Introduction

According to property monism, there is only one ontological category of properties. It opposes property dualism, which considers properties as falling into two categories, physical and mental. Experiential monism is a panpsychist property monist metaphysics that itself comes in at least two forms: weak experiential monism (WEM), which claims that all properties are experiential, and strong experiential monism (SEM), which claims that all being is experiential. The former allows for non-experiential bare particulars, the latter does not.

Though strong experiential monism has many alluring virtues — parsimony being just one of them — it faces a deep problem: in virtue of what are the external experience of observing something and the internal experience of being that seemingly same thing actually experiences about the same thing at all? If there is nothing else to the being of an object than experience, no underlying substance to act as the carrier of properties, then there is nothing that could act as the ontological commonality between the experientially distinct internal experience of being something (its ‘self-experience’) and the external experience of observing that putatively same thing. If I were to meet you on the street, for example, the way you experience yourself and I experience you would be, as experiences, highly dissimilar. The phenomenal properties of your experience of yourself and the phenomenal properties of my experience of you would really have no overlap whatsoever — none of the phenomenal properties of your experience of yourself would figure as parts of my experience of you. If there is no object outside of the experiences themselves, it seems like these two experiences have no overlap whatsoever — they merely parallel each other in a functional wayFootnote 1. If there is no object over-and-above the experiences and the phenomenal properties involved, they seem to lack a shared ontological referent.

This, is, I feel, already a psychologically relatively distressing conclusion: that the thing I see were I to meet you on the street is an altogether different being than the being you are — that they have, quite literally, nothing in common. However, the conclusion is logically and conceptually acceptable; and one could argue along these lines that though my experience of you and your experience of yourself are about two ontologically distinct existents, what my experience is about is still a representation of your self-experience. Since your self-experience and its constituent phenomenal properties do not appear as a part of my experience, my experience of you can be at most just a representation of your self-experience; I simply don’t experience your self-experience directly. The relationship between that representation and your self-experience would then be the next question. A causal connection between them would be the natural choice here — that the representation is somehow ‘informed’ about your self-experience through a causal connection. However, since in SEM there is no physical space and physical matter to instantiate that causal connection, the connection, if such there be, would have to work through a brute and massively bloated nomology. Without an instantiating structure for the causal connection, there is little chance of reducing causality of this sort to a limited set of natural laws, such as the generalized laws of theoretical physics. Instead, what is needed is an infinite amount of specific and brute laws of the type: experience type X in field of experience A causes a representation type Y in field of experience BFootnote 2. This is hardly a satisfactory solution, leading essentially to a metaphysics closely resembling the monadology of Leibniz with its pre-established harmony between otherwise windowless monads.

This is the binding problem for strong experiential monism: experiences that seemingly have the same referent seem to lack the glue that would bind them together so as to make them actually be about the same referents at all. That glue could be either a shared object, so that the same ontological existent appears in both experiences (much like common sense considers the same physical object to appear in both experiences) or some other explanatory connection between them, such as a nomologically generalized, instantiated causal connection — in any case some kind of connection between them that does not necessitate an infinite amount of brute and arbitrary natural laws. I argue in this article that SEM is unable to accommodate any such connecting factor in its metaphysics. The experiential monist has to either (1) accept bare particulars to act as the shared commonality of the referents, relinquishing strong experiential monism; (2) accept that the referents of the internal and the external experience are ontologically distinct, and therefore embrace a kind of brute parallelism between the two; or (3) face a collapse back into property dualism.

But why care about strong experiential monism at all? Because strong experiential monism is the logical result of a combination of three theses all of which seem attractive to manyFootnote 3 philosophers: panpsychism, the view that phenomenal properties are ubiquitous; the Powerful Qualities view, a kind of property monism; and no bare particulars, that is, a unity of substance and property. I shall begin my elucidation of the fundamentals of this motivation with an exploration of the contrast between property dualism and property monism. I will then move on to explicate the two kinds of experiential monism and the problem facing the strong variant of experiential monism. Following this is a discussion of the alternatives for avoiding the problem. I conclude the article with some final remarks and a tentative nod towards weak experiential monism as the technically most appealing way forward for the experiential monist.

Property Dualism and Property Monism

Property dualism is distinguished from the somewhat more traditional substance dualismFootnote 4, which sees the physical and the mental as two separate kinds of substance. In substance dualism, minds and bodies are separate objects; there is an immaterial mind and a material (but as such mindless) body, and these two are separate substances, separate objects that at best interact with each other. According to property dualism, on the other hand, there is only a distinction between physical and mental propertiesFootnote 5. Both kinds of property can be carried by the same substance. Property dualism is therefore an ontologically monist view, that is, an example of substance monism. In the property dualist framework, the one and the same object, such as a human brain, or a person, can have both physical properties (pertaining to any of its features that are perceivable from the empirical third-person perspective, and available for consideration in the language of physics), as well as mental properties (such as feelings, pains, and other phenomenal properties perceivable from the personal first-person perspective). One way to express the property dualist idea (as well as define what ‘physical’ and ‘mental’ properties actually denote) is through a distinction between dispositional properties and qualitative propertiesFootnote 6 (e.g. Mumford 1998). Dispositional properties are the dispositions of the object having the properties to act or behave in specific ways in specific circumstances. Fragility is a common example: the glass being ‘fragile’ means roughly that it has the disposition to shatter when subjected to force or droppedFootnote 7. A more scientifically minded example would be the disposition of a negatively charged object to repel another negatively charged object. Dispositional properties are powers: powers to cause effects in other things, and powers to be affected by other things. In this way it can also be seen how dispositional properties are essentially causal in nature; dispositions are ways things have causal effects on each other in different circumstances.

Physical properties — the properties of things perceivable from the empirical, third-person perspective, and potentially available for consideration in theoretical physics — are often considered to be essentially dispositional (e.g. Stoljar 2001, Ney 2007, Pereboom 2015, Alter 2016). Whatever properties we can perceive from the empirical perspective seem always to be tied to the effects things have on our sensory organs and whatever measuring apparatuses we have at our disposal. Nothing can be perceived if there is no interaction between the perceived and the one perceiving; and interaction rests on things doing certain things in certain circumstances, that is, dispositionality. This idea forms the basis of the Russellian insight that the empirical sciences, including physics, can only grasp the dispositional, causal, or structural properties of reality, while remaining silent about its qualitative propertiesFootnote 8.

So what are qualitative properties? They are what’s left once you take out all the dispositionality: the way the thing exists in itself, its intrinsic nature. Dispositionality is potential — how things act and interact if specific circumstances take place — but qualitative properties are always actual — how a thing itself is. Mental (or phenomenal) properties are prime examples of qualitative properties: they are actual properties that a being has in itself. They furthermore do not seem to have any impact on how the world looks like from the empirical perspective — whether or not a bat, for example, has phenomenal properties and is as such a conscious, experiencing being has no impact on how the bat appears to an outsider. Qualitative properties seem to lack causal powers to affect other things.

This fact has led proponents of property dualism — those who uphold a relatively strict ontological distinction between qualitative and dispositional properties — into a pretty troublesome situation. It seems that all causal effects in the world can be explained without any reference to qualitative properties. This is usually expressed by way of referring to the causal closure of the physical (e.g. Kim, 2011): the domain of physical (and thereby dispositional) properties seems to be causally closed. There is neither need nor even space for mental (and thereby qualitative) properties to have any causal effect on anything. Property dualism is therefore susceptible to epiphenomenalism, or the causal impotence of the mental. Though some writers (e.g. Lyons, 2006; Baysan, 2020) do not find epiphenomenalism all that problematic, many think it to be an unfavourable outcome. Chalmers (2016 p. 193), for example, calls it an ‘unattractive choice’, whereas McLaughlin (2016 p. 320) describes it as ‘abhorrent’ and a ‘truly unhappy consequence’.

Though there are ways for the property dualist to explicate the picture so as to alleviate this troubleFootnote 9, this and other problems have led many to abandon property dualism and its distinction between the dispositional and the qualitative in favour of the property monist view. In property monism, there is only one category of properties. There are at least a few ways of building a property monist view. One is dispositionalism or the Pure Powers view, which dispenses with qualitative properties altogether, and claims that dispositions are all there really isFootnote 10. Another way is to dispense with the distinction itself and claim that the dispositional and qualitative features of the world are ontologically identical: the very same property is both a qualitative property and a dispositional property at the same time. It is unclear whether these two approaches are as different from each other as it would initially seem — Taylor (2018a) argues that both approaches result in the same ontology and that their difference is purely superficial. But I will in any case focus on an exploration of the second alternative, that of the identity between the qualitative and the dispositional, also called the Powerful Qualities view.

The Powerful Qualities view — spearheaded by Martin and Heil (1998, 1999), and later defended in one form or another by at least Strawson (2008, 2021)Footnote 11, Heil (2012), Howell (2013), and Taylor (2018a,b) — takes the difference between dispositional and qualitative properties to be purely epistemic and conceptual in nature. There are qualitative properties, and these qualitative properties have causal powers — thus, they are powerful qualities. It is not so that the same object has qualitative properties and on top of those it has dispositional properties. Neither is it so that the qualitative and the dispositional are separate aspects of the one self-same property — this would still introduce a divide between the two and, among other things, make the resulting view susceptible to epiphenomenalism of the qualitative aspects. The dispositional properties (and so the physical properties as they are traditionally understood — mass, spin, and charge) are not only ontologically reducible but in fact identical to qualitative properties, which carry the relevant causal powers qua qualitative properties. Another way to put this point is that the qualitative properties of an object serve as the truthmakers for its dispositionality. The only way to distinguish between the two is through abstraction in the traditional Lockean sense, that is, by considering the property only in part, on a conceptual level (see e.g. Heil, 2012 p. 52–54). No real distinction can be made between them. The Powerful Qualities view is thereby an excellent example of a deeply monist view on propertiesFootnote 12.

Experiential Monism, Weak and Strong

Many contemporary panpsychistsFootnote 13 are drawn to something akin to the Powerful Qualities view, and understandably so: it represents the same kind of tendency for unification and monism as panpsychism itself. The combination of panpsychism and Powerful Qualities, in turn, begins to tread on experientially monist territory. The Powerful Qualities view postulates that all properties are qualitative properties, and at least the most common varieties of panpsychism state that qualitative (i.e. intrinsic, categorical) properties are phenomenal propertiesFootnote 14. The combination therefore implies that all properties simpliciter are phenomenal properties. This qualifies the resulting view as an example of experiential monism. I shall in this article make a distinction between two forms of experiential monism: weak experiential monism (WEM) and strong experiential monism (SEM). Both forms are trivially committed to panpsychism: in both views all properties are experiential, and since properties are ubiquitous, so is experientiality. However, not all panpsychist views are examples of experiential monism. Property dualism is entirely compatible with panpsychism.

Weak experiential monism agrees with just the following conclusions: first, that all properties are (powerful) qualitative properties, and second, that all qualitative properties are experiential properties. This weak form of experiential monism still allows for an ontological distinction between substances and properties, since it only claims that all properties are experiential. Substances — ontologically distinct carriers of properties — can still be non-experiential. In this way, WEM allows for a kind of non-experiential being, even if this being is reduced to an empty, contentless carrier or binder of experiential properties — that is, a bare particular.Footnote 15

Strong experiential monism, on the other hand, discards all non-experiential being altogether. It is essentially the conjunction of weak experiential monism and the separate unificatory view of the identity between substances and properties, that is, between being and qualityFootnote 16. In this view, there is nothing to an object over-and-above its experientiality, no thin or bare particular, no carrier substance. There is no non-experiential being, no divide whatsoever between the experiential and the non-experiential; everything exists purely experientially, and this experientiality has causal powersFootnote 17. Strawson (2016, 2021), being perhaps the foremost contemporary exponent of strong experiential monism, encapsulates this idea neatly in his German dictum Ansichsein ist Fürsichsein, ‘being in-itselfness is being for-itselfness’; that is, intrinsic existence is experiential existenceFootnote 18.

SEM might at first glance appear quite difficult to understand, perhaps even absurd. But on closer scrutiny, it reveals itself to be quite an elegant view of the world. It is, for one, remarkably parsimonious: we already have indubitable evidence of experiential being, and by discarding all non-experiential being from the picture we are left with only this one ontological category. It removes the final traces of Cartesian dualism from our ontological picture. It dispels mysterianismFootnote 19 about qualitative properties, intrinsic natures, and thin (or bare) particulars. And furthermore, it gives us causally powerful qualitative properties without requiring a carrier theory of causation to answer questions about mental causation.

Accepting that the only indubitable kind of existence we are faced with in reality is the existence of experience, it might feel natural to seriously question whether we need anything else. Prima facie the view does not seem to face all that many problems either: it does not directly contradict any of our cherished beliefs in an objective outside world, in the multiplicity of objects, or in the existence of other beings save ourselvesFootnote 20. The only thing dropped away is the belief in non-experiential being, in that age-old metaphysical enigma of things existing somehow in themselves, but yet not for themselves.

Or so it seems on the surface. Deeper reflection on the matter shows that there is actually one pretty uncomfortable problem hiding under the waters, and that problem is the main subject of this paper.

The Binding Problem for Strong Experiential Monism

The problem is, in essence, the following: If all things exist in themselves as experience, and experientiality is the fundamental ground of being — that is, there is no other category of existence that grounds experience, or on which experience supervenes — in what way can the internal, first-person ‘self-experience’ of something be said to be about the same thing as the external, third-person experience of that thing? In other words, how is your experience of being you an experience of the same thing (i.e. you) as the experience of your friend who is looking at you? What serves as the ‘ontological glue’ that makes these things be about the same thing? What is their commonality?Footnote 21

Before I go on to unpacking these questions, let me add a further example that might help elucidate the idea. Imagine a person undergoing open-brain surgery in a doctor’s office. Their skull has been opened, yet they are still wide awake, as is common for open-brain surgery. A camera is filming the operation and is linked to a monitor in full view of the patient. The patient therefore sees their own brain via the monitor. Now, suppose for the sake of the argument that we humans are at least partly our brains, and that our experience of ourselves is in part an experience of our brains; it’s an experience of how it is to be this or that brain, and in that sense, it’s an experience about the brain. So the patient has an experience about their brain in virtue of being, in part, that brain. The patient also has an experience about their brain in virtue of seeing it via the monitor linked to the camera filming their brain. These are, then, two entirely distinct and phenomenally different experiences seemingly about the same ontological thing.

Keeping this scenario in mind, and taking into account that in strong experiential monism everything exists fundamentally as experience, in virtue of what is it that these two experiences of the patient are actually about the same brain at all? In virtue of what does the same brain, the same ontological object, appear in both these experiences? Furthermore, if the brain exists fundamentally as experience, what kind of experience is it? Does it exist as the first-person experience of being the brain, that is, the self-experience of the brain? Or does it exist as the third-person experience of someone observing the brain? The essential point here is that it cannot be both, since the first-person experience of the brain and the third-person experience of the brain are obviously distinct as experiences; as experiences they really have virtually nothing in common.

The same holds for every single object: the first-person experience of being that object (its intrinsic experiential being) and the third-person experience of observing that object externally cannot be identical as experiences. However, if there is fundamentally nothing else to that object than experientiality — there is no bare particular, no distinct substance — it seems that there is really nothing there in virtue of which the object seen from the external, third-person perspective, and the object experienced in the first-person perspective are the same object. Again, what would be the ontological commonality between those experiences? And what kind of being would that commonality have, if there is no non-experiential being, and experiential being is already accounted for by the first-person and the third-person experience under considerationFootnote 22? It seems like something else is needed, something of which both experiences are about, that carries all the relevant properties.

So we would need a kind of bare particular, some further intrinsic or categorical nature to act as the ‘object’, the thing that both experiences are about, so as to make them be about the same ontological thing at all. If we make this move, strong experiential monism naturally fails, since the bare particular will then be an example of non-experiential being that furthermore has to be at least as fundamental — if not more fundamental — than experiential being. Weak experiential monism — that there are only experiential properties — could still survive with such a bare particular, a substance with no additional properties of its own that simply carries the experiential properties. But this would require the experiential monist to relinquish what Strawson (2021) calls the ’fusion of the categories of substance and quality’, i.e. the idea that the existence of any object is nothing over-and-above its qualities at any given time.

Causal Connections Between Experiences

One could attempt to solve the problem by answering that the third-person experience of a given object represents the first-person self-experience of that objectFootnote 23. If you and I were to meet on the street, my experience of you would be a representation of your self-experience of yourself — and in that way both experiences could be seen to refer to the same object, your self-experience. This answer leaves open the important question of the actual relationship between the representation and the represented self-experience. Why do they parallel each other at least seemingly with such high fidelity? One prima facie appealing answer would be that there is a causal relationship between the representation and the represented experience, through which the representation is ‘informed’ (in a wide sense) about the represented experience. This answer has its own deep problems, however. In our common sense, physicalistically inclined view of the world, fields of experience (or minds) are attached to physical bodies, which are built of vast quantities of a relatively limited set of elementary particles, which have a relatively limited set of properties and fundamental interactions with each otherFootnote 24. These elementary particles and their interactions are, in turn, governed by a relatively limited set of generalizable natural laws. Because in the physicalist (or the property dualist) picture minds and experiences can be seen to supervene on physical objects, and the physical objects themselves are governed by a limited set of laws, causal connections between separate minds can also be explained with a limited set of laws.

However, in SEM there is no physical space over-and-above the experiences themselves, nor is there physical matter. There is no non-experientially existing ’container’ in which non-experientially existing physical objects could interact. Without any such container we can’t appeal to our everyday notions of spatial connectivity, continuity, or proximity to explain causal connections between fields of experience. Nor is there any structure out there beyond experiential properties that could instantiate the kind of generalizable causal connections that the physicalist (or property dualist) picture can point to. The key point here is, perhaps, that SEM diverges much more from ‘common sense’ property dualist metaphysics than might seem at first glance. Reducing existence to experience and cutting out any connecting structure over-and-above experience means that relations of material constitution also become suspect. Is the field of experience I am currently experiencing — ‘my’ consciousness — constituted of other fields of experience of a lower order? If not, we can’t reduce any causal connections between, say, your experience and my experience, to the causal connections between smaller and less diverse simples. Material constitution no longer makes sense, and we no longer have recourse to a limited set of causal laws between a limited set of different simples, as in the physicalist or the property dualist picture. Without a limited set of laws to govern causal interaction, we are faced with a massive over-proliferation of laws of an entirely arbitrary and brute kind to explain the seeming parallels between different experiences of putatively the same things.

The parallelism required here is at least somewhat reminiscent of Leibnizian panpsychist idealism, that is, monadology. In monadology, the world is composed of a vast hierarchy of souls (or monads), each of which is an experiencing being that perceives the universe in its own particular wayFootnote 25. There is no real existence besides these souls. Each soul is causally separate from the others — monads ‘have no windows, through which anything can enter or go forth’ (Leibniz, 1714/1867, section 7). Instead, they are governed by a pre-established harmony, set in place by God; and in virtue of this pre-established harmony all monads, though they have no direct effect on each other — their perceptions arising spontaneously ‘from their own depths’ — yet have a ‘perfect conformity relative to external things’, that is, relative to each other (Leibniz, 1989, p. 143)Footnote 26. In much the same way, the strong experiential monist would have to accept the following: that though the referents of the external and the internal experience are ontologically distinct, and though they share no direct causal link to one another, they still mirror each other perfectly with a lawlike reliabilityFootnote 27.

But this would indeed be a very brute kind of parallelism, without any further explanation of why and how it is in place. Without reference to the will of God or some other divine or cosmic entity as the explanation for this parallelism, we need an absurd amount of natural laws to uphold the harmony. Without reference to theism or a significantly more sophisticated theory on how such a fundamental parallelism could hold, I feel that this already somewhat unsatisfying view remains deeply problematic.

Other Potential Answers

One might try to find another solution to the binding problem in a kind of cosmopsychism, i.e. a view according to which the experientialities of any subsets of the universe are all grounded in the experientiality of the universe as a single whole entity. Bernardo Kastrup (2017), for example, seems to defend a cosmopsychist kind of strong experiential monism. Both my experience of being me and your experience of seeing me are, in Kastrup’s system, grounded in the cosmic consciousness’ experience of me. Perhaps the cosmic consciousness’ experience would then be the binding factor between the other two? The cosmic consciousness could, in essence, be the container in which other fields of experience are contained, as well as the medium through which they are connected.

This might work in principle, but only if the cosmic consciousness’ experience of me contains, as an experience, both your experience of being you and my experience of seeing you as they are in its own experience. For if there is nothing else to the two experiences other than their immediate phenomenality, no underlying substance, they would have to appear to the cosmic consciousness as they appear to you and me, for otherwise the cosmic consciousness could not be said to contain our experiences. If there is nothing else to experience than experience, then for A to be the same experience as B, A has to be identical to B as an experience. It is not enough that the cosmic consciousness would include a representation of our experience since this representation would be another experience qualitatively and numerically distinct from our own. And since our respective experiences of you both have a kind of inherent boundedness, a particular point-of-viewness, it is difficult to see how they could co-exist as such within a single conscious experienceFootnote 28. If they could be seen to do so in an understandable way, the cosmopsychist experiential monist might have recourse to the same answer as the bundle theorists: that the compresence of these experiences in the larger cosmic consciousness binds them together. But this also has its problems, since all experiences whatsoever would be compresent in the cosmic consciousness — and this would hardly help us explain the identity between just the two particular subsets of the contents of cosmic consciousness. All in all it thus far seems to me that, though the cosmopsychist solution could perhaps help explain the sustained parallelism between experiences, it is not sufficient to explain or ground the identity of their referents.

Weak experiential monism, on the other hand, does not face the problem, since it accepts that there can be a bare or thin particular, an underlying substance that acts as the binding agent and the referent of both the first-person and the third-person experience. Property dualism likewise faces no problem here. In a property dualist framework, even though the experiences are completely distinct as experiences, they are both about the same object carrying or manifesting the same physical properties — so in this case the physical properties act as the ontological glue. The same holds for dual-aspect theories, which consider the experiential as just the inner, intrinsic aspect of being, while the physical remains the extrinsic or outer aspect of beingFootnote 29.

What to do, then, in the light of these conclusions? I am personally still a bit torn between the alternatives. Technically I feel that the most promising way out of this impasse would be to accept bare particulars and weak experiential monism. Gregg Rosenberg’s (2004, 2016) Theory of Natural Individuals seems to me like the most elegant and explanatorily powerful weak experientialist model thus far explicated. It postulates that all causally effective properties are experiential properties, which are bound together in particular receptivities, or receptive connections. Receptive connections and effective properties bound together form causal nexuses, and these nexuses are conscious subjects. What makes the model an example of weak experiential monism is that the receptivities are ontological entities distinct from the effective properties themselves; the ‘point-of-viewness’ of the conscious subject stems from this receptivity, which resembles, in some ways, the kind of contentless awareness found in Hindu wisdom traditions, such as Advaita VedantaFootnote 30. Thus, the model keeps up the distinction between being and quality, or properties and their carrier. Rosenberg’s theory is nevertheless highly elegant and seems to me to handle many of the problems surrounding panpsychistically minded theories quite readily.

On the other hand, I feel a strong pull towards models that do away with a distinct subject of experience altogether: that is deflationist views on subjectivity. Subject-deflationism rests essentially on the unity or non-duality of being and quality. Dual-aspect theories of at least a panqualityist type such as they have been explored by e.g. Coleman (2016) are one possible candidate for a working subject-deflationist panpsychist metaphysics. But I also feel the pull of property monism, and especially experiential monism; and it seems to me that any experiential monist view that also embraces a deflationism about subjectivity would necessarily count as an example of strong experiential monism, and thus inherit its problems. I cannot but rest tentative as to this matter, though with a particular nod towards Rosenberg’s weak experiential monist model and its remarkable elegance.

Conclusion

Based on the considerations given in this article, it seems to me that strong experiential monism has to either (1) retreat to weak experiential monism and accept the existence of bare particulars; (2) accept a possibly untenable and brute kind of parallelism; or (3) face a collapse into property dualism or a dual-aspect monism. Out of these three I tentatively find (1) weak experiential monism to be technically the most elegant view. (3) Property dualism or dual-aspect monism can preserve a deflationist view on subjects while escaping the binding problem unscathed, which is a clear benefit to those who are drawn to subject-deflationism. The Leibnizian kind of parallelism of (2), on the other hand, seems to me to be a highly counter-intuitive view and technically difficult to work out, and though I feel it remains conceptually possible, I wouldn’t embrace it without a very good reason, since it would necessitate a multiplication of brute natural laws beyond all proportion, leading to a kind of arbitrary pre-established harmony. Other avenues of escape from the trouble facing strong experiential monism and the related forms of objective idealism might reveal themselves in due time, but as things stand right now, I believe the view faces a significant challenge.