Conscious Experience

    Conscious Experience

    ''The best concerted voice to date 
    from the philosophers' camp''

    Susan Greenfield, review in the 
    Times Higher Education Supplement

Conscious Experience, ed. T. Metzinger (Exeter: Imprint Academic, 1985), pp. 3-37

Thomas Metzinger

Introduction: The Problem of Consciousness

I. Do we need a new science of consciousness?

How can consciousness arise in a physical universe? Is it at all imaginable that something like conscious experience could emerge from a purely physical basis? Is it conceivable that subjective sensations and the emergence of an inner perspective are part of the natural order or are we now confronted with an ultimate mystery, a grey area on the scientific map of the world that may in principle have to remain grey?

Today, the problem of consciousness perhaps together with the question of the origin of the universe marks the very limit of human striving for understanding. It appears to many to be the last great puzzle and the greatest theoretical challenge of our time. A solution of this puzzle through empirical research would bring about a scientific revolution of the first order. However, in this case meeting the challenge may require a completely new type of intellectual revolution, for a number of reasons. To begin with, when we look closely, it is not at all clear what the puzzle of consciousness actually is, and what we would accept as a convincing solution. Second, the problem in a strong sense concerns ourselves: it is always our own consciousness, among others, that we want to understand. Therefore the problem of consciousness is also a problem of self-knowledge. It affects all of us, not just philosophers or scientists. Third, such a revolution if it actually took place might for this reason have greater social and cultural ramifications than any previous theoretical upheaval. This could be due to the consequences of a radically changed picture of ourselves, or to the impact of new technologies that might result, for instance, from progress in the neurosciences or in artificial intelligence research. These three reasons have recently led to an increasing restlessness in the sciences as well as to a growing interest among the general public in questions concerning the connection between consciousness and the brain.

It has become obvious that, for quite some time, we have been reaching for a new theory of mind. Such a new theory will, among other things, be a theory about the nature of conscious experience. It also has the potential to be the first such theory in the history of mankind to rest on a solid empirical basis. There seems to be a theoretical revolution in the air, which can be expected to affect the way in which we see ourselves in a hitherto unknown fashion. Although empirical research has given only a few indications of this so far, the puzzle of consciousness has already advanced to become the ‘secret research frontier’ in a number of scientific disciplines. This development in turn has met with great interest among philosophers.

For the last ten or fifteen years philosophers have shown a rapidly increasing interest in the problem of consciousness. A number of new journals and academic organisations have been founded, and large meetings on the subject are taking place with increasing frequency. Interest has been further stimulated by the emergence of serious claims about consciousness in disciplines ranging from quantum physics through neurobiology to cognitive science. In the philosophy of mind many of these proposals have been watched closely. At the same time, more and more empirical researchers are beginning to realize that philosophers have been dealing with this theoretical problem for many centuries, and that philosophical analysis has a central role to play in its resolution. In its origins the concept of consciousness is a philosophical concept. As a consequence, there is now increasing interest in a serious and empirically-informed philosophy of mind in the general public and in the neuro- and cognitive sciences. This is also revealed by the simple fact that a number of prominent scientists researching the brain have published popular books with philosophical content.1

This rising interest is also shown in the increasingly firm connections between philosophy and the adjoining areas of research in neuroscience, cognitive science and computer science. Many hold that the necessary revolution can only occur when our understanding of the subject transcends disciplinary boundaries and links between the relevant areas of research are drastically increased. However, it has also become obvious that a systematic integration of research activities is necessary as well. This situation has led to a call for a new science, the science of consciousness,2both from empirical and theoretical researchers. In considering this fascinating idea, one must bear in mind two caveats. First, the idea of a ‘science of consciousness’ is anything but a new idea, especially from the viewpoint of the philosopher. For example, the whole phenomenological movement (and its demise) can be understood in these terms. In a more general sense, philosophy as the love of wisdom and queen of the sciences has always been the science of consciousness. The ideal of self-knowledge is a classic ideal of philosophy. So professional philosophers may well regard the current euphoria over consciousness with a degree of scepticism, seeing it as merely the latest intellectual fashion. Secondly, the success of such an undertaking is by no means assured. One has to ask: does the concept ‘consciousness’ really define an independent and coherent domain, a subject area that could correspond to an autonomous area of research?3

What would be the subject, methodology and aim of such a research area? This brings us back to our starting point: what is the real problem of consciousness? Can it be approached by means of the natural sciences at all? What exactly do we want to know? These questions especially the last one, the setting of the epistemic goal are typical philosophical questions. As philosophers we also want to know how it is possible that a phenomenon as complex as consciousness could arise in a physical universe: we are looking for a conceptually convincing analysis of the phenomenon and its relationship to the objective world. The first aim, therefore, is conceptual clarity and freedom from contradiction. As empirical researchers, on the other hand, we want to know how all this actually happened: we are interested in the history of the phenomenon in our own world. Is there a neural correlate of consciousness? Which forms of information processing in the brain lead to those states which we call conscious experiences? Such questions are central to the second goal on our way to a better understanding of the phenomenon of conscious experience. This goal is the achievement of an empirically meaningful and informative theory of consciousness.

How can the emergence of conscious experience be reconciled with the laws of nature that dominate this world? As far as we know, consciousness is a very recent phenomenon in the physical history of the universe. Once again we can say, in a certain sense, that we ourselves are the phenomenon in question. The emergence of organisms with highly organized nervous systems, and soon afterwards of human beings, are events that from a cosmological viewpoint have only just occurred. The idea that, in a strong sense, we ourselves are the phenomenon in question, leads to a third provisional answer to the question what we actually expect from a satisfactory theory of consciousness. To be convincing, such a theory must not only be conceptually coherent and empirically plausible: we must also be-able to accept this theory as a theory about our own inner experiences. It must account for the subtlety and phenomenological wealth of this experience and take seriously the inner perspective of the experiencing subject. Above all, it must explain the connection between one’s own first-person perspective and the third-person perspective of science operating from the outside. If it turns out that our intuitions about consciousness and the interpretation of these intuitions by folk psychology prove radically wrong, then such a theory must offer a detailed explanation of why we are so wrong about the matter. Fortunately, there is a wide consensus that a serious theory of consciousness must account for the phenomenological wealth, colourfulness and variety of our inner life. In the philosophy of mind at least, there are only a few examples of naive and ideological forms of reductionism: it has long been clear that a primitive scientism, attempting to bulldoze the subtlety and depth of our conscious experience simply by introducing new materialistic jargon, would be evading the real problems.

The problems on the path to a convincing theory of consciousness differ fundamentally from other unsolved problems in the natural sciences. Although physics, chemistry and biology have solved many of the fundamental puzzles in their respective domains, a considerable number of grey areas on the map have remained. These sciences are far from being able to describe exhaustively the parts of reality which form their own subjects of research. However, where there are lingering mysteries in physics, chemistry and biology, it is at least clear what would be accepted as a solution of the problems. This is not so for the problem of consciousness, for a number of reasons.

To be able to speak seriously about a science of consciousness, a number of fundamental questions would have to be answered. It is interesting to note that with the emergence of consciousness, private worlds spaces of inner experience are opened up. These spaces, however, are individual spaces: ego-centres of experience that suddenly appear in a centreless universe.4Each such centre of consciousness constitutes its own perspective on the world. This perspective is what philosophers sometimes like to call the ‘first-person perspective’. A phenomenal world of its own is tied to each of these perspectives. These individual worlds of experience also possess a historical dimension: almost always a psychological biography emerges together with them what we call our ‘inner life’. This too can be seen as the history of the genesis of a world, or a phenomenal cosmology:

within each of us a cosmos of consciousness unfolds temporarily, a subjective universe develops. The first part of the problem is to understand how such a variety of subjective universes can constantly form and disappear in our objective universe. Empirical research on the evolution and neurobiological genesis of consciousness is relevant here, but there is also a philosophical aspect to the problem. This consists in understanding how we our selves can be such subjective universes, and above all in understanding the meaning of it all. Do we really understand what we are saying when we describe ourselves as dynamic subjective universes, that have something like a centre and temporarily light up in an objective universe? I do not think we do.

Before our general ideas and epistemic goals can be turned into concrete research projects, we need a careful conceptual analysis of the problem. Empirical practice and philosophical meta-theory must go hand in hand, allowing the various disciplines to work together systematically and productively. This is the context of Owen Flanagan’s call for a ‘Unified Theory of Consciousness'5and it ties in with the fact that from a number of very different fields calls for the establishment of an independent academic discipline of consciousness research have been heard. Such a desire for cooperation, existing alongside suspicion and wrangling both within and between disciplines, is typical of situations in which great theoretical advances are on the horizon. It also leads into the second feature which makes the problem of consciousness a special problem, because it raises questions of methodology: what are the appropriate methods with which to approach the problem of consciousness? What relationship have these methods to one another? Again, a typical philosophical objection could be as follows: if we wish to take seriously our own consciousness as a phenomenon bound to individual perspectives of experience, we cannot as a matter of principle approach it through objective methods, since the essence and the strength of these methods consists precisely in moving as far away as possible from any purely individual perspectives.6 But if we then ask what it would mean to treat conscious experience seriously as a subjective phenomenon, we will be led back to our original question: what exactly is it that we want to know?

The final aspect of the question is that in the present state of interdisciplinary research on consciousness the explananda still remain undefined: it is not at all clear what it is that has to be explained. Conscious experience is not a single problem, but a whole cluster of problems. One may ask: What are the components of this cluster? How, if at all, are they related? Is it really even a single cluster at all? As long as there are no convincing answers to these three aspects of our original question, there is a danger that the present enthusiasm for the subject will degenerate into a euphoria that ultimately fails to produce a tangible advance. One has to admit that consciousness research is still in its infancy. It has not yet left its pre-paradigmatic phase:

so far there is no uniform theoretical background against which a science of consciousness could develop. To create such a background, we must address not only general questions like the ones concerning the epistemic goal, the methodological canon and the catalogue of explananda discussed above, but also a whole range of detailed problems. This is essential if we are to show that the notion of a unified science of consciousness is not only a fascinating but also a coherent idea. I will discuss some of these detailed problems in the remainder of this introduction, which is an attempt to approach the problem of consciousness in three steps.

The first step deals with the most important phenomenological characteristics of consciousness. This aims at presenting an initial description of three concrete and global properties of conscious experience, which shows why this phenomenon is indeed a special problem. The second step moves from the phenomenological plane to an analytical one, producing a list of the conceptual problems that result from the global description. I will present a short account of these problems, as they appear from the perspective of the philosophy of mind. The third and final step is to engage with the current debate, which will lead us directly into the present collection of texts.

II. Transparency, Perspectivalness and Presence:

Concrete Properties of Conscious Experience

1. Pure experience. phenomenal content, phenomenal properties, phenomenal states
Nothing is simultaneously so close and yet so distant as our own consciousness. To start with, nothing in the world seems to be more intimate to us than the contents of our own consciousness: our sensations, our feelings and our thoughts are all given to us in a very direct and self-evident way. True, we can sometimes be deceived about the true causes of our sensory perceptions, and our conscious thoughts and feeling can also mislead us when we have views of the world that cannot be justified. But nevertheless there is nothing in our subjective experience that could be more obvious and natural than the simple facts of experience itself: the fact that I now sense the cover of this book to be blue; the fact that I am now of the opinion that a fundamentally new image of man is emerging; or the fact that just now I am led on by curiosity. One cannot, apparently, doubt pure experience itself.

In the philosophy of mind this pure experience is termed the phenomenal content of our mental states. Consciousness in this sense is therefore frequently termed ‘phenomenal consciousness’ by philosophers. States of consciousness, according to this understanding, are always phenomenal states, because they are those states in virtue of which the world appears to us in a certain way, they determine how we experience the world and ourselves. We are thus dealing with a special meaning of the word ‘consciousness’ that is not immediately related to common phrases such as ‘feminist consciousness’, ‘ecological consciousness’, or ‘awareness’ in the spiritual or psycho-therapeutic sense. When one speaks of phenomenal consciousness, one is always referring to pure subjective experience. It is precisely this aspect which is considered in this book.

The philosophical core of the following discussions is formed by the simple fact that some mental states have more than information content: they also have a certain feel. The most popular recent formulation of this point was given by Thomas Nagel: there is something it is like to be in these states (cf. Nagel 1974). This property of what-it-is-like is the subjective character of experience. But what exactly is meant by saying that many of our sensory or mental states possess a subjective, experiential character? Let me illustrate this experiential character with a simple example. Take a moment to sit back and contemplate the background colour which we have chosen for the cover of this book. The name of this colour is Pantone Blue 72. If you allow the colour to make an impression on you, you can experience directly the meaning of ‘phenomenal content’: in the visual experience of the colour of the cover this phenomenal content is the subjectively felt property of blueness, that is, the quality of Pantone Blue 72 now appearing in the flow of your conscious experience. You perceive a certain object and in the very process of perception the subjective sensation of blueness is formed: the book appears to be blue. Because Pantone Blue 72 is a property of appearance, one also speaks of ‘phenomenal properties’. Such isolated phenomenal properties are the simplest examples of the subjective, experiential character of consciousness. Of course, there are higher forms of phenomenal content: Gestalt-awareness, object formation, self-consciousness, or the conscious experience of situations and contexts. More about this later. Let us for the moment keep to simple properties.

Among English-speaking philosophers, the mental states associated with such simple phenomenal properties are sometimes identified with the concept of raw feeling.7 Here again, the aspect of pure sensation, the aspect of pure experience, is decisive. One can easily find a variety of examples of such qualities: the painfulness of pain, the scent of sandalwood, the taste of Bourbon-Vanilla or the extraordinary sound quality in the tone of a cello. But the sense of weight that you are just experiencing in your body, or the feeling of ‘smoothness’ that you have when you run your hand over the cover of this book, are equally examples of such qualia. The subjectively sensed heaviness of one’s own body (the ‘heaviness-quale’) and the subjectively experienced smoothness of the book-cover (the ‘smoothness-quale’) are phenomena that only exist when there is also consciousness. This subjective aspect of conscious experience, it seems, is something which cannot be measured: phenomenal content is qualitative content.

Qualitative content is frequently described as a phenomenal property. However, if one speaks of properties in this way, one is immediately faced with the first serious philosophical problem. What is the thing of which the phenomenal properties are features? What are the logical subjects to which we assign such properties? Or, rephrasing the question: is Pantone Blue 72 a property of a non-physical entity, a phenomenal individual, that only exists in the field of conscious experience? This is an unattractive option, because it confronts us with the classic mind—body problem: the well-accepted principle of the causal closure of the physical world is not compatible with the assumption that conscious experience understood here to imply the occurrence of one or more non-physical entities has an effect on our behaviour and our actions. How should one conceive of the interaction of such non-physical agents with our brain? If conscious experience is interpreted as the mental act in which we grasp a mysterious kind of non-physical entity, then the meaning of such a description becomes notoriously unclear: what exactly is meant by the term ‘to grasp’?

Perhaps there is a way to avoid this classic dilemma. Is the subjectively experienced blueness perhaps a feature of a process rather than a thing, in this case the process of visual perception? The scientific investigation of human colour vision is now yielding detailed descriptions of such perceptual processes.8 Unfortunately, nothing figuring in such objective descriptions ever possesses a concrete property such as blueness in the sense required here: nothing in the eye or in the brain is blue in this sense. Might one therefore suggest that it is really the book ‘out there’ that is blue? ‘Out there’, however, there are only electro-magnetic oscillations that are reflected from the surface of the book in a certain mixture of different wave-lengths. In any case, one can have blue-colour experiences even in a dream, when no rays of light fall on the retina. (Perhaps you will be dreaming tonight of Pantone Blue 72 . . .) Many other examples contradict the idea of phenomenal properties actually being concrete, first-order properties of objects in the environment. When we look into a green light-flash and then close our eyes, we see a red after-image. But what is it that has the property of redness? Such common observations already suggest that phenomenal properties may not be objective properties. Whatever they are, they do not seem to be ‘out there’.

The problem emerges again when we look at sets of phenomenal properties ‘phenomenal states’ or states of consciousness. When one or more of the properties in such a set change, then we are dealing with subjective events; and when chains of such events link successive phenomenal states together, then we are dealing with phenomenal processes: processes of consciousness. All this appears to be clear and simple. But the question still remains: what is it that changes here? What are ‘phenomenal states’ states of? Are they states of the ‘soul’ or states of an ‘ego’? Are phenomenal states simply states of the brain? Or are they something intermediate information-processing states in the central nervous system, perhaps, states of a large data structure which in the waking and dreaming state is episodically being activated by our brain?

All these attempts to impute possible answers in advance, by the way in which the question is posed, are more than problematical. One might try to interpret them as research programmes for the future, as open questions, to which empirical answers potentially exist. However, such research programmes would only be conceptually plausible if it were made clear that our traditional, formal ontology could be projected on the phenomenal ontology of conscious experience. This would require that everything which exists in conscious experience, as seen from the perspective of the first person, can be adequately described with the linguistic and conceptual means at our disposal. Only then could there be an abstract analysis of phenomenal states and properties that could, at least in principle, be complete. But it is precisely this point which has been frequently doubted by philosophers. 1 shall therefore return to this later.

For the moment we should take the first step, and, slowly and carefully approaching the problem of consciousness, turn to concrete features of phenomenal states. Having clarified the concepts of phenomenal content, phenomenal properties and phenomenal states, we can now ask what the interesting features of these states actually comprise. We are now ready to take an initial look at the phenomenology of consciousness. This first step will once again demonstrate that the problem of consciousness is indeed a special problem. Phenomenal states are quite different from physical, chemical, biological or neurobiological states: they are transparent, they are perspectival and they are present.

2. The transparency of phenomenal states
Phenomenal states are infinitely close to us. Before the formation of any philosophical concept takes place, conscious experience is already immediately given to us: nothing could be more natural or more familiar than this. I have already pointed out that, in a certain sense, we ourselves are this experience —what would human beings be without phenomenal consciousness? Human beings without phenomenal consciousness would not be persons, but ‘zombies’, roaming dead. Such zombies would probably not have a problem regarding consciousness, either at a scientific or at a philosophical level. For us, on the other hand, the simple facts of consciousness have been fixed even before we can start to think about the problem of consciousness. To put this insight another way: from the perspective of the experiencing self the field of phenomenal consciousness is transparent.9 This simply means that we do not experience phenomenal states as phenomenal states, but that we, as it were, look through them. They seem to bring us into direct contact with the world, because we perceive the content of these states in the mode of immediate givenness. We do not have the feeling of living in a three-dimensional film or in an inner representational space: in standard situations our conscious life always takes place in the world. We do not experience our conscious field as a cyberspace generated by our brain, but simply as reality itself, with which we are in contact in a natural and unproblematic way. In standard situations the contents of pure experience are subjectively given in a direct and seemingly immediate manner. It is precisely in this sense that we can say: they are infinitely close to us. This infinite closeness is the first major phenomenological characteristic of consciousness.

How could one explain this fact scientifically? Can one take the transparency of consciousness seriously by making it the subject of an empirical theory? If as the majority of empirical researchers assume today conscious experience is indeed a phenomenon based on information processing and mental representation in the brain, then one could perhaps suggest that the data structures active at a given time are not recognized as such by the system. They do not convey the information that they are indeed such data-structures. They contain no variables and are almost always ‘fully interpreted’. Therefore we simply feel ourselves to be directly in contact with the content of our conscious states, because our functional design forces us into a naive realism. And this naive realism may, in turn, have its roots in our biological history: we are naturally formed information-processing systems, that have been configured and optimized over millions of years of evolution, and optimal solutions often are simple solutions. Such considerations are fascinating because they not only point towards an explanatory outline of how consciousness ‘works’, they also illuminate that central paradox: how our own consciousness can so quickly be transformed from something infinitely close into something strange and far distant, when we try to understand it empirically. For what could be more alien to my own experience than ‘active data structures in the brain’? What could be more likely to divide me from my own consciousness than the dubious attempts of scientists to explain its transparency by information-processing properties in my central nervous system?

So no wonder, if we strive for a conceptually convincing philosophical understanding of a global phenomenological property like ‘transparency’, that consciousness easily turns from something infinitely close to something infinitely distant. The Latin tag esse est experiri can be applied here and interpreted: ‘the reality of phenomenal qualities lies in their being experienced’; meaning among other things that we fully grasp the essential character of phenomenal states just by experiencing them.10 But if this is really true, then phenomenal states are not only transparent, they are incorrigible: they can never deceive us. Here a series of simple but momentous questions arises again. Is something which could not deceive us therefore also something that we know? Is phenomenal transparency really the same as certainty? Is there any guarantee that conscious experience is a form of cognition? Or, to put it another way: how do we know that the simple facts of consciousness are really facts at all?

3. The perspectivalness of phenomenal states
A second important feature if one wishes to understand what phenomenal consciousness actually is is that experiences always appear to be experiences for an experiencing ego. It is I myself who experiences these feelings and sensory perceptions in a certain way. It is I myself who discovers in my sensory perceptions certain subjective qualities say the ‘blueness’ in a colour or the characteristic scent of sandalwood. Pure sensory perceptions of these simple qualities are thus always tied to a subjective perspective of experience. It is always true, at least for the normal states of consciousness, that subjective sensory perceptions always are my sensory perceptions, they possess their specific character of experience for me. Nobody knows what it is like for me to hear the tone of a cello or to look at the special blue of the book cover. And I don’t know what it is like for a bat to hear the tone of a cello or what it is like for one of my readers to look at the Pantone Blue 72 of the book cover. One can call this characteristic of phenomenological consciousness its perspectivalness: in standard situations phenomenal content is always accompanied by a phenomenal point of view, by an experiencing ego. This experiencing ego turns conscious experiences into its experiences. It is also true of this ego that it is infinitely close to itself. Before reflecting about the theoretical problem of self-consciousness in philosophical terms, it is essential to understand an interesting and conceptually puzzling feature to which every convincing theory of consciousness must do justice: a very simple being, that can only feel and not think, might also possess this characteristic. Philosophers sometimes call it ‘prereflexive self-intimacy’ (cf. Frank 1991). This quality arises from the fact that states of phenomenal self-consciousness are also transparent in the previously explained sense.

Our consciousness is a centred consciousness, since it almost always has a centre. We ourselves are the centre, the focus of consciousness. We ‘possess’ ourselves prior to all intellectual operations, we have always been intimate to ourselves. It is not just that phenomenal states are tied to an individual perspective; this individuality is also apparent at the level of conscious experience itself. We are infinitely close to ourselves, because a large part of the content of self-consciousness is experienced as being immediately given. Now, it is particularly interesting that in this way our experiential space as a whole not only gains a central focal point of consciousness, but a perspectival structure as well. It is thus at the level of conscious experience that we find the origin of what philosophers call the ‘first-person perspective’. There is also a further mental phenomenon, originating together with this structural feature of the perspectivalness, which any reputable theory of consciousness must take seriously. This phenomenon is called inwardness.

Let us briefly return to the simple phenomenal properties already mentioned en passant. The qualitative content of mental states the blueness of blue or the painfulness of pain is not only experientially ‘transparent’, it also seems to be the defining characteristic of such states: a pain that is not painful is inconceivable; a blue experience without the subjective quality of colour is simply not a blue experience. It is, however, questionable whether such ‘private’ properties of mental states could ever be tied in conceptually with public and objective properties of the underlying physical states. Could the subjective taxonomy of these states be projected in principle on an objective categorization such as could be supplied by the cognitive- and neuro-sciences for example in terms of information-processing states in the brain? Their existence lies precisely in their being experienced, in the way in which they appear to us: an experience of pain that is not painful is just not an experience of pain and a theory about pain from which this qualitative characteristic were missing would not be a theory about pain at all. Such a theory would be utterly remote from us, it would omit precisely what concerns us. This is one of the reasons why a number of philosophers consider the phenomenal content of qualia as an irreducible property of such states.

Some philosophers have a similar attitude to our second feature of consciousness —perspectivalness. This is also widely held to be an essential characteristic of conscious states: that they are always tied to a subjective perspective. Seen from the inner perspective, this structural feature of conscious experience is again transparent: what is more natural and self-evident than the fact that only I myself know how it feels at this very moment to have exactly those conscious experiences that I am just passing through? And what could be simpler to appreciate than the fact that I myself am the irreplaceable centre of my conscious field; that although I can always try to empathize with the subjective states of another being say a bat using the resources of my consciousness, I shall never know what it is like to be a bat?

The prereflexive self-intimacy of the phenomenal ego is perhaps the best illustration of the fact that our own consciousness is infinitely close and at the same time infinitely distant. Yet although we speak as if we were infinitely near to ourselves in experience, and as if there were indeed something like the perspective of the first person, no one actually knows what all this is really supposed tomean. ‘Closeness’ is a spatial metaphor, that alludes to a relationship between spatial objects. (Although it is not clear what it would mean to claim that, say, a chair was infinitely close to itself.) The ‘perspective of the first person’, by contrast, is the combination of a spatial-visual metaphor with a grammatical one. It may just be acceptable to call our experiential space ‘perspectival’, since this relates to sight, which is our dominant sense. Our visual experience of the world has indeed been constructed around a centre, because as seeing people we experience the world apparently from one viewpoint, and this viewpoint seems to lie behind our eyes and is the centre point of our visual experiential space. We even speak of ‘the world around us’. But who then is ‘the first person’ supposed to be a little man in our head, looking out into the world through the windows of our eyes? Here again, numerous questions and difficult theoretical problems follow on quickly and obviously.

For the final step, let us again consider first the simple phenomenal properties. If it is correct, that the special phenomenal property of ‘blueness’ of Pantone Blue 72 is an intrinsic and essential characteristic of certain conscious colour experiences, then the question arises whether there can ever be a scientific theory about conscious colour experience at all. We have said that in the physical outside world there are only electro-magnetic oscillations of certain wavelengths, and nowhere a property called ‘blue’. A variety of physical processes can trigger in us a sensation of blue, but there does not seem to be anything to connect all these processes in the physical outside world with each other. On the retina also we search in vain for the property called ‘blue’: the firing of the optic nerve is not blue, any more than the firing of neurons in those regions of our brain which are responsible for the sensation of pain is painful. All we find are myriads of subtle electrical impulses. Again, the patterns of excitation that build up from those impulses are spatially and temporally very complicated in just those areas of our visual cortex of which brain-researchers say that they are absolutely necessary for our colour-perception: but they are not blue. The concreteness of consciously experienced ‘blueness’ let us say of Pantone Blue 72 cannot be retrieved from the mathematically abstract theories with which scientists will soon be offering a precise description of the relevant patterns of neural activity. Many, therefore, ask themselves privately whether the phenomenal property of ‘blueness’ really exists in this world: is there a point of contact between the inner world of consciousness and the outer world of physics? We all want scientific psychology to take our consciousness seriously. But can it really do this? If already simple phenomenal properties like the consciously experienced blue of the book cover appear to elude the objective grasp of science, what would a theory of phenomenal consciousness be like, which took this really seriously and offered convincing explanations for what we have declared to be the defining characteristic of subjective states their qualitative character?

Such questions become sharper still when we take account of perspectivalness, the second concrete characteristic of phenomenal consciousness. Given the generally accepted view, that the association of conscious experience with a subjective inner perspective is an essential property of the phenomenon, a satisfactory explanation of what it actually means to ascribe an inner perspective to a system is indispensable. When I confront a completely alien being, and ask myself whether this being is really conscious, I do not only want to know, in the sense of Thomas Nagel, what it is like to possess the mental states of this being. I want to know whether this being has a genuine inner world, whether in its experiencing it enjoys the immediate self intimacy described above, whether it really takes on a subjective perspective in relation to its mental states. This inwardness of phenomenal consciousness, resulting from the emergence of an ego-centre, would have to find a place in any scientifically convincing theory of consciousness. However, the principle of objectivity, on which the empirical sciences are based, means avoiding all subjective perspectives and distancing oneself as far as possible from all individual points of view. This gives rise to a fundamental problem: can a means of investigation, whose leading characteristic is the elimination of all subjective perspectives, help us to approach our own consciousness? If we are serious about the project of a science of consciousness, then we must build bridges from the outside world to the inner world, that is, to where we already are. On closer inspection, however, this proves to be an incoherent idea.

Thomas Nagel devoted himself to this problem as few other philosophers in the last few decades have done. He illustrated the point like this: even if some future physics achieved a complete description of the spatio-temporal world, it would still not explain which time was now and which place was here. Moreover, this would still be true even if we included an all-embracing description of the inner states of all conscious beings at all times and in all places. The most important of all facts to each of us at least according to Nagel would still be missing from such an objective picture of all subjective experiences and states of consciousness: namely, the fact that I am identical with one of the persons contained in this picture and that the inner experiences of this person are my experiences. In the objective picture of the world I would only be what Nagel aptly calls ‘one person among countless others in oceans of space and time’ and ‘a momentary blip on the cosmic screen’ (Nagel 1986: 61). A fundamental question arises from this: can the centreless conception of the world developed by science account at all for the phenomenal content that is tied to many individual perspectives of consciousness? Does not consciousness from a scientific viewpoint inevitably change from something very close to something quite distant, from something indisputably real to an illusion? One can quite reasonably doubt whether Thomas Nagel correctly formulated this aspect of the problem. So far no one has demonstrated that the inwardness of phenomenal consciousness is a mystery in the face of which the ideal of scientific investigation has in principle to fail. On the other hand, Nagel is quite correct in stating that it is never merely a matter of how we should express ourselves in philosophy, of what we shall say (cf. Nagel 1986: 56). The question is persistent, it constantly appears in new forms: could we ever really accept a new scientific theory however plausible about the emergence of a phenomenally conscious ego-perspective as a theory about ourselves?

4. The presence of phenomenological states
The first major phenomenal characteristic of consciousness was transparency. The second is its perspectivalness. Let us now turn to the third characteristic: the presence of phenomenal content. I said at the outset, that, according to experience, there was nothing more obvious than the simple facts of consciousness itself: for instance the fact that I now experience the cover of this book as being blue. It is this subjectively experienced ‘Now’ which represents the third important characteristic of phenomenal consciousness. This means that the characteristics of transparency and immediate givenness also possess a temporal aspect. It is interesting to note that, just as the transparency and closeness are accompanied by an equally obvious aspect of ineffability and cognitive impenetrability, so also our consciousness of time is simultaneously both infinitely close and infinitely distant. In a famous passage in chapter 14 of the eleventh book of his Confessions (written c. 400 AD), St. Augustine wrote: ‘What then is time? If no one asks me, I know; If I wish to explain to one who asks, I know not.’ That still rings true. The relationship between subjective and objective time is even today one of the most intractable variants of the mind—body problem.11

This comes about because however diverse and varied our subjective feelings may be, they are always given to us in the unity of an experienced moment. That is how we talk and think. But when we look at the matter more closely particularly in the light of recent empirical material we have to ask whether we actually understand what we are saying. It seems that my feelings and ideas, my bodily sensations and all other qualities of my sensory consciousness, including the ineffable wealth of subjective nuances and minutest distinctions, are always connected by the fact that they are components of an experienced present. One might simply express this observation as follows: phenomenal content is present. In saying this one does not want to imply that phenomenal consciousness is some rigid structure, in which there is no flow and no change. This could not be true, since, in a strong sense, it is precisely the flux of subjective experience (in the sense of the German word Erleben) that makes us alive. Rather, one is wanting to spell out, as an essential feature of consciousness, that its contents are always presented to us within a concretely experienced present. Pantone Blue 72 always is Pantone Blue 72 now. According to subjective experience, the taste of Bourbon-Vanilla, the scent of sandalwood or the feeling of curiosity are now immediately given to us. We do not ourselves construct these subjective qualities of experience: they are without doubt real facts of consciousness. Of course, we can re-present conscious contents for instance, when remembering the scent of sandalwood or looking forward to the taste of Bourbon-Vanilla. But then what is present in our consciousness is simply the memory of a certain joss-stick or the pleasure in anticipating custard. The conscious experience of these higher cognitive processes, like memory or future planning, still always takes place within a phenomenal present. Thirdly then, phenomenal content always possesses an aspect of being present. Moreover, this aspect of phenomenal presence also refers to the self, the centre of the experiential field: the experiencing ego is also present, because, in a maximally natural and self-evident sense which precedes all philosophical reasoning, it is a present ego. There would be no first-person perspective without the concretely experienced presence of the ego.

The contents of consciousness only become real facts through this general experience of presence. When we wake up in the morning, reality presents itself to us: with the activation of phenomenal consciousness a phenomenal reality is always formed. This phenomenal reality is my world, the world in which I live. I, along with the world around me, am phenomenally present in every single moment, that is, the phenomenal content of self and world is experientially represented as real right now. At first, nothing could be more unproblematic and more obvious than this presence of my reality, because from the perspective of the first person it, too, is transparent. The conscious present is infinitely close to us.

We have known for a longtime, however, that the presence of a conscious reality can be created artificially. Episodes of conscious experiences that are absolutely real and present from the first-person perspective can, for instance, be created in subjects by directly stimulating the brain with the help of electrodes (although it is in these situations mostly possible to keep up a critical ‘meta-perspective’; cf. Penfield 1975). States following the ingestion of certain psychoactive substances, or severe mental illnesses, are further examples of situations in which ‘alternative realities’ can arise within us through outside intervention. In a dream, a very natural and recurrent state of consciousness, reality appears to us as present and real. Despite the dreamworld’s being full of contradictions and bizarre changes, we do not know, that we are dreaming.12 In a number of psychiatric disorders, complex hallucinations occur that make it impossible for the patients to realize that they are actually hallucinating. Again, something like an ‘alternative present’ arises. There are also various disorders in which the experience of time and present itself is distorted or even destroyed. Even in the healthy state, we all know situations in which the phenomenal presence of conscious reality can diminish temporarily, perhaps after a moment of shock and panic or in a situation of danger. Here, the world attains a dreamlike and ‘unreal’ quality. This shows that the experience of presence can be subject to various fluctuations. As neuropsychological research has clearly demonstrated, these fluctuations are determined by factors that lie outside consciousness, such as complex processes in the affected person’s brain.

This understanding consequently concerns the actuality of the simple facts of consciousness: from the outside, there is mounting evidence that our conscious experience of reality as reality is a construct. But if this is true, then it calls into question the transparency of phenomenal consciousness. That is, empirical research concerning the subjective experience of presence strongly suggests that the traditional assumption that the contents of consciousness are given to us directly and immediately must be wrong. Let us look at an example. Patients suffering from a certain type of disorder known as ‘anosognosia’ have a deficit in their consciousness, but cannot consciously experience it as such. They lack the higher-order insight into the fact of illness, because of an injury to the brain that is, because of purely physical damage. A particularly impressive example is Anton ‘s syndrome (cf. Anton 1899, Benson & Greenberg 1969). Patients who suddenly lose their sight through a lesion in the visual cortex in some cases stubbornly insist that they can still see. They collide with furniture and other obstacles, and show all symptoms of functional blindness, yet behave as if they are not consciously aware of the subjective disappearance of the visual world. Thus they produce false, but consistent, confabulations in response to questions about their surroundings: they seem to believe the tales which they tell about nonexistent phenomenal worlds, and they deny any functional deficit in their faculty of seeing. These and many other empirical results have given rise to strong doubts at the philosophical level as to whether our subjective field of consciousness, experienced as transparent, really is transparent.

Once more it becomes clear why I said at the outset: nothing is simultaneously so close and yet so distant as our own consciousness. The more closely we look into our own consciousness, the more puzzling and mysterious it becomes. Nowadays, the inner perspective increasingly comes into conflict with the outer perspective. Scientific advance, brought about by methods operating strictly from the third-person perspective, now invades our sphere of ‘mental privacy’, since it casts doubt upon the transparency of our conscious space, and thus questions the authority of the first-person perspective altogether. Is Anton’s syndrome, the lack of insight into one’s own blindness, a neuropsychological metaphor for the cultural situation in which we find ourselves? Are we systems that produce consistent confabulations, that stubbornly play certain traditional language-games and cling to our folk-psychological image of ourselves, although we increasingly collide with obstacles resulting from new empirical material? The irrevocable loss of the (illusory) confidence in the certainty of introspection13 is making us all unsure of our own subjective experience our own consciousness, apparently so close to us, becomes remote, shadowy and puzzling. It is interesting to note that Descartes’ basic assumption of the self-transparency of consciousness which, through Brentano’s notion of the ‘evidence of inner perception’, has exerted a strong influence right into our own century, has become untenable precisely because psychology, from an empirical standpoint, became an autonomous discipline.

If one now, finally, retransfers the theoretical loss of transparency and presence to the second major phenomenological feature of consciousness the perspectivalness of our experiential field a dramatic consequence results. On closer inspection we no longer know what it means that we ourselves experience ourselves as being present. For if the presence of the contents of our experience is a construct, then so are the contents of conscious self-experience, including the experience of what in philosophy of mind is called the ‘phenomenal self. Further, if introspection is not a reliable means of understanding since looking into one’s own consciousness can lead to unnoticed deceptions then the inner space of self-consciousness also darkens, since the experienced transparency of this space too could at any time turn out to be an illusion. But whose illusion would that be? Nor is it any longer clear how to understand the the notion of our grasping the ‘simple facts of consciousness from the perspective of the first person. In any case, who is this first person? What do we refer to when speaking of the ego and its perspective? We speak of our conscious experiences, as if we ourselves had these experiences and as if we were present subjects of experience in an unproblematic sense. But our initial stock-taking of some very general and concrete properties of phenomenal states already showed that we do not really know exactly what we are talking about here. Now, at the end of our first careful exploration of these global phenomenological features, there is already a danger that not only consciousness but also we ourselves will be transformed from something infinitely close into something infinitely distant.

The headlong development of the empirical side of research has consequences for the philosophical treatment of our theme. Today it goes without saying that a philosophical theory of phenomenal consciousness must not conflict with empirical knowledge. The problem of consciousness has become an interdisciplinary problem. This is an advance in many ways, but it has also blurred the concept of consciousness. By straying into the most diverse disciplines, consciousness has become divorced from its philosophical origins. A semantic inflation has been the unavoidable consequence. This in turn has led to a state of uncertainty at the theoretical level. I have already identified the core of this insecurity: we are frequently not clear about the epistemic goal of our efforts, about what we actually want to know when we investigate consciousness. As the concept of consciousness has become more firmly embedded in empirical theory formation, the diverse facets of the problem have become particularly evident. There is no such thing as the problem of consciousness. ‘Consciousness’ is really only the summary title for a whole cluster of problems and possible research programmes. In such a situation it is urgently necessary, as a first step, to create a solid communications base. Each of the disciplines potentially participating in the project of a unified science of consciousness must therefore develop a systematic catalogue of its own questions. Each discipline has to determine its own set of explananda, the accepted methods of approaching these explananda, the respective explanatory basis and what —from the perspective of this discipline counts as an explanation.14 This also applies to the philosophy of the mind. Of course, this introduction is not the place for establishing such a catalogue, but in the following section I will offer a brief selection of the most important questions constituting the problem landscape today. I hope it can help the readers of this book in their first attempts at an orientation in this landscape.

III. The Problem Landscape: A Catalogue of Questions

1. Consciousness as a conceptual problem
A convincing theory of consciousness must be conceptually coherent. This part of the project unambiguously falls in the area of philosophy of mind. There, the concept of ‘consciousness’ is only rarely interpreted today in the sense of a non-corporeal substance or a non-physical individual. It is no longer a question of ‘the consciousness’, but of consciousness in the sense of a property. What is, however, the correct analysis of consciousness as a property?

(a) ‘Conscious’ as a one-place or two-place predicate
If the word ‘conscious’ is interpreted as a one-place predicate, then it appears as a primitive property of some mental states, which resists further analysis. There is no non-circular definition for this concept of phenomenal consciousness. Consciousness in this sense appears to be an intrinsic property of mental states that cannot be resolved into a set of relations between entities on a lower level of description. Consciousness, as a primitive property of mental states, would be irreducible. But it is this interpretation of ‘conscious’ that is central to the debate on phenomenal consciousness.

If ‘conscious’ is analysed as a two-place predicate, then one confronts the problem of the intentionality of the mental. In this form, consciousness is always ‘consciousness of something’ . Franz Brentano in 1874 described the basic feature of consciousness as the directedness of mental acts towards an object (cf. Brentano 1973 [1874]: 124 f. This notion of a relation between intentional acts and their objects led to a conception of consciousness as a psychological act that determined many theories of the late 19th and early 20th centuries. Edmund Husserl abandoned interpretations of this directedness in terms of operation or activity. He introduced the term ‘intentional experience’ [intentionales Erlebnis] in the Logical Investigations in order to avoid the concept of a ‘psychic phenomenon’, but retained the concept of act, in order to accommodate a use of language which was already deeply rooted. The distinction between act and object is still found in current discussion: for example, when it is asked whether the consciousness of a state is explained by its becoming the object of a higher-order thought (for instance in David Rosenthal’s theory of Higher-Order Thoughts),15or by the fact that it is an act (in the sense of a teleofunctionalistic theory of mind) with the help of which an organism attains knowledge about its surroundings (see Dretske submitted). It also plays an important part in the discussion of higher-order forms of self-consciousness. 16

(b) State consciousness and system consciousness
It can be said of certain states of an agent that they are conscious, using a one-place predicate. Such states are phenomenal states. The logical subject to which properties are attributed here are states of a system, for instance mental states, that may or may not need to possess the property of consciousness. David Rosenthal calls this kind of consciousness state consciousness or intransitive consciousness.

A difficulty arises, when one wishes to apply ‘conscious’ in the second sense (that is, ‘consciousness of something’ or transitive consciousness) to mental states. One can well say that mental states often possess an intentional content and with it help a person relate to objects in the outside world. What cannot be said is that these states themselves possess a consciousness ‘of something’: the states themselves are not epistemic subjects; they do not know anything and they do not possess an awareness of the relation of intentionality. Rather, these states are more like aids with the help of which persons or systems regarded as a whole attain a ‘consciousness of something’; instruments with which they can realize the relation to intentionality (again, see Dretske submitted). One must therefore ask what are the semantical rules which govern the application of these two interpretations of the predicate ‘conscious’ to persons or systems as a whole.

We also wish to know in which sense one can describe a person, an animal or perhaps even an artificial system, as conscious (creature consciousness). The interesting sense of the word, once again, is that of conscious experience rather than of unproblematic concepts like alertness, perceptual awareness or the presence of an orienting reaction. A system possesses phenomenal consciousness if and only if one can ascribe to it conscious states in the first sense. This does not mean, however, that these states will necessarily help in relating to the world in the sense of a realization of the relation of intentionality. A person or system has intentional consciousness if and only if it possesses phenomenal states with intentional content. These states are states of the second type, they support the person in building up and maintaining epistemic relations with the world. A simple and natural solution for this case seems to exist in the statement: conscious states are states through which we become conscious persons. On the other hand, this analysis does not cover such phenomenal states as do not possess an intentional object or a biological function. For this reason, the four interpretations of ‘conscious’ sketched here also require a careful analysis of the relationship between intentional and phenomenal content.

(c) The strength of the modal relation between consciousness and its explanatory basis
The task of empirical consciousness research is to set limits to the explanatory basis for phenomenal consciousness. This may relate to consciousness in general or the basis for instantiating particular phenomenal properties. Besides, such research projects will always be related to a certain class of systems. Such classes of systems may be people, animals or artificial systems. Within such classes it is possible to differentiate even further: human beings, for instance, can be investigated in a dream-state or awake, in different life-phases, or as members of differing cultures. Human beings in altered states of consciousness may be investigated: perhaps those who are afflicted with schizophrenia, ‘Multiple Personality Disorder’ or certain brain lesions; but also human beings in the state of deep meditation or madly in love. Let us assume, that consciousness in the sense of a minimal materialism is supervenient on a certain set of physical properties.17It is then possible to ask a multiplicity of questions: are the properties that give rise to phenomenal properties under the conditions of the natural physical laws, only intra- or also extra-organic properties? Are they quantum physical, neurobiological or neurocomputational properties? Are they functional, teleo-functional or representational properties? It is always a matter of describing as accurately as possible the minimal supervenience basis of the supervenience relation in question.

The philosophical project, in this context, is the closer investigation of the analytical strength of this relation. There is a strong modal intuition (a wide-spread assumption about what would be logically possible), that implies an unconscious Doppelganger for each system (whether described as physical, functional, or representational), which indeed has the same physical, functional or representational properties, but no phenomenal consciousness. Whichever cognitive phenomenon of consciousness is chosen for a closer investigation, the inevitable question always arises whether all this would not also be possible without consciousness.18Causal explanations and functional analyses of cognitive processes seem to be immune to the property of ‘consciousness’. In other words: there does not appear to be a necessary connection between phenomenal content and certain forms of its physical realization. In recent discussion, three important arguments have crystallized the philosophical debate surrounding this type of issue:

i. The Modal Argument
For any given system with physical, functional and phenomenal properties, there is always a conceivable Zombie Twin, a double without consciousness.19 One recent formulation of this view states: phenomenal properties, even though they are in our world naturally supervenient on physical or functional properties, are by this fact alone not logically supervenient on them (cf. Chalmers 1996).

ii. The Absent-Qualia Argument
For any conscious system one can imagine a functionally isomorphic twin-system that does not possess qualia, or is realized in such a bizarre way that the assumption of qualia becomes strongly counter-intuitive. If this is correct, it suggests that functionalism in the philosophy of the mind which after all still forms the meta-theoretical background of the information-processing approach in cognitive science and in cognitive neuroscience is systematically blind to the phenomenon of conscious experience.20

iii. The Inverted-Qualia Argument
It is also possible to develop a weaker version of this argument. This denies only the necessity of tying certain kinds of qualitative content to certain functional states. Thus, one can perhaps assume that one of the readers of this book possesses a systematically inverted colour spectrum in her subjective space, that is, she will always experience yellow, when we experience blue, and green where we experience red, etc. Of course, the person with the inverted colour qualia will always say that she has an experience of blue, whenever we have an experience of blue. In reality, however, a yellowquale appears in her conscious space. It is unclear how we could determine the existence of such a situation, but the crucial question is whether it is possible in principle.21

2. The epistemic asymmetry
This is the modern name for the most fundamental epistemological problem associated with consciousness. There are two fundamentally different methodological approaches that enable us to gather knowledge about consciousness: we can approach it from within and from without; from the first-person perspective and from the third-person perspective. Consciousness seems to distinguish itself by the privileged access that its bearer has to it. This poses the question of who actually possesses the epistemological authority over the facts of consciousness: the experiencing subject, or science that takes hold of the phenomenon from outside?

This epistemic asymmetry has found its expression in the history of ideas in two fundamentally different ways of approaching consciousness. The classic example of a philosophical investigation purely operating from the first-person perspective is René Descartes’ approach in the Meditations (see also Williams 1978). Toward the end of the last century, this strategy turned into what we now call the phenomenological approach. Outside of philosophy it turned into what is often called the introspectionist phase of scientific psychology. The phenomenological movement was the first comprehensive attempt to establish the conceptual and epistemological basis for a systematic, autonomous science of consciousness. That is its merit. Its failure was mainly due to its reliance on the evidence of inner perception. This proved to be an untenable approach of data collection because it lacks a reliable procedure for eliminating false observations and statistical inconsistencies, especially in the case of conflicting statements.

The alternative strategy is that of a naturalistic objectivism, the attempt to approach the problem of consciousness from without. Here, data collection is exclusively limited to information that is objective and accessible from the outside perspective, for instance, human behaviour. The facts deployed in any explanation of consciousness are thus exclusively public facts. The analytical behaviourism of Gilbert Ryle (cf. Ryle 1949) corresponded to the psychological behaviourism on the philosophical plane. Ryle had analysed statements concerning mental states as statements concerning possible behaviour. One of the reasons for the failure of this strategy lay in the fact of dispositional analyses not being able to explain the ‘processuality’ of phenomenal states: when I have conscious experiences, then something happens in me. Functionalism preserved important insights of this phase by extending its explanatory strategy to the investigation of internal states of systems as a form of ‘microbehaviour’.

Both kinds of research programme have become ideologies leading to irrationalism and methodological radicalism, to fear of contact and bouts of silence. With the advent of cognitive psychology and the ‘computer model of the mind’, the situation changed in an encouraging way. One of the reasons why the current debate is so attractive is that the old front lines between phenomenology and analytical philosophy of mind have disappeared, and the subject of consciousness has been accepted even by the best analytical thinkers as a serious and promising area of theory formation. Even advocates of strictly reductionist approaches admit that a theory of consciousness must be characterized by a maximum of phenomenological plausibility. On the other hand, many authors with a background in phenomenology have now started to discover cognitive science. This gives rise to hope. For what we need are answers to the fundamental epistemological problems that take the first-person perspective seriously and that at the same time integrate our new empirical knowledge in a conceptually plausible way.

In spite of this, for many the first reaction to the epistemic asymmetry will be one of scepticism.22 Does the essential ‘inwardness’ of consciousness mean that all attempts to study and understand it in the ‘public’ arena are bound to fail? Does it entail a class of facts that must a priori remain closed to the methods of the natural sciences? When I have a visual experience of Pantone Blue 72 for the first time, do I then know something about the world that cannot be known in any other way?

i. The Knowledge Argument
The Knowledge Argument is an important focal point of the discussion about phenomenal consciousness from the viewpoint of epistemology.23 The core of this idea, as developed by Frank Jackson, is the intuition that no knowledge about the neurophysiological or physical facts concerning a person is sufficient to derive the conscious experience of the person. The argument also has an ontological version, suggesting that this implies there are non-physical facts and that, therefore, physicalism must be false. Another way of interpreting our obvious epistemic limitation concerning the consciousness of other beings is to see it as an explanatory gap in our general scientific picture of the world.

ii. The Explanatory-Gap Argument
In 1983 Joseph Levine transformed a well-known modal argument against the identity theory by Saul Kripke24 into an epistemological version. Since nothing in the physical or functional correlates of a phenomenal state helps us to understand why this state subjectively feels in a certain way, an intelligibility gap opens up.25 This is the root of Cartesian intuitions which the modal arguments by Kripke and others attempt to explain at a formal level. Reductive strategies to explain qualia seem to leave a gap in the explanation, in that, strictly speaking, such explanations cannot really be understood. They do not seem to tell us, in principle, what we want to know.

3. Demarcation of explananda
All this underlines the fact that consciousness is a special problem. In spite of the fundamental epistemological problems, an attempt must also be made to characterize the explanatory concepts that are supposed to be the subject of a theory of consciousness. If there is such a list of concepts, then the way is open for differentiated investigations of the methods and targets necessary to approach this explanatory concept.26In this area philosophy need not confine itself to a conceptual commentary on the progress of empirical exploration. Philosophers can play a key role in defining the concepts that every convincing empirical theory must explain. Their detailed knowledge of the problem (which is, after all, many centuries old) qualifies them to intervene, actively and critically, in the construction of theories within individual sciences. The dialogue of philosophy with neuroscience has achieved at least one aim in the last two decades: there is hardly a researcher in the neurosciences who has not yet heard of the problem of ‘qualia’ and many are now quick to admit that this is indeed a truly deep problem.

4. Qualia
Qualia are the darlings of philosophers of consciousness. In the philosophy of the mind, qualia are mental states that possess a certain elementary kind of phenomenal content. The subjective quality of Pantone Blue 72 in colour perception, or of ‘painfulness’ in an experience of pain, are examples. Subjective qualities of experience create an inner taxonomy of mental states: they appear to be the essential features through which we individuate part of our own mental states by introspection. It is, however, debatable whether such ‘private’ properties of mental states can ever be connected to the public and objective properties of the underlying physical states. Qualia are problematic because they are:

· Difficult to verbalize. We cannot convey the meaning of redness to a person born blind.27

· Apparently private properties. Only public properties can be accessible to a scientific examination. This point concerns the epistemic asymmetry, as well as the ineffability of qualitative content.

· Possibly the intrinsic core of a state. That is, we may not be dealing here with properties that are relational, and therefore could be accessed by a relational (e.g. functional) analysis. Arguably, non-relational properties cannot be naturalised reductively: they cannot be traced back to relations between elements on more basic levels of description.

· Homogeneous, i.e. ‘grainless’ or ‘ultrasmooth’ on the level of subjective experience. Simple phenomenal properties prima introspectione do not possess an inner structure. They are, therefore, experienced as indivisible, as phenomenal atoms. This problem is closely connected to the grain problem.28

· Transparent and present. They appear to be immediately given to the subject of phenomenal states.

5. Higher-order phenomenal properties and structural characteristics of phenomenal space
The field of conscious experience possesses a complex inner structure. Here are a few examples of particularly succinct properties of this structure:

· Object formation. Simple phenomenal properties do not appear in isolation, but as components of integral complexes. This creates the problem of binding and integrating phenomenal properties into a whole. It is, however, quite unclear exactly what ‘holism’ means in this context.

· Relation-types. This concerns an important part of what is called ‘naive physics’. We require a theory of the types of relations between objects and events that our conscious model of reality knows.

· Experience of time. How does subjective time arise from objective events? What is phenomenal ‘simultaneity’ and how does the experience of ‘succession’ arise?

· Experience of space. How are spatial relationships between phenomenal objects constituted? What are the embedding relations, which enable us to experience consciously one spatial object as part of a larger object? What is the relationship between the part of the human self-model which is phenomenally represented as having spatial properties (body image) and that part, which subjectively possesses only temporal, and no longer any spatial properties (conscious thought, ‘the thinking subject’)?

· Experience of causality. Under what conditions do we experience an event as the cause of another event? How does this experience influence our higher cognitive operations?

· Situatedness. What does it mean that conscious experience is always embedded in situations and contexts? How does implicit knowledge influence the content of our explicit phenomenal states?

· Embodiment. What precisely does it mean that our form of consciousness is almost always integrated into a bodily model of the self?

· Possible phenomenal worlds. We are beings who can create in our consciousness ‘virtual’ worlds of experience, e.g. through fantasy, imagination and planning. What are the neurobiological and evolutionary conditions such that we can imagine some things but not others? How do these natural limitations of our mental imaginative space affect our cognitive operations, e.g. in consciously forming concepts of logical possibility or in the grounding of certain theoretical intuitions? If the presence of the phenomenal ‘Now’ can be shown to be an illusion, is it then correct to say that our conscious model of reality as a whole is only a virtual world?

6. Perspectivalness and phenomenal subjectivity
Here we meet again the perspectivalness of phenomenal states, this time not as a problem of epistemology, but as a concretely experienced structural property of the phenomenal space. To begin with, an important element of every theory of phenomenal consciousness will always be phenomenal self-consciousness. Therefore, conscious experience of one’s own identity is a potential subject for empirical explanation too. The neural and functional correlates of self-consciousness may be central here, but so may be the extent to which its content is determined by social interactions. What is needed is a comprehensive theory of the phenomenal self. We also need to explain its influence on other forms of phenomenal content. What exactly does it mean to say that our consciousness is a centred consciousness? A convincing theory about the origin of a ‘first-person perspective’ might also help to resolve some of the epistemological problems I have mentioned.

7. The Unity of Consciousness
If one looks at the history of the concept of consciousness,29then two fundamental semantic elements stand out in many of its theoretical precursors: the concomitant (i.e. accompanying) and the synthesizing functions of consciousness. These still play a major role in the contemporary discussion. The view of consciousness as an activity that accompanies mental states by creating higher-order states is reflected for instance in theories of ‘metacognition’ in cognitive science. In the philosophy of mind, it appears in numerous theories of inner perception and higher-order thought.30 Similarly, the classic problem of the unity of consciousness in the sense of a synthesis combining the different contents of consciousness into a holistic entity appears in the contemporary philosophical discussion as the question of the integration of phenomenal content and in the empirical sciences as the binding problem.31

Our field of consciousness has an undeniable holistic quality. The different forms of phenomenal content that are active in it stand in part—whole relationships to this field. This holism is a higher-order property of consciousness, just like its transparency, perspectivalness and presence. The unity of consciousness is a highest-order property of the phenomenal model of reality active at a certain time. This global unity of consciousness seems to be the most general phenomenological characteristic of conscious experience, and is therefore difficult to understand on a conceptual level.

8. Consciousness as the core variant of the mind—body problem
In the post-war discussion, the classic mind—body problem has blossomed with many variants. Here are some of them:

· The nomological incommensurability of personal and sub-personal levels of description. It is not possible to give detailed descriptions of causal chains across these levels, since the logical subjects of the personal and sub-personal level of description differ. There are therefore no law-like generalisations to be refined, as there are no strict (homonomic) psychophysical laws.

· Mental universals. A philosophical theory of mind is most interesting when it explains the nature of certain types of mental states. The philosophical project consists in the search for a ‘universal psychology’. According to a widely accepted view, however, mental states can be physically realized in principle by a wide range of physical states, just as the same computer program can run on different machines (cf. Putnam 1967). So type physicalism seems to be false, while token physicalism gives no access to mental universals.

· The liberalism—chauvinism dilemma of classical functionalism: When we individuate mental states according to their causal role, it appears that the resulting criteria for attributing such states are always either too strict or too liberal. Abstract descriptions of mental states, for instance by program listings or by Turing machines, do not capture the subjective concreteness, temporal dynamics and parallelism of phenomenal states.

· Mental causation. Non-reductive forms of materialism such as supervenience theory (just like classical interactionist dualism or epiphenomalism) cannot explain ‘downward’ causation by mental states. The strong variants of supervenience collapse into a reductionist identity statement, the weak forms of global supervenience do not conceptually grasp the fact that conscious experience is a phenomenon bound to individual systems or organisms.

· Brentano ‘s Problem. Many mental states possess intentional content. They are directed to a part of the world and contain it in a mysterious sense (‘mental inexistence’). It is not easy to explain how this meaningfulness or referentiality of mental states could be grounded in relations in the physical world.

· The rationality of the mental. How can reasons be causes? In order to keep on believing in our own rationality as well as in the rationality of our theories, e.g. concerning the relationship between mind and body, insight into reasons must play a causal role in our behaviour. Can one be a realist about the rationality of conscious thought? Do attributions of rationality refer to mental processes, or only to a certain coherence in externally observable patterns of behaviour?

· Personal identity. Are there firm criteria by which we can delineate the transtemporal identity of persons? Is there an enduring core-property in the temporal development of such systems to which we wish to attribute the property of personhood?

· Parallelism versus seriality. What is the relationship of serial cognitive processes with symbolic and propositional content to subsymbolic processes on the ‘microcognitive’ level of neural information processing?

· Subjective versus objective time. How can an inner time order arise from individual events on the physical level? What is the relationship between physical and psychological time?

· Theory-neutral inner experience. What is the epistemological status of introspection? Is there a kind of knowledge about mental states that is independent of all verbal descriptions and not ‘theory-laden’? What are the falsification criteria in the introspective attribution of psychological properties? Does the subject possess an epistemicological authority over the content of its own phenomenal states?

This short list of mind—body problems could easily be extended. One might well differentiate these questions in greater detail, and thus even more exactly.32 They also suggest that the problem of consciousness is the core variant of the mind—body problem. Thomas Nagel, in his classic essay of 1974, pronounced this in a sentence that has been widely quoted:

Without consciousness the mind—body problem would be much less interesting. With consciousness it seems hopeless (Nagel 1974: 261).

Nagel is right in that a solution to the problem of consciousness would lessen most people’s interest in the questions listed above. If there were an empirically, conceptually and above all intuitively convincing theory of how phenomenal consciousness in its full content that is, our own conscious experience can be part of the natural world, then these questions might suddenly be looked upon by many people as merely technical problems for analytical specialists or brain researchers. On the other hand, a complete and detailed answer to all these questions would leave us unsatisfied, if it did not tell us what conscious experience actually is. Conscious experience is almost always what we really want to understand. It is not only the intuitive root, but also the theoretical core-variant of the mind—body problem.

9. Developing a comprehensive theory of phenomenal content
Approaching the theoretical core of the problem is not only a matter of isolating individual explananda by describing them in as precise and differentiated a way as possible. It is now also necessary to place such analyses into a systematic general context. What has to be worked out is a general theory of phenomenal content, a comprehensive theory of conscious experience.

Such a theory would have to yield a ‘mark of the phenomenal’, by offering demarcation criteria for the domain of conscious experience. (This was Brentano’s original problem.) This concerns first and foremost the distinction between conscious and unconscious mental states: what exactly is it that makes a mental state the content of conscious experience? In what way do unconscious mental states influence the structure and the explicit content of phenomenal states? Empirical research has shown that the distinction between ‘conscious’ and ‘unconscious’ is not simply all-or-nothing.33So a theory of phenomenal content must investigate more accurately what the property of ‘phenomenality’ actually is. A second important point is the development of criteria concerning the ascription of phenomenal states: under what conditions can one assume that a given system has conscious experiences?34 We need to develop empirically plausible and pragmatically satisfactory criteria for the epistemological Other-Minds Problem. It may be that descriptions of phenomenal states with the help of concepts such as ‘information’, ‘representation’ or ‘content’ will soon be given up, perhaps in favour of the terminological apparatus that is offered to us by non-linear dynamics. Until this should happen, every theory of phenomenal content must investigate the relationship of this special form of content with other forms of content. This particularly concerns the intentional and the unconscious content of mental states.

Empirical consciousness research approaches the problem of consciousness at many levels of description. This leads to the question of which level is the most interesting for a philosophical theory of consciousness, allowing maximal conceptual precision. What we may need are mathematical models of the phenomenal ontology of the human brain. Which science offers us the best abstract descriptions of the structure of the phenomenal space and of the dynamics of phenomenal states? Can mathematical models of the neural correlate of consciousness be tied to those theoretical entities which are interesting from a philosophical viewpoint? This question remains open. Should it be shown, for instance, that the structure and dynamics of our consciousness can be plausibly described by a connectionist approach supplemented by principles of temporal coding, then one could call the project of an objective phenomenology as formulated by Thomas Nagel, a neurocomputational theory of phenomenal states’.

The main difficulty lies in linking the often ill-defined mental categories of folk psychology and classical philosophy of mind to the newly-emerging conceptual apparatus of an empirical psychology (such as that modelled on information processing). The philosophical project of a universal theory of mind, however, goes much further: it demands that ‘consciousness’ should be reconstructed as a mental universal. The philosophical dream consists not only in a general, but also in a universal theory of conscious experience. This means that, using hardware- and species-independent criteria of ascription, one develops a theory of consciousness that would answer the question of what makes consciousness consciousness in all possible systems endowed with that property. Yet it could be that this very project has to be given up in the course of a naturalization of the mind, since the ‘bottom-up constraints’ necessary for the clarification of the concept may be too strong and so entangle us in an anthropocentrism.

IV. This Volume

This collection of articles offers a cross-section of the current debate, carried out at a high level.35I have tried to keep a broad perspective, deliberately including a wider range of topics, and to provide readers with the means to go on and pursue their own interests effectively. The articles are for this reason cast at various levels of abstraction and with differing degrees of interdisciplinarity. All these articles are original, and all mark important forward-looking lines of enquiry in the current debate. They are summarized in short introductions to each of the seven main parts of the book. These introductions also include short selections of recommended reading, which I hope the reader will find useful.


1 A selection of these can be found in section 1.3 of the bibliography in Appendix I of this book.

2 Cf. Penrose 1994:7 ff. Further important contributions to the debate concerning an independent science of consciousness are Baars 1988, Chalmers 1996, Flanagan 1992: 213 ff., Revonsuo 1994: 249 ff., the collection of texts by Hameroff et a!. 1996 and the ongoing discussion subsequent to Chalmers 1995. See also Greenfield 1995, Scott 1995 and Velmans in press.

3 Concerning sceptical considerations of this point, cf. the papers by Kathy Wilkes, Martin Kurthen and David Papineau in Part 2 of this book.

4 In the more recent discussion no one has highlighted this point as clearly as Thomas Nagel. Cf. Nagel 1974, 1979, 1980, 1986.

5 See Flanagan 1992: chapter 11.

6 Once again, thanks are due to Thomas Nagel for raising this point very clearly in an unfavourable climate, first in Nagel 1974. See also some of the texts listed in section 3.7 of the bibliography. For criticisms of the assumption of perspectival ‘first-person facts’ see Lycan 1987: chapter 2, Malcolm 1988, Metzinger 1993: chapters 4 & 5, Metzinger 1995b.

7 Other frequently used expressions are qualia, secondary qualities, sensory qualities, subjective qualities of experience, experiential properties and the subjective character of experience. See e.g. Kirk 1994 and Clark 1992.

8 Hardin 1988 offers an excellent introduction, tailor-made for the discussion of the underlying philosophical problems.

9 Regarding the concept of ‘semantic transparency’ see Metzinger 1993, Van Gulick 1988 a, b.

10 See e.g. Bieri 1981: 206.

11 See the paper by Eva Ruhnau in the third part of this volume and the references given in the introduction to this part. A good introduction to the empirical aspect of this problem can be found in Pöppel 1988, see also Pöppel 1978

12 This need not be so. The importance for consciousness research of so-called ‘lucid dreams’, in which we develop a critical relationship to phenomenal reality, may have been underestimated in the past. For further references see the introduction to Part 7 of the expanded German version of this book and Metzinger 1993. See also Flanagan 1995 and Revonsuo 1995a. In researching conscious experience, dreams are of great interest for at least two more reasons. First, it may be possible to approach empirically the question of phenomenal coherence and integration of content (see my contribution to this volume) by measuring the bizarreness of dream content; cf. Mamelak & Hobson 1989, Revonsuo & Salmivalli 1995. Second, it might be of great importance to delineate the common neurobiological and functional substrate of both waking and dreaming, as both are the paradigmatic types of phenomenality in our own case. As it turns out, waking consciousness may be looked upon as a special form of online-dreaming. See Hobson & Stickgold 1993, 1994, Llinás & Pare 1991, Llinás & Ribary 1994, Revonsuo 1995a.

13 Concerning the decline of introspective research programmes see Lyons 1986.

14 See Robert Van Gulick’s contribution in Part 1 of this volume.

15 See Rosenthal 1986, 1993 and especially Rosenthal 1996 as well as his contribution in Part 6 of this volume.

16 This is for instance true for those forms of an act—object equivocation dubbed the banana-peel-fallacy by William Lycan, which he diagnoses in his criticism of Nagel’s concept of an ‘objective self and also in Kripke’s essentialist argument against the identity theory.

17 For the concept of ‘supervenience’ see Kim 1993.

18 See the opening essay by Peter Bieri in Part 1 of this volume.

19  See Campbell 1970, Kirk t974 as well as some of the texts listed in sections 3.2 and 3.8 of the bibliography at the end of this volume.

20 See Shoemaker 1975, Block & Fodor 1972, Block 1980a, b, the contribution by David Chalmers in Part 5 of this volume and the references listed in section 3.8 of the bibliography.

21 See Lycan 1973, Shoemaker 1982, Block 1990, Horgan 1984, Hardin 1996, Nida-Rumeun 1995, the text by David Chalmers in Part 5 and the references listed in section 3.8 of the bibliography.

22 See McGinn 1989 and, from other perspectives, the contributions in Part 2 of this volume.

23 See Jackson 1982, Nagel 1974, the contributions by Martine Nida-Rümelin, William Lycan and David Papineau published in this volume, as well as the references given in the introduction to Part 4 as well as in section 3.7 of the bibliography.

24 See Kripke 1972, 1977. Here, Kripke put forward the argument that psychophysical identity-claims, using rigid designators like ‘my conscious experience of type P’ and ‘my brain-state of type X’ on both sides of the equation, had to be false, since identity in this sense is a necessary identity which holds across all logically possible worlds. Since we all can imagine that we have the mental state in question e.g. the quale of Pantone Blue 72 without being in the respective brain state, this applies, at most, to contingent statements of identity and, therefore, in this sense to false statements of identity.

25 The texts by David Papineau and Robert Kirk in this volume are attempts to contribute to closing this gap.

26 Robert Van Gulick does this in his contribution to this volume.

27 Regarding the important problem of the ineffability of phenomenal content, I refer readers to Diana Raffman’s contribution in this volume. This paper also contains a very striking example of a particular red; the name of this colour is Pantone Red 032. See page 298 below.

28 See Thomas Metzinger’s contribution in Part 7 of this volume.

29 Many non-trivial misunderstandings in the ongoing discussion arise precisely because the long history of the concept ‘consciousness’ is both very extensive and full of semantic complexity. The problem of consciousness is the most persistent obstacle in the way of a scientifically complete picture of the world, largely because it stems from a vast and dense network of roots planted in cultural history. Since lack of space would preclude anything more than an inadequate sketch, I am not attempting in this introduction any historical presentation of the development of the modem concept of consciousness. Comprehensive literary references to the conceptual history of ‘consciousness’ in philosophy and psychology can be found in Diemer 1971, Grauman 1966, Guzeldere 1995a, 1995b.

30 See the texts in Part 6 of this volume and the references given in the introduction to that section.

31 See the contribution by Thomas Metzinger in Part 7 of this volume.

32 See Metzinger 1985, 1990, 1991.

33 See e.g. Kilhlstrom 1993.

34 See Dieter Birnbacher’s text on this question at the end of this volume.

35 An excellent compilation of articles from the recent Anglo-Saxon debate can be found in Block et al. 1996. The anthology published by Sybille Kramer gives an overview of the present state and main lines of the German discussion; see Krämer 1996. Two important interdisciplinary collections are Marcel & Bisiach 1988 and Davies & Humphreys 1993.


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