Physicalism and the Privacy of Conscious Experience Miklós Márton Eötvös Loránd University Budapest Center for Theory of Law and Society János Tőzsér Research Centre for the Humanities Hungarian Academy of Sciences Biographies Miklós Márton (Phd) is currently assistant professor at Eötvös Loránd University Budapest, Center for Theory of Law and Society. His research interest includes the philosophy of mind and philosophy of language. János Tőzsér (PhD) is currently senior researcher at the Research Centre for the Humanities, Hungarian Academy of Sciences. His research interest includes the philosophy of mind and metaphilosophy. Acknowledgements We are grateful to Gergely Ambrus, Katalin Farkas, Gábor Forrai, Zoltán Jakab, Dávid Márk Kovács for their helpful comments on an earlier version of this paper, and to the participiants of the "Consciousness Conference" in Flint October 2015 for their inspirative comments. The research leading to this paper was supported by OTKA (Hungarian Scientific Research Fund), grant no. K109638, and by Bolyai Research Scholarship, grant no. BO/00028/13/2. Publication Details Journal of Cognition and Neuroethics (ISSN: 2166-5087). April, 2016. Volume 4, Issue 1. Citation Márton, Miklós, and János Tőzsér. 2016. "Physicalism and the Privacy of Conscious Experience." Journal of Cognition and Neuroethics 4 (1): 73–88. Journal of Cognition andNeuroethics 74 Abstract The aim of the paper is to show that the privacy of conscious experience is inconsistent with any kind of physicalism. That is, if you are a physicalist, then you have to deny that more than one subject cannot undergo the very same conscious experience. In the first part of the paper we define the concepts of privacy and physicalism. In the second part we delineate two thought experiments in which two subjects undergo the same kind of conscious experience in such a way that all the physical processes responsible for their experiences are numerically the same. Based on the thought experiments and their interpretations we present our argument for the inconsistency of the privacy of experience with physicalism in the third part of the paper. In the final part we defend our argumentation against some objections. Keywords Privacy of Conscious Experiences, Subjectivity, Physicalism, Property Dualism, Modularity of the Brain Introduction In this paper we would like to show that the privacy of conscious experience is inconsistent with any kind of physicalism. However, we do not conclude from this that physicalism is mistaken, we merely generate a dilemma. On the one hand, if one is a physicalist, then one has to deny our common sense conviction that only one subject can have a specific conscious mental state, that is, more than one subject cannot undergo the very same conscious experience. On the other hand, if one does hold this common sense conviction, one has to accept substance dualism and claim that conscious experiences are modifications of the immaterial mind. Our paper divides into four parts. In the first part we define the concepts of privacy and physicalism and we formulate our thesis. In the second part we delineate two thought experiments and interpret these. In the third part we present our argument for the inconsistency of the privacy of experience with physicalism. In the final part we defend our argumentation against some objections. Physicalism and the Privacy of Conscious Experience Miklós Márton and János Tőzsér Márton and Tőzsér 75 1. The thesis In order to formulate the thesis, we need to clarify the two concepts in question, so we have to define privacy and physicalism shortly. There are two different senses of the concept of privacy. In one sense of the term, conscious experiences always belong to a subject. As Gottlob Frege puts it in his 'Thought': It seems absurd to us that a pain, a mood, a wish should rove about the world without a bearer, independently. An experience is impossible without an experient. The inner world presupposes the person whose inner world it is. [...] [I]deas need a bearer. (1918/1956: 299.) In other words: conscious experiences always need an owner for their very existence. They cannot exist in their own right, that is, without the subject. To wit: for every experience e there is at least one subject S who has e and e cannot exist without S having it. This kind of necessary ownership constitutes the first kind of privacy or subjectivity of conscious experiences. However, according to some philosophers (for example Michael Tye or Ronald de Sousa) there is nothing extraordinary about the privacy of conscious experience in this sense, since it is not just her pains, fears and anxieties which belong necessary to a subject S, but her laughter, walk and state of health, as well. What is more, the falling of a stone belongs necessary to the stone in question. If this is true, then it will show that this conception of privacy has nothing to do with the 'mental' per se. Conscious experiences or occurrent states are events that happen, just as the above examples. So, according to these philosophers, they are private or subjective entities just because they are occurrent states of the owner, not because they constitute some special kind of entities (see e. g.: Tye 1995, 84-92; de Sousa 2002). Nevertheless, we rather focus on the other sense of the term, since we think this second sense of the concept of privacy plays a more essential role in the common sense conception of conscious experience. In this sense of the concept, every conscious experience can belong only to one subject. For example: Mary's pain can be felt only by Mary and Juliette cannot feel it, or vica versa, Juliette's pain can be experienced only by Juliette and Mary cannot experience it. As Frege wrote it a few paragraphs later: It is so much of the essence of each of my ideas to be the content of my consciousness, that every idea of another person is, just as such, distinct from mine. [...] No other person has my idea but many people Journal of Cognition and Neuroethics 76 can see the same thing. No other person has my pain. Someone can have sympathy for me but still my pain always belongs to me and his sympathy to him. He does not have my pain and I do not have his sympathy. [...] [E]very idea has only one bearer, no two men have the same idea. (1918/1956: 300.) To wit: for every experience e there is at most one subject S who has e. What does constitute this essential character of conscious experiences? The well-known answer is that only S can directly experience her conscious mental states; only S can undergo her particular conscious experience, and so, only S can access her conscious mental states in a way that nobody else can. In other words, whereas it can be true that anyone can have access to S's pain in some way, only she can feel it. That is, S has a private path to it. This kind of private access constitutes the second kind of privacy or subjectivity of conscious experiences. One clarification: The sentence "S has private access to her conscious mental states" does not say that there are two entities, namely S and her conscious mental state, whereas both of them exist in their own right and S has private access to the latter one. That is, private access is not a relation between two independent entities, since the entity to which the subject has this special kind of access, cannot exist without the subject having the access. In a certain sense, in the case of conscious experiences the very act and the result of it are just two aspects of the same thing. When we speak about private access we mean the first one, and when we speak about the entity to which the subject has private access we mean the second one. Let us compare the two conceptions of privacy. While the first one states that the occurrence of every conscious mental state presupposes a subject as an owner of it; according to the second conception, only one subject can experience a conscious mental state directly. So, while the former one does not exclude the possibility that more than one subject could experience the very same conscious mental state, the latter one does exactly that.1 1. There are several other formulations of the common sense thesis of privacy. For example, one can speak about the necessary subjective quality or inalienability of conscious experiences, or the esse est percipi character of them. In our opinion, these phrasings are either more opaque or ambigious than the two above, or can be subsumed under them. For example, it seems obvious that to claim that conscious experiences have an esse est percipi character is nothing more than to claim that they cannot exist without a subject who experience them, which is exaclty the content of the 'necessary ownership' sense of the thesis. Márton and Tőzsér 77 Let us turn to the concept of physicalism. As it is well-known, the precise content of the physicalist thesis is difficult to explicate, but everyone agrees about the following. Physicalism is the metaphysical thesis that every phenomenon in our world is physical. Naturally the above-mentioned difficulty arises from the fact that we have no consensual answer to the question of what physical properties are. Consequently, we do not wish to take a stand on the debate concerning the details of physicalism, so we will work with the following modest conception. There are fundamental physical phenomena (for example bozons, fermions and spin or charge), and all other phenomena depend on them for their existence. This dependence could be ontological. This means that the fundamental physical entities or some configuration of them bring about all the others with metaphysical necessity. In the usual phrasing: there is no possible world in which all facts about these fundamental physical entities hold, but some facts of the actual world do not. According to the orthodoxy, this ontological dependence of mental phenomena on the physical can be understood in three ways. (1) Mental properties are identical to physical (neurophysical) properties. As the good old (and empirically false) example says: pain = C-fiber firing. (2) Although mental properties are not identical to physical ones, there obtains a necessary supervenience or constitution relationship between them. In this conception, mental properties depend on physical ones in the sense that metaphysically there can be no difference in the former ones without a difference in the latter ones. In other words: if one determines a neurophysical entity together with its properties then one eo ipso determines the mental entity supervene on it together with its properties. (3) Mental properties necesseraly supervene on or are constituted by not merely some neurophysical properties, but also by further relevant facts of the external world. In a strict sense only these three conceptions should be called physicalism. However, there is another form of dependence that can be found in the theory of property dualism. Here is a standard formulation of the main thesis of this view: [C]onscious experience involves properties of an individual that are not entailed by the physical properties of that individual [...]. Consciousness is a feature of the world over and above the physical features of the world. This is not to say it is a separate „substance" [...] All we know is that there are properties of individuals in this world - the phenomenal properties - that are ontologically independent of physical properties. (Chalmers 1996: 125, italics in the original) Journal of Cognition and Neuroethics 78 As we can see from the quotation, property dualism is also a substance-monist theory which differs from strict sense physicalism in the sense that this theory does not commit itself to the ontological dependence of mental properties on the physical ones. Some kind of dependence nevertheless obtains between the two kinds of properties in this conception as well, namely there must be contingent psychophysical laws which connect them. As Dean Zimmerman says: „[t]here would have to be laws governing causal relations between microphysical events and emergent mental properties, laws that are sensitive to differences in microphysical duplicates [...]" (2003: 506.). In a word, the property-dualist denies the ontological dependence, but states nomic dependence instead (see also: Chalmers 1996: 240.). In spite of the main difference between any kind of strict physicalism and property dualism, they agree on a crucial point. Namely both claim that physical phenomena determine the mental ones, that is, mental properties depend on the physical ones. Therefore, they cannot allow the following: (1) there are mental states that do not connect to any physical entities at all (such as Descartes' clear acts of thinking); (2) there are mental states which connect to physical entities merely randomly. From this point of view, the difference of the two theories consists merely in the fact that in property dualism the dependence of mental properties from physical ones is assured by some nomological relation rather than a metaphysical one. We are finally in a position to state our thesis. The common sense conviction that a subject has private access to her conscious mental states, that is, for every experience e there is at most one subject S who can undergo e, is inconsistent with any theory according to which mental phenomena depend on physical ones in one of the above senses. Consequently, the privacy of conscious experiences is inconsistent with all three versions of strict physicalism and property dualism as well. 2. Two thought experiments and their interpretation Let us imagine that the parts of Mary's brain which are responsible for the pain in her lower back are damaged, so Mary cannot feel this kind of pain when she has lumbago. And let us also imagine that Mary's nerves are wired across to a healthy person's brain. Let us call this person Juliette. The wiring works in the following way: when Mary's nerves have been pinched in her lower back the neural information arrives from her waist to the parts of Juliette's brain which are responsible for lower back pain. Then, the information flows further to those parts of both Juliette's and Mary's brains which are, in the case of healthy people, Márton and Tőzsér 79 directly connected to the parts responsible for the pain in question. In this situation Mary feels her lower back pain through Juliette's brain. (Of course, poor Juliette would also feel lower back pain, since her appropriate brain-parts would be active.) Or imagine that in the future, technology of neurosurgery can produce a device (an implant) which is suitable to satisfy the function of the damaged brain parts. So, imagine that there will be such an implant which is wired into Mary's brain so that this device will be responsible for Mary's ability to feel lower back pain when her nerves would have been pinched in her waist. However, Mary and Juliette, who suffer from the same condition, will get a common implant which is wired into the both of their brains. From here, the story is similar to the one told above: when Mary's nerves have been pinched in her lower back, the neural information first travels to the common implant, and then flows further to those parts of both Juliette's and Mary's brains which are, in the case of healthy people, directly connected to the parts responsible for the pain in question. In this situation both Mary and Juliette feel lower back pain through the implant. We have created these thought experiments in such a way that in both of them each physical entity responsible for Mary's and Juliette's pain is the same. One and the same physical entities (the brain parts, the implant, the wiring, etc.) are responsible directly for their conscious experiences. The empirical plausibility of this claim hangs on the modular make-up of the human brain. According to this (oversimplified) modularity thesis, when a conscious experience occurs, only a certain part or parts of the brain and their connections are responsible for it. Or, in a reverse formulation: there are parts of the brain the activities and interconnections of which are not necessary conditions of the occurrence of a certain conscious experience (though they can be necessary for the occurrences of other kinds of experiences). In our case this means that we have to suppose merely that the other, uncommon parts of the two subjects' brains do not play any role in the occurrence of the conscious experience in question.2 However, one can say that the conception of the modular brain in itself does not support our interpretation of the thought experiments, namely that in the case of Mary and Juliette every relevant physical factor is common. While it can be true that the local 2. The modularity thesis can be supported by considerations concerning the possibility of evolutionary psychological explanations of mental functions. As Leda Cosmides és John Tooby write: "[...] natural selection will ensure that the brain is composed of many different programs, many (or all) of which will be specialized for solving their own corresponding adaptive problems. That is, the evolutionary process will not produce a predominantly general-purpose, equipotential, domain-general architecture". (Tooby – Cosmides 2005: 17) Journal of Cognition and Neuroethics 80 neural or "implantic" bases of their conscious experiences are the same, these common bases have different connections. In the situations described above, the same physical entity is connected to two different brains, therefore their appropriate relationships are quite different. The modularity thesis actually claims that it is not only particular parts of the brain, but also their appropriate relationships to other brain-parts which are necessary for the occurrence of some conscious experience. Consequently, since the latter differs in Mary' and Juliette's cases, it is not true that all the relevant physical factors are common. We think this possible objection misses its target. Remember for example the famous case of Phineas Gage (see e. g.: Damasio 1994: ch. 1). As it is well-known his brain's left frontal lobe was seriously damaged, and that injury had strange effects on his personality and behavior for the rest of his life. What is the moral of Gage's case? On the one hand, even such complex properties of a person as his personality or behavioral patterns can be associated to particular parts of the brain and their interconnections. It is plausible to suppose that this is also true in the case of a much more simple particular experience. On the other hand, and this is more important, in Gage's case the most remarkable fact is that his basic and several complex mental abilities remained intact after the injury. For example, according to the testimonies, his basic cognitive, linguistic and practical abilities survived the brain-damage to a great extent. One have to infer from this that the seriously damaged parts of his brain and its connections were not necessary for these abilities to work. In other words, Cage's case shows that a mental ability can survive the loss of some parts of the neural network, therefore the latter and its connections to other parts were not responsible for this ability. It is not the whole brain with its extremely complex neural interconnections which serves as the physical basis of a mental state. Consequently, it must be possible that two subjects share the relevant physical bases (brain parts plus interconnections) and individually possess only those brain-parts and connections (wirings) which are not necessary for the mental state in question. The above thought experiments show exactly this arrangement, so our interpretation seems to be empirically plausible in light of modularity.3 3. There may be a further worry about the correctness of our interpretation if there is some part or parts of the human brain which are necessary for all kinds of conscious experiences, even for all kinds of mental functions. If such a central processing unit really exists (and it is questionable) then it will be more difficult to conceive that each parts of the brain that are responsible for the pain in question are common, since in this case even this central universal parts must be shared by the two subjects, so we have to conceive them as totally incapable of any conscious experiences, or even any mental functions before the operation. However, we think that this possibility has no serious theoretical impact for our argumentation, though we acknowledge that it makes our interpretation empirically less plausible. Márton and Tőzsér 81 In sum, we interpret the thought experiments as follows. It is true in both fictive cases that (1) the two subjects equally feel pain and feel this pain as coming from her lower back; and (2) all the physical processes responsible for the experiences are the same. (1) seems phenomenologically evident in light of the situations described in the thought experiments, and (2) seems obvious, if we commit ourselves to the modularity of the brain. Of course, although our example of conscious experiences in these thought experiments was a certain kind of pain, you can substitute it for any other kind of conscious mental states (e. g.: perceptual experiences, moods, other bodily sensations, etc.).4 3. Arguments for inconsistency As we defined in the first section, the privacy of conscious experiences in the second, more interesting sense is the thesis that every experience can be directly experienced or felt only by one subject, and not more. It follows from this thesis that if two subjects have some conscious experiences, as in the above cases of Mary and Juliette, these experiences are numerically different. There are two conscious pain-experiences, rather than Mary and Juliette feeling literally the same pain. Let us see, why this common sense conviction is incompatible with any kind of physicalism, including property dualism. Maybe, the result will seem strange to some theorists since it is a widely held view in the literature that the question of privacy and that of physicalism are conceptually independent ones (see e. g.: Farkas 2008b: 15). Our argument against this view is, in a certain sense, quite simple: since every mentioned theory is committed to some kind of dependence of mental properties on some physical phenomena, and because all the relevant physical factors are the same for the two subjects' experiences, they depend on the same physical basis and therefore cannot differ from each other. Let us see the details of the argumentation. (i) As for the case of identity theory, it seems obvious that this kind of physicalism is incompatible with the privacy of conscious experiences. The situation is this: if two properties, a mental and a physical one, are identical, then any instances of them are also 4. We think that all this is not just empty philosophical phantasy. Consider the case of craniopagus conjoined twins. They are joined at their head and so have some smaller or larger brain parts in common. One of the most famous cases is that of Krista and Tatiana Hogan. According to the medical reports, the tickling of one of them triggers laughter in the other, or one of them stops crying when somebody puts a teat into the other's mouth (see: Dominus 2011). In our opinion this situation is similar to the fictive cases delineated in our thought experiments: two subjects have supposedly similar conscious experiences and one and the same physical (neurophysical) processes are responsible for them. Journal of Cognition and Neuroethics 82 identical. For example, any instances of pain are identical to an instance of C-fiber firing. Now, Mary's and Juliette's experiences, as instances of a certain type of pain-experience, are equally identical to the common physical basis responsible for them. It follows from this that the two subjects' experiences will be also identical to each other, regarding the transitive nature of the identity-relation. So, they feel numerically the same pain. In other words: since there are no physical differences between the facts relevant for their experiences, one can say that they undergo the same experience. Consequently, the supposition of the obtaining of the identity-relation is conceptually inconsistent with the privacy thesis. (ii) As for the case of supervenience or constitution theory, it is important to see that these views are also committed to the identity of instances of mental and physical properties. A particular conscious experience as an instance of a certain type of mental property is identical to a particular neurophysical state or event in the subject's brain. Therefore, in the cases of particular mental events such as a conscious experience, it does not matter, whether one is a supporter of reductive or non-reductive physicalism, because both theories accept the thesis concerning the identity of property instances. The only difference between them lies in the fact that non-reductive theories allow the possibility that different instances of the same mental property can be identical to (realized by, supervene on, etc.) different instances of different physical properties. Now, since the cases in our thought experiments are about particular experiences, such a supervenience physicalist have to think that an identity relation obtains between Mary's and Juliette's pain-experiences on the one hand and the appropriate common physical basis on the other hand. (This latter could be some neurophysiological process of the relevant brain parts of one of the subjects, or some state or process of the common implant.) Consequently, the situation in the case of these kinds of physicalism is the same as it was in the identity theory. Given the transitivity of the identity relation, the two subjects undergo numerically the same experience, therefore non-reductive physicalist theories are also inconsistent with the privacy thesis. (iii) There are other physicalists, who think that although particular conscious mental property instances depend on some physical entities, these entities are constituted not merely by inner neurophysical states or processes but by some further physical factors of the environment, too. Such an externalist physicalist can therefore claim that Mary's and Juliette's pain experiences will differ due to these factors. In order to examine this possibility, we have to distinguish between two kinds of externalist approach. The first one is usually called 'phenomenal externalism', and the essence of it can be summerized in the claim that the phenomenal quality which is, Márton and Tőzsér 83 at least partly, constitutes the conscious experience in question, can be found in the experienced object itself, not in the subject's state of experience (see e. g.: Dretske 1996, Fish 2009). The second one is usually called 'content externalism', and the main thesis of it consist in the famous claim that the content of a mental state, which is also a constitutive part of it, is not in the subject's head. As for the former one, we think it is easy to see that this theory does not threaten our thesis. Such phenomenal externalist physicalism should include the objects of experiences and their qualitative properties in the physical factors on which conscious mental property instances are supposed to depend. And it is plausible to suppose that the object of the experience is nothing more than the causal starting point of the sensational process. However, in the case of Mary and Juliette, these factors are all common. There is only one pinching of a nerve, which serves as the common causal starting point of both subjects' pain-experience. Or, if we would take an example of sensory experiences rather than pain in the thought experiments above, then there will be only one object that serves as the causal starting point of their sense impressions. Consequently, the situation is the same as in the case of the internalist physicalist: all the physical entities to which the subjects' conscious experiences are identical are common; therefore, they are identical to each other, too. The situation is a little bit more complicated in the case of content externalism. There surely can be some facts in the context of the occurrences of conscious experiences in question which differ from each other. To mention just the most obvious one: the content of Mary's and Juliette's pain-experience is the same in a sense, that is, both Mary and Juliette experience that their lower back hurts. However, these contents are, at least for the content externalist, different in another sense: the content of Mary's experience is that Mary's lower back hurts and the content of Juliette' experience is that Juliette's lower back hurts. So the two contents differ, which makes their experiences also different. We think this consideration has nothing to do with the concept of privacy we are interested in. As it was stated in the first section, this concept is connected to the notion of access, and makes conscious experiences private insofar as the subject's access to them is private. If we keep this in mind, it will become obvious that the difference between the two subjects' experiental content is a difference to which the subjects have absolutely no access. The concept of the content to which we have access is the concept of narrow content, and narrow content is, in turn, the content on which the external factors of the context have no impact. For example, Mary and Juliette have the same narrow experiental content in the above situation, namely: "my lower back hurts". As it is wellknown, all external considerations are concerned with the broad content, but this very Journal of Cognition and Neuroethics 84 broad content (if there is any, see: Farkas 2008a, Pitt 2013) does not make the conscious mental states private. (iv) If we are right, strict physicalism is incompatible with the private nature of conscious experiences due to the simple reason that all forms of strict physicalism are committed to the identity of conscious mental property instances and the appropriate physical entities. However, we think that property dualism is also incompatible with the privacy thesis. What should a property dualist say about the case of Mary and Juliette? Perhaps the following: There are two numerically different conscious experiences present in these situations, that of Mary's and that of Juliette's, because there are two numerically different phenomenal properties which have emerged from the common physical causal basis. Nevertheless, these conscious experiences as phenomenal properties of some physical phenomena are connected by some contingent psychophysical laws to it, in our case to some physical properties of the common brainparts or the implant. Given the above, we think the property dualist has to commit herself to the claim that Mary's and Juliette's experiences are qualitatively alike. They feel their lower back hurting in equally the same way. Although psychological laws are contingent ones according to the property dualist, there could not be two different laws in the same world, i. e. two psychophysical laws by which two different kinds of phenomenal properties emerge from the same physical ones. In this respect there is no difference between nomic and ontological dependence. In sum, since Mary and Juliette live in the same world and all the relevant physical bases of their experiences are the same, their experiences are of exactly the same kind. So, the property dualist opponent has only one thing to say, namely that although Mary and Juliette have qualitatively identical conscious experiences, these experiences are numerically different. The two subjects' conscious mental states differ from each other merely in a numerical sense. It seems to us that this idea is a rather implausible one. If the defender of property dualism claims that the conscious experiences of Mary and Juliette are merely numerically different while qualitatively identical, then she will have to allow that the same could be true in the case of one single subject, for example you. She has to allow that when the nerve in question pinches in your waist, two qualitatively identical but numerically different phenomenal properties will emerge; or, in simpler words; you will have two qualitatively identical but numerically different conscious experiences, so you will feel two qualitatively exactly the same but numerically different pains. There is no theoretical difference between your case and that of Mary's and Juliette's, so if Márton and Tőzsér 85 someone allows the possibility in question in the latter case, then she will also have to allow it in the former one. However, it is not the end of the story yet. If one allows that you have two exactly alike pain-experiences, then one will have to allow that you have more, say 342 or 234 or 454627 merely numerically different, but in all other respects exactly alike conscious experienes. In a situation like this you could not tell the pain experiences apart, because they are qualitatively identical and so there is nothing by which you could distinguish one from the other. We do not assert that the obtaining of such a situation is logically or metaphysically impossible. It is without any doubt metaphysically possible for a subject to have several indistinguishable and merely numerically different pain-experiences at the same time.5 It is rather implausible for the reasons of phenomenology and theoretical parsimony. Why should we include several qualitatively identical but numerically different conscious experiences or phenomenal properties in our ontology if this move is not supported either by any consideration regarding the phenomenology of our mental life or by any theoretical benefit? Consequently, if we do not want to commit ourselves to such implausible claims then we have to acknowledge that even from a property dualist point of view, Mary and Juliette undergo numerically the same conscious experiences. In a word: it is not just strict physicalism, but also the other substance materialist theory, that is property dualism, which is inconsistent with the privacy of conscious experiences. If our argument succeeds in supporting the incompatibility thesis, then it has two important consequences. Firstly, it shows that from a physicalist point of view, the private nature of conscious experiences is not a conceptual truth, but only a contingent feature of our physical make-up. We, normal human beings are built in such a way that the physical bases of our conscious mental states do not extend over our bodies. The neural networks which are responsible for conscious experiences are usually not connected to any other fellow's brain, and this is the reason why at most one subject can undergo a particular experience. Secondly, from an anti-physicalist point of view, the incompatibility of privacy and physicalism can serve as a possible starting point of a new kind of argument against 5. As far as we can judge it, one needs to argue for the metaphysical impossibility of this situation in the following way: we have such special access to our conscious experiences that excludes any error in the individuation of them. That is, if it appears to us that a conscious experience A is identical to another one, namely B, then they will be necessary identical. The appearance of identity implies the identity of appearances, so to say. However, this kind of argumentation does not work. To see this, one only has to think about the examples of the phenomenal sorites problem. Journal of Cognition and Neuroethics 86 the latter. The hitherto presented and much debated arguments usually allude to the supposed subjective nature of conscious experiences. They regularly emphasize that the very existence of conscious experiences supposes a subject with a special viewpoint who undergoes such experiences, and this essential subjectivity cannot be explained by any physicalist theories. In contrast to this, our argument alludes to the other sense of privacy, namely that more than one subject cannot undergo the same experience, and claims its incompatibility not just with physicalism in the strict sense, but with property dualism as well. 4. Some downright objections and replies As far as we can see it, our argument can provoke some prima facie plausible or downright objections. We consider the following three of these. (i) One can say that in order to individuate mental states, we have to allude to the whole stream of consciousness of the subjects. If this is true, our interpretation of the thought experiments will be false, because Mary's and Juliette's pains are parts of different streams of consciousness. So they cannot be the same. Reply: nothing exludes the possibility that a part of a stream of consciousness be also a part of another one. The fact that a conscious experience essentially belongs to some subject's stream of consciousness does not imply that it is conceptually inconsistent for it to belong to more than one stream. In other words, streams of consciousness can be shared with each other, and we think that in the cases of Mary and Juliette the situation is exactly that. (ii) Probably, Mary's earlier mental life was quite different from Juliette's. Perhaps Mary is an elite soldier who was trained to tolerate heavy pains, so her pain threshold was raised to a very high level. Therefore, even if her appropriate brain part happened to be damaged, and her brain has been wired to Juliette's, who is an ordinary person, in the way described in the first thought experiment, her pain experience will be much less intensive than Juliette's. Naturally, since the intensity of a pain belongs to the phenomenal features of this experience, Mary undergoes a phenomenologically and so numerically different conscious experience. Reply: we suppose that a physicalist cannot accept the assumption that the level of Mary's pain threshold remains the same after the operation. If this was true, then this mental ability would not depend on any particular neurophysiological state or process. Therefore, if the objector adheres to the story above, she will have to deny physicalism. Márton and Tőzsér 87 Consequently, this objection is not directed against our inconsistency thesis but rather acknowledges it. (iii) Mary and Juliette cannot undergo the same pain-experience as Mary feels her pain in her1 lower back, Juliette feels pain in her2 lower back, that is, Mary feels her pain as a state of her own body and the same is true for Juliette. This difference can be explained in intentional terms. Mary's and Juliette's pains have different intentional structures, namely their intentional objects are different. Mary's experience directs to Mary's waist; Juliette's experience directs to Juliette's waist. Reply: The consideration behind this objection is very much like the one we handled in the argumentation for the inconsistency of privacy with the externalist version of physicalism. As we explained there, the concrete particular factors of the context of a conscious experience belong to the broad content of the mental state in question. However, broad content is actually the content to which the subjects do not have any direct access, so one cannot allude to it in order to support the private nature of conscious experiences. All that can contribute to this private nature rather belongs to the narrow content. We illustrated this by the example of the particular experiencing subjects. The concrete identity of the experiencing subject does not belong to the narrow content; therefore, if two subjects feel the same kind of pain, then the narrow content of their experience will be the same regarding the subject of the pain. Both feel the pain as their own. The same is true for the objects of conscious experiences: the identity of the particular object belongs to the broad content and the narrow content will be the same: both subjects feel their lower back pain as belonging to their own lower backs. 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