. There is noquick way to adjudicate this dispute; a careful assessment of eachcase is required. Still, a comparison of neutral monism with itsclosest mentalist alternatives can be a useful first step.

Panpsychism holds that every basic entity—usually understood asa physical entity—is also a mental entity. The physical nature(if present) and the mental nature of the basic entities arefundamental, i.e., irreducible (to each other or to anything else). Onthe face of it, panpsychism and neutral monism are strikinglydifferent. Neutral monism takes mental and physical phenomena to bederivative, panpsychism does not; neutral monism holds that basicreality is neutral, panpsychism does not. And neutral monism iscompatible with the view that most physical objects, and all theirparts, are absolutely nonmental, whereas panpsychism is not. This ishow things look on theNeither View of neutrality. On theBoth View of neutrality, things are less clear. The mostnatural reading of theBoth View will, however, not yieldpanpsychism—the view that every basic physical entity has amind—but a dual aspect view—a view according to whichevery basic entity has a physical and a mental aspect or side. In anycase, Mach, James, and Russell resisted panpsychist or dual-aspectinterpretations of their views, though there is evidence suggestingthat James adopted panpsychism sometime after 1904.

Phenomenalism has been defended as a doctrine about language, aboutfacts, and about things. Taken in this last sense, it attempts to“reduce material objects to sensa, that is, to explain them asconsisting solely of sensa or as being primarily groups or patterns ofthem” (Hirst 2006: 271). We might substitute the terms“sense-data”, “sensations”,“percepts”, “experiences”, etc. forHirst’s term “sensa”. As noted above, thetraditional neutral monists supposedly purged such terms of theirusual intrinsically mental dimension. Moreover, neutral monism is notlimited to those entities that are sensations, perceptions, and soon—this is particularly clear in the case of Mach and Russell.The existence of vast majority of neutral entities is inferred fromthe minute set of elements that, due to their causal-functional roles,happen to be sensations and perceptions. These inferred elements areoutside of all minds. These are the strong anti-phenomenalist (andanti-idealist) positions. How successfully the various neutral monistsdefend 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 ofmentalism. But a number of contemporary philosophers argue that it isbest understood as a form of physicalism.

We have noted how Landini’s interpretation of the notion ofneutrality (according to (3)—thePossible ConstituentView) allows him to argue that neutral monism is compatible withphysicalism. As he sees it, Russell is committed to such a version of(four-dimensionalist) physicalist neutral monism. Accordingly, Russellis engaged in the project of “constructing both [minds andmatter] out of orderings of physical events that are theirstages” (Landini 2011: 280). After a detailed discussion,Landini reaches the conclusion that Russell’s basic transientparticulars (or events) “are without intrinsic phenomenalcharacter” (Landini 2011: 297). This makes it possible to regardthem as physical entities in good standing. Their neutrality consistsin the fact that they are the building blocks of both mental andphysical continuants (see Landini 2011: 292).

Similarly, Erik Banks (see section 5.6 below) presents his so-called“realistic empiricism” as a direct descendant oftraditional neutral monism (Banks 2014, viii). But he also holds thathis neutral monism is “a kind of physicalism” (Banks 2014:7, 142). Banks takes himself to follow Russell embracing an ontologyof events as manifestations of underlying powers orenergies—such as electromagnetism, gravitation, and nuclearforces (Banks 149), as well as neural energies (Banks 2014: 142). Butevent particulars such as these, Banks insists, “aresophysicalistic in nature that there does not seem to be any reason toassume that these natural qualities in physics have anything at all incommon with our sensations, which are qualities of a very differentorder… [involving] events in the human nervous system at a verydifferent scale of complexity and size” (Banks 2014: 156).

After a careful survey of the development of Russell’s neutralmonism, Donovan Wishon observes that Russell’s post-1940 versionof neutral monism “has a greater affinity to RussellianPhysicalism than any genuinelyneutral monism” (Wishon2015: 114–5). Among other things, Wishon draws our attention toRussell’s report that “I find myself in ontologyincreasingly materialistic” (Russell 1946: 700). He also pointsto Russell’s remarks that “I should regard all events asphysical” and that “the distinction between what is mentaland what is physical does not lie in any intrinsic character ofeither, but in the way in which we acquire knowledge of them”(Russell 1958: 12). Hence, Wishon concludes, “mental events willturn out to be a subclass of the physical events that make upreality”—only their special epistemic accessibilitydistinguishes them from the other physical events (2015: 112). But hedoes not quite answer whether Russell, in this late period, took allevents to be physical due to their intrinsic natures or merely due totheir relations to other events.

These attempts to combine neutral monism with physicalism, or toreinterpret neutral monism along physicalist lines pose a seriouschallenge to neutral monism as usually understood. They deny thecentral claim that the fundamental building blocks of the world areneutral in the sense of being both nonmental and nonphysical.^[18] But no less noteworthy is the degree to which these versions ofphysicalism depart from more standard forms of physicalism. Inparticular, they agree with neutral monism that physical theories donot fully capture the nature of the world they describe. This suggeststhat the difference between neutral monism and this sort ofphysicalism 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 criticshold that neutral monism shares a common failing with materialism:namely, that it cannot accommodate experience. The argument is drivenby two deep metaphysical convictions. First, experience cannot bereduced to or constructed from the non-experiential; second, radicalemergence is unintelligible. Assuming that the neutral must benon-experiential, it follows that the neutral monist world has no roomfor experience. Galen Strawson has wielded this argument against allforms of traditional materialism, and occasionally also againstneutral 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): Heargues that there is no path that leads from the qualities weexperience (but which are not experiential themselves) to theexperience of those qualities. Having an experience is a matter ofhaving phenomenal qualities. Phenomenal qualities involve awareness ofqualities. But “no instantiations of qualities ever necessitateawareness of qualities” (Chalmers 2015: 273). This“quality/awareness gap” (Chalmers 2015: 273) shows that nostructure of qualities can add up to experience.^[19]

The traditional neutral monists would all agree that the mereinstantiation of qualities (which are intrinsically neutral) does notnecessitate any awareness of qualities. After all, such awarenessresults only when a number of such qualities are appropriately relatedto each other. However, they depart from Chalmers in holding thatsuitable relations among qualities are not only necessary forawareness, 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 ofcomplex causal-functional relations to other neutral events in anorganized system of which it is a member (i.e. the bundle-self). Thus,the traditional neutral monists would reject the view that nostructure 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 furtherquestion suggested by Strawson’s argument above: is experience,thus understood in a neutral monist setting, a feature that isemergent in an objectionable way?

4.5 The Problem of Emergence

The basic idea of emergence has to do with the fact that complexsystems may display interesting novel properties—properties notpossessed by their parts. This idea has been regimented in variousways. Following Chalmers, we can distinguish between“weak” and “strong” emergence. Weakly emergentphenomena are merely unexpected, given our knowledge of the domainfrom which they arise. Strongly emergent phenomena are not justunexpected; they cannot (not even in principle) be deduced from thedomain from which they arise (see Chalmers 2006: 244). There areongoing debates, however, about whether epistemic and/or logicalnotions such as deducibility are reliable guides to the nature ofreality. Such issues can be set aside by instead using the notion of“radical” emergence for cases in which complex systemsdisplay novel features that are not wholly grounded in the nature,features, or relations of their parts (Strawson 2006). Weak emergencewill, no doubt, be a wide-spread phenomenon in the neutral monistworld. It poses no problem. In contrast, neutral monism appears torule out radical emergence. Things are less clear in the case ofstrong emergence. Russell’s method of logical construction isincompatible with strong emergence, but it is an open question whetherother neutral monist accounts are.

Landini adopts the contrary view that radical emergence plays acrucial role in Russell’s neutral monism. If the fundamentaltransient physical particulars (or events) have no phenomenalcharacters (or qualia), such qualities must be understood as radicallyemergent 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 brainstates…. (Landini 2011: 302–305)

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

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

All of these events are percepts—the only kinds of events we canknow without inference. And, pace Landini, all of these events do havequalitative characters. When Russell speaks of emergence, he has inmind the relationship between physics’ abstract mathematicaldescriptions of events and their intrinsic qualitative characters. Andhe maintains that latter are (strongly) emergent with respect to theformer because there is no inferential path leading from abstractmathematical descriptions of an event to its intrinsic qualitativefeatures. For such abstract and mathematical considerations

cannot conceivably…prove that there are visual events, orauditory events, or events of any of the kind that we know byperception. (Russell 1927b: 295)

But it is debatable whether such qualities radically emerge fromanything. In any case, this is not how the neutral monist (who followsRussell’s lead) starts out. Our percepts are our initial data,and they are qualitative through and through. We simply do not have todeduce the existence of quality from other known facts, such as thoseinvolving complex brain states. Qualities are there, in the form ofour percepts, at the very foundation of Russell’s theory. Suchother events as we may believe in, whose intrinsic qualities we do notknow, are all inferred from this qualitative bedrock. Even so, theneutral monist insists, it doesn’t follow from the fact thatsomething is irreducibly qualitative that it is intrinsicallymental—or mental at all.

4.6 The Dualism Suspicion

Another common objection to neutral monism is that it constitutes aform of property dualism or dual aspect theory. The argument isstraightforward: On the neutral monist picture, physics describescertain relations—namely, the physical ones—among thebasic entities without capturing their intrinsic qualities, or thoseof the complexes of which they are parts. These latter features arerevealed to us only in the case of our sensations, percepts, and othermental episodes. This suggests that the basic entities exhibit twofundamentally different kinds of aspects or properties: extrinsicphysical relations and intrinsic mental qualities. But there’snothing properlyneutral about either kind of feature, andneither is reducible to the other. At best, they are two radicallydifferent aspects of an underlying reality which, in itself, isneither 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 thislabel is that there is an underlying reality that we can grasp asmental or as physical, depending on the point of view from which weapprehend it. Each one of us can know their own brain under each ofthese aspects—via introspection and (scientific) observation.But the claim of the theory is quite general: everything there is isto be understood as consisting of an underlying reality that has thesetwo aspects.

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

But if neutrality is understood according to theBoth View,the case for identifying neutral monism with dual-aspect theory isstrengthened. Much will turn on how the details of theBothView are articulated. It must not, for example, be understood asproposing the identification of mental and physical properties. Forthe dual-aspect theory insists that the two aspects are fundamentaland irreducible to each other. Another question concerns therelationship of the aspects and properties. If aspects are understoodas properties, dual-aspect theory may simply collapse into propertydualism (see, e.g., Van Gulick 2014). Dual-aspect theorists havepushed back by insisting that aspects are not properties (see Skrbina2014: 228–29). But this may, in turn, block the project ofreconciling the dual-aspect theory with neutral monism. For theneutrality, on theBoth as well as on theNeitherView, is understood in terms of mental and physicalproperties.

There is a lively debate concerning the relationship between neutralmonism, property dualism, and dual-aspect theory (see, e.g., Velmans2008; Skrbina 2014). The decision about these theories—whetherthey are identical, distinct but compatible, or incompatiblerivals—is still out.

4.7 Error

The neutral monism of Mach and James is committed to a naivelyrealistic account of perception according to which our perceptualexperiences and the aspects of the world perceived are one and thesame. 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 theworld does not contain the relevant entities.

But traditional neutral monism provides an ontology rich enough toaddress this problem. The pink patch experienced by the drunkardexists: it is a portion of pure experience (James), or an element(Mach). This much naïve realism is true, even in the cases ofnonveridical experience. But the pinkrat that the drunkardtakes himself to see does not exist—the pink patch he sees isnot a member of a group of neutral elements constituting a materialobject such as a rat. There is nothing wrong with the drunkard’svisual experience; what is mistaken is his assumption about how whathe sees is connected with the rest of the world. Russell (though not anaïve realist himself) states this point most succinctly:“There are in fact no illusions of the senses, but only mistakesin interpreting sensational data as signs of things other thanthemselves” (Russell 1948: 149–50). Mach puts the point asfollows:

When we consider elements like red, green, hot, cold and the rest,which are physical and mental in virtue of their dependence on bothexternal and internal circumstances, and are in both respectsimmediately given and identical, the question as to illusion andreality loses its sense. Here we are simultaneously confronted by theelements of the real world and of the ego. The only possible furtherquestion of interest concerns their functionalinterdependence…. (Mach 1905: 7–8)

5. New Directions for Neutral Monism

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

5.1 Information as Ultimate Reality

Anticipating the current revival in interest in neutral monism by afew decades, Kenneth Sayre (1929–) published his main ideas onneutral monism in the 1970s. Unlike thebig three, Sayrefinds the neutral base of his system not in experience, but in theform of pure information, where information is understood in thestrict information theoretical sense. His proposal must not bemistaken for the uncontroversial claim that that mental and physicalprocesses can bedescribed in information theoretic terms.Sayre puts forward an ontological claim about the ultimate nature ofreality—that ultimate reality consists of informationalstates:

If the project…is successful, it will have been shown not onlythat the concept of information provides a primitive for the analysisof both the physical and the mental, but also that states ofinformation…existed previously to states of mind. Sinceinformation in this sense is prior to mentality, but also implicatedin all mental states, it follows that information is prior also in theontological sense…Success of the present project thus will showthat an ontology of informational states is adequate for anexplanation of the phenomena of mind, as distinct from an ontology ofphysical events. [And Sayre adds:] It is a reasonable conjecture thatan ontology of information is similarly basic to the physicalsciences…. (Sayre 1976: 16)

One of the greatest challenges faced by the traditional versions ofneutral monism is to show how basic entities that are derived fromexperience can be neutral, rather than mental. By choosing an ontologyof informational states as his “neutral stuff”, Sayreelegantly bypasses this problem. But while the neutrality ofinformational states may be taken for granted, the question of therelationship of this seemingly abstract “stuff” toconcrete world of physical and mental entities becomes all the more pressing.^[21]

Sayre’s main work on neutral monism is his 1976. In recentdecades, David Chalmers has similarly explored the idea of aninformational ontology (see Chalmers 1996). And the related ideas thatultimate reality is purely structural (Ladyman and Ross 2007; Floridi2008, 2009), is a computational process (Fredkin 2003; Lloyd 2006), oris purely mathematical (Tegmark 2014) are the subject of a livelydiscussion.

5.2 Mind & Matter: A Merely Conceptual Distinction

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

because it carries with it the implication that there is anasymmetry in the identification of mental qualities withmaterial qualities: the mental is supplanted by the material. (Heil2013: 242)

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

Neutral monism includes the denial that there is amental–material chasm to be bridged. The mental–materialdistinction is, as Spinoza and Donald Davidson contend, a distinctionof conception only, not areal distinction, not a distinctionin reality. (Heil 2013: 242)

An example will help to illustrate the gist of this view. Take thewell-worn claim that pain is c-fiber stimulation.^[22] According to Heil, this says that the predicate “c-fiberstimulation” and the predicate “pain” apply to thesame things: all things that are truly described as being c-fiberstimulations are also truly described as pains. But—and this isa crucial claim that cannot be developed here—this doesnot mean that there is a property of being a c-fiberstimulation and a property of being a pain, and that these twoproperties stand in some interesting relationship, such as identity,reduction, realization, etc. What this does mean is that there is acomplex object that makes both of these claims true. The nature of theparts of this object, as well as their intricate relationships to oneanother, is the reality that is correctly described as a c-fiberstimulation and as a pain.

This part of Heil’s account appears to be consistent with thespirit of traditional neutral monism. One is reminded ofRussell’s pithy description of his project: “What I wishto do in this essay is to restate the relations of mind and brain interms not implying the existence of either” (Russell 1956a:145). This can easily be read as a denial of the existence of a chasmbetween mental and physical properties. And Heil’s complexobjects that serve as the truth-makers for mental and physicaldescriptions may be seen as analogous to Russell’s groups(logical constructions) of events that can properly be described inphysical and mental terms.

But there is an additional part to the neutral monistpicture—one that (arguably) all of the mainline neutral monistsagree on—that Heil rejects. When he tells the “deepstory” (Heil’s term) about the nature of the complexobjects (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 theyare like,how they are. (Heil 2013: 201–2)

So the basic constituents of the world—the substances and theproperties—that Heil describes arephysical. Here theneutral monist (who is committed to theNeither View ofneutrality) parts company with Heil. The deep story of the neutralmonist has it that the fundamental entities—be they events, bitsof information, substances, properties, etc.—are not physical(and not mental, of course). That is to say, they are not the sort ofthing that fundamental physics (or psychology) reveals.

5.3 Complex Basic Entities

In his bookMind and Cosmos (2012) Thomas Nagel asserts that“the weight of the evidence favors some form of neutral monismover the traditional alternatives of materialism, idealism, anddualism” (2012: 5). Neutral monism is understood as a view that“accounts for the relation between mind and brain in terms ofsomething more basic about the natural order” (2012: 56). Thisyields a picture of a “general monism according to which theconstituents of the universe have properties that explain not only itsphysical but its mental character” (2012: 56). Borrowing aconcept from Tom Sorrell (whom Nagel cites approvingly), we can saythat these basic constituents of the universe are “transphysicaland transmental” (2012: 57). All that has been said up to thispoint supports the view that Nagel endorses neutral monism.

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

Everything, living or not, is constituted from elements having anature that is both physical and nonphysical—that is, capable ofcombining into mental wholes. So this reductive account can also bedescribed as a form of panpsychism: all the elements of the physicalworld are also mental. (Nagel 2012: 57)

Note that the description of the basic constituents has changed from“transphysical and transmental” to “physical andmental”—from theNeither View to theBothView. And Nagel is very much aware that these are not the samething. In an earlier paper in which he considers neutral monism, hewrites:

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

Is there a way to overcome this apparent tension and to see these twoviews 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 merelymental, but something that was necessarily both physical and mental,(or protomental); but since this necessary connection can’t holddirectly between the physical and the mental as we conceive them, itwould require that the real character of these fundamentalconstituents be something more basic that accounts for their beingboth physical and (proto)mental.

The resulting picture is this. Described at the most fundamentallevel, the constituents of the world have properties that are neithermental nor physical. These neutral properties of every fundamentalentity give rise to physical and mental (or protomental) properties.Thus each fundamental constituent is complex: it has mental (orprotomental) properties, it has physical properties, and it has thesetwo sets of properties as a necessary consequence of its having athird set of properties—the neutral properties.

The foundational role played by the neutral properties (in theNeither Sense) can be taken to suggest that the view is aform of neutral monism. This interpretation can be further supportedby arguing that the neutral properties ground the other properties ofthe basic entities, and that grounded properties make for “noaddition to being”. However, it departs from traditional neutralmonism in holding that the basic constituents haveintrinsicphysical and mental (or protomental) properties in addition to theneutral ones.^[23] Thus, it is perhaps best characterized as a form of the dual-aspecttheory or property dualism. But others might call it a panpsychismsince 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 becaptivated by the thought that we know less about matter than we arecommonly led to believe. All we do (or can) know are the dispositionalproperties of matter.^[24] When combined with the insight that dispositions need categoricalgrounds, one discovers that one’s conception of matter isradically incomplete. In addition to all the dispositional propertiesthat physics treats of, matter must have intrinsic categoricalproperties. Since the nature of these intrinsic properties is unknown,the following bold thought suggests itself. Perhaps these propertiesplay a dual role: in addition to grounding to dispositional propertiesof matter, they also serve as the grounds of our conscious experience.These are the kinds of considerations that may lead one to Russellianmonism, the view that

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

The basic constituents of the world of Russellian monism are thefundamental entities of physics. But their most fundamental propertiesare not ones fully captured by physical descriptions, but rather theintrinsic 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, inpart, to varying views about the relations between the intrinsicproperties, on the one side, and the mental and material properties,on the other. But the central disagreement concerns the nature of theintrinsic properties. They have been held to be physical (Stoljar2001; 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 monistversions or Russellian monism. The precise nature of thesedisagreements is difficult to pin down, given the number of differentnotions of the mental and the material that are in play in thisdiscussion (see 1.1 above).

The best-known version of Russellian neutral monism ispanprotopsychism—a view that David Chalmers has explored overmany years (Chalmers 1996, 2015). In the standard form of this theorythe postulated intrinsic properties are characterized as being neitherexperiential (they areproto-psychic) nor physical (they lackthe structural/dispositional nature of physical properties). Thatmakes them neutral (in theNeither Sense). But this purelynegative characterization of the intrinsic properties has struck manyas unsatisfying.

A variant of this theory—panqualityism—addresses thisproblem in a most satisfactory manner, while creating new problems ofits own.

It takes redness, greenness, sweetness, roundness, etc.—theprimitive sensory qualities given to us in experience, and consideredjust as such—to be the intrinsic properties of the fundamentalphysical entities (cf. Chalmers 2015: 272). One’s being aware ofredness—i.e., the property of phenomenal redness—ismental, but redness itself is not. Nor is (this kind of) redness aphysical, structural property. So panqualityism has secured a neutralbase with which we are intimately familiar. Chalmers conjecturesthat

versions of [panqualityism] were popular among the neutral monists ofthe early twentieth century, including William James (1904b), ErnstMach (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 ofinteresting new directions and restored its place within ongoingdebates about the metaphysics of mind (Coleman 2014, 2017a, 2017b,2022).

Of course, one may wonder how an electron’s being red can groundits negative charge or share Wilfrid Sellars’ (1963: 35) viewthat it makes no sense to think that fundamental physical entitiesmight have primitive sensory qualities like redness. One may alsoshare Chalmers’ worry that qualities, no matter how intricatelyarranged, cannot ground the phenomenal properties of our experiencefor “no instantiations of qualities ever necessitate awarenessof qualities” (Chalmers 2015: 273). Whether panqualityists canoffer satisfactory replies to such objections is a matter of ongoingdebate.

What the consideration of Russellian Monism shows is that (i) newversions of neutral monism are currently being developed thatemphasize somewhat different considerations than those of traditionalneutral monism; (ii) but even the most promising and well-developedversions of Russellian neutral monism face considerablechallenges.

5.5 Realistic Empiricism: Powerful Qualities

In his bookThe Realistic Empiricism of Mach, James, and Russell.Neutral Monism Reconceived (2014), Erik Banks presents a criticalsurvey of the big three of neutral monism, as well as his owndevelopment of the doctrine.

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

individual events are neutral, neither mental nor physical. Neutralevents make up “physical” systems and extensions and“mental” sensations in minds through different functionalrelations. (Banks 2014: 203)

This is very much in keeping with the traditional version of neutralmonism, especially Russell’s event-based version. But Banksembeds this core idea into a larger metaphysical framework. Theresulting theory is thoroughly original.

In a first step, Banks explains how this core idea fits into a largera posteriori physicalist picture. Physicalism, according toBanks, is best thought of as the view that mental supervenes on thephysical. Standard physicalism focuses on the question of how mentalproperties and relations depend physical properties and relations. ButBanks sides with the Russellian monist in holding that standardphysicalism does not specify the nature of the entities that exemplifythese properties and relations. So-called “enhancedphysicalism” goes beyond standard physicalism in specifying thenature of the entities that bear or instantiate the relevantproperties and relations:

In enhanced physicalism…the instantiation of all physicalproperties are individualized event particulars in causal-functionalrelations to each other. (Banks 2014: 147)

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

In a second step, Banks provides us with an account of events. Eventshave, and are individuated by, intrinsic characters or concretequalities. None of those qualities are mental; but experiencefamiliarizes us with some of them (see Banks 2014: 6). These qualitiesare the ways certain powers manifest themselves in events (see Banks2014: 6). Examples of such powers (or energies) includeelectromagnetism, gravitation, and nuclear forces, and, most relevantin the present context, neural energy—the internal energies inneurons (see Banks 2014,149, 203). Manifesting itself qualitatively atthe level of the single neuron, this energy may yield an electricaldischarge event; but manifesting itself at the level of a complexbrain event—an event that is “somehow‘composed’ of neurons firing in some kind ofcluster” (Banks 2014: 147)—this very same neural energymay yield the event that is a sensation of blue. This closes theapparent chasm between the experience of blue and the firing of abunch of neurons:

the quality blue and the individual electrical discharges are justdifferent and mutually exclusive manifestations of the same naturalpowers which we mistakenly see as belonging to totally differentcategories of event. (Banks 2014: 164)

This is an attractive picture, but it is difficult to see it as amonism of neutral events. Wherein does the neutrality of theseevents—an experience of blue and neural firingevents—consist? Moreover, it is not obvious that events (whetherneutral or not) play a fundamental ontological role. In their place wefind the powers/energies that give rise to events.

Banks addresses this second problem head on: Powers are identical withtheir token manifestations, identical with the events that consist inthe individual qualities wherein these powers manifest themselves (seeBanks 2014: 149).^[25] There is no separate, more fundamental level of reality underlyingthe events. The powers justare the events. The neural energyis identical with the event that is the electrical discharging of agroup of neurons; this same neural energy is also identical with theevent that is a sensation of blue. But, and this is a crucial part ofBanks’s view, the discharge events are not identical with theblue sensation. Banks sees the question that this poses clearly:

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

This is not an easy knot to unravel. Perhaps an appeal to thecontroversial notion of “relative identity” might help.

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

the event of seeing a blue patch and the event of having all theconfigured neurons fire in the region of the brainresponsible for seeing the blue patch. (Banks 2014: 164)

To the naïve 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 typeof physical event among others in nature. The separate category ofmental phenomena simply ceases to exist, except as a provisional wayof talking. (Banks 2014: 164)

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

6. Concluding Remarks

The traditional versions of neutral monism of Mach, James, and Russellreflect the empiricist outlook of their authors. This leads them toturn to experience when searching for their neutral entities. And thisleads many to see their doctrines as tainted by mentalism. Perhaps thetraditional neutral monists were wrong to forge this tight linkbetween the neutral entities and experience. Or perhaps critics aretoo quick in dismissing their efforts to discover a neutral base inthis area. The rising interest in the history of analyticphilosophy—especially the gradual rediscovery of Russell as ametaphysician and epistemologist—offers some hope for betterunderstanding of traditional neutral monism.

But the fortunes of neutral monism are no longer exclusively tied tothe fate of traditional neutral monism. The neutral monist strandwithin the broader movement of Russellian monism is a promising newdevelopment. And some contemporary attempts to revive neutral monismare 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 ametaphysical system is being actively developed by philosophers andscientists alike.

Until quite recently it seemed that neutral monism (in its traditionalform) was a brief and unimportant sideshow on the grand stage ofmetaphysics. Notwithstanding the fact that the three mainprotagonists—Mach, James, and Russell—were importantfigures, whose ideas profoundly influenced many areas of contemporarythought, their speculations about neutral monism seemed to havevanished from the philosophical scene. But the current wave ofinterest in the traditional versions of neutral monism, as well as thelively development of alternative versions of neutral monism, indicatethat neutral monism is, once more, becoming a live option in theongoing efforts to explore the metaphysics of mind.^[26]

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Acknowledgments

We would like to give a special thanks to Galen Strawson for manyinsightful discussions about neutral monism and to an anonymousreferee for a set of very useful comments. We also thank the late ErikBanks, whose numerous writings on Mach and the history of neutralmonism taught us much about the view.

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