Philosophy, unlike most other subjects, does not try to extend our knowledge by discovering new information about the world. It tries to deepen our understanding by reflection on what is already closest to us—the experiences, thoughts, concepts, and activities that make up our lives, and that ordinarily escape notice because they are so familiar. Philosophy begins by finding utterly mysterious the things that pervade our everyday lives, such as language, perception, value, and truth. For everyday purposes we don’t have to know how these things are possible: we talk, we see what is in front of us, and we judge that this action is wrong or that assertion true. But it is possible, in the tradition deriving from Plato, to stop and think about what we are really doing, not for a practical purpose but just in order to understand what lies beneath the familiar surface of life.

Human consciousness, which is at the core of everything we do and think, is one of the principal targets of such philosophical understanding. In our time the advance of physical and biological science makes possible the search for some of its physiological conditions, but there is a more basic understanding of consciousness that remains a philosophical task, and that is surprisingly undeveloped—an understanding that we can pursue only from within. It has occupied phenomenologists like Husserl and Sartre, and empiricists like Hume and Russell. Even if empirical science starts from the evidence provided by conscious experience, understanding the nature of that starting point is still mainly the concern of philosophers.

Brian O’Shaughnessy’s philosophical career has been occupied with our most basic relations to the world. Twenty years ago he published another enormous book called The Will, about how we connect to the world in the outward direction, when we act. His new book, going in the other direction, examines the world’s impact on us through perception—though he emphasizes that we are anything but passive when we perceive the world around us; that, too, involves the will.

O’Shaughnessy’s approach to perceptual consciousness is distinctive in being simultaneously physical and phenomenological. He takes us and other animals to be physical beings—parts of the physical world—each of which has a perspective on that world and an inner life of some kind. An inquiry into conscious experience cannot be based merely on the observation of external behavior; but it also cannot be carried out in abstraction from our physical nature. The understanding of the inner life of the person who is conscious must include the physical body from the start. In his new book O’Shaughnessy develops an account of human consciousness as a continual process by which we come to know about the vast physical world around us through awareness of what is going on in a small part of it, namely our own body.

It could hardly be otherwise, since all information about the rest of the world reaches us through our bodies. But when I gaze out my window toward the Hudson River and watch the planes coming down to land at Newark Airport, and see the stars emerge in the sky as night falls, the complexity and radical indirectness of the process is completely hidden from me. I seem just to see these things directly, and of course I do not. A full account of what actually happens would be extremely intricate, and we have only part of it. Since Plato and Aristotle began to worry about the question a great deal has been learned about what takes place between the stars and the retina, and much is currently being learned about the physical effects of retinal stimuli on the brain; but what goes on in the mind remains very difficult to describe, even though it is in a sense closer to us than anything else could be.

To his credit O’Shaughnessy, unlike most contemporary philosophers, uses the term “consciousness” in its correct English sense, to mean a state of awareness of the world around us—roughly, the state of being awake. For some reason the term is commonly used more broadly in philosophical discourse to refer to any psychological state that has a subjective, experienced character. For this latter category O’Shaughnessy uses the term “experience,” which is more accurate. Dreams, for example, are experiences we have when we are not conscious. (Other psychological states, like knowing, believing, and intending, need not be immediately experienced to exist; nor are they usually present in consciousness, though they can have large conscious effects, as when my long familiarity with the Beatles causes me instantly to recognize the melody of “Yesterday.”)

The stream of consciousness is what we all live in. That expression is now associated with a literary form in which a character’s inner monologue of thoughts and associations is presented accurately and is very different from the orderly outward forms of his life in the world. But the true stream of consciousness is far richer and far less verbal than anything described in Ulysses. Think about what happens to you during any two minutes spent walking on a city street—the flood of sensations, perceptions, and feelings that courses through you, most of them hardly drawing your attention. The multiplicity and density of detail is far greater than even the richest collection of verbalized thoughts or conversations with yourself that may have been going on at the same time. The process by which the world impinges on us at all times and the constantly shifting apprehension of our relation to it are too enormous for us to fully grasp.


O’Shaughnessy wants to analyze this process insofar as it is accessible to the heightened self-consciousness of philosophical reflection. Consciousness as he conceives it is filled with experience, but experience of a special kind. Unlike the experience in dreams or imaginings, the experience of consciousness is subject to a ceaseless rational control that tries to make sense of the surrounding world and our place in it: it is shaped by the requirement of reality—of placing ourselves as physical beings in a physically real world. It is necessarily aimed at the truth.

Even when its contents do not change, consciousness is never static but always proceeding in time, so that it apprehends both change and changelessness. Whether you are crossing a street, reading a letter, making a phone call, or merely staring at the ceiling, your experience includes at every point the sense of what is immediately past and the readiness for what may come next; consciousness prepares us to act in the light of what is happening and about to happen. It gives us our only acquaintance with time and, through changes in the relation between ourselves and other things over time, our knowledge of space as well, by tracking our movements and the change in how things look and feel as we move around them. In all this, our unformulated sense of the location, posture, and boundaries of our own body plays an essential part. The accomplishments of an ordinary pedestrian are technically as amazing as those of Fred Astaire.

Most of O’Shaughnessy’s book is about perception, especially visual perception. He presents a theory that is rather old-fashioned and these days rarely defended: the theory that all our perception of the outer world depends on awareness of psychological items in our own minds called sense-data. We don’t think of perception in this way, he argues, but that is what is really going on. We never “just see” any physical object: to see it we must see the light reflected from it that strikes our retinas. And to see that light we must become aware of the visual sensations, as he calls them—describable in terms of color and shape—that are directly caused by the chemical effects of the light on the retina and their effects on the optic nerve and the brain. (O’Shaughnessy’s view is consistent with the hypothesis that visual sensations are in some sense identical with certain physical effects in the brain, but this aspect of the mind–body problem is not his subject.)

The full set of your visual sensations at any time comprises your “visual field.” O’Shaughnessy maintains that you can be visually aware of these psychological items, as you can be visually aware of an afterimage, which exists only in your mind and cannot be seen by other people; even if they looked into your brain they would see only the physical features of the brain event that corresponds to it. If the chain of causal links is sufficiently reliable, we get what he calls the Transitivity of Attention: “Presence in the visual field of a round orange sensation is presence in the visual field of a cylindrical beam of orange light, which is in turn presence therein of an orange sphere…. [The attention] has merely to make any one of these three listed items its object, to make the other two simultaneously its object.” Perception of the orange is not, in other words, accomplished by inference (I need not believe that it is an orange in order to see it). It is guaranteed by the strict causal link from object, to light, to nerves, to sensation. It is that causal link that makes it possible for a single event to be all these things at once: the awareness of a visual sensation, the seeing of light, the seeing of the near surface of an object, and the seeing of the object.

Without sensation we would not see, but it is another important feature of O’Shaughnessy’s view that there is vastly more in the visual field, and in our other sensory data, than reaches our attention—and that even when we are not aware of these sensations, they exist. The attention has a severely limited capacity at any moment, and it must select first of all from the plethora of content in our own minds.


What kind of existence do unnoticed sensations have? The question is a new version, transferred to the interior of the mind, of the old philosophical chestnut, whether a tree falling unheard in the forest makes a sound. What about the ticking of the clock when you don’t notice it? The question is not whether the clock ticks, but whether it produces an auditory sensation in you whether you notice it or not. O’Shaughnessy uses the example of tinnitus, a condition with which I am familiar: in my case the symptom takes the form of a constant, faint hissing noise that no one else can hear. Most of the time I don’t notice it, but I only have to listen for it and it is there, at any time of day or night. Obviously its physical cause, whatever that may be, is present whether I hear it or not (like the sound waves in the forest); but is the sensation itself present even when I don’t notice it? O’Shaughnessy says yes, and that the same can be said for a large part of what makes up your visual field or your tactile sensations at any moment. If you now direct your attention to the pressure of your left shoe on your heel you will become suddenly aware in experience of a sensation that was already there, unnoticed and—paradoxical though it sounds—unexperienced.

So O’Shaughnessy has introduced a threefold distinction among phenomena to which many people have indiscriminately applied the term “consciousness”: sensations (which need not be experienced), experience (which need not be conscious), and consciousness. In the case of vision, he believes the selectivity of awareness with respect to sensation is extreme, because of the richness of the sense-data. When you see a field of daffodils, each daffodil registers its contribution to your visual field through direct action on the retina, but you can’t possibly attend to all of them. What your attention grasps is the whole field, and that is the content of your conscious experience, even though images of the particular daffodils are present in the visual field and you could become aware of them if you attended to them individually. But until you do that, the images of particular flowers are “a noticeable and unnoticed part of the noticed.”

The selectivity and limited capacity of attention is a pervasive fact of life. But do these auditory, visual, and tactile sensations that fall outside the scope of our limited attention at any time have real psychological existence, as O’Shaughnessy claims? Or does their existence consist merely in the possibility of their being noticed? And how much can philosophical reflection do to answer the question? Introspection won’t settle it, since what you notice when you attend to a sensation doesn’t tell you what happens when you don’t attend to it. Does attention have the effect of making sensations spring into existence that weren’t there until you looked or listened for them? When I shift my attention to listen to the ticking of the clock or the conversation at the next table in a restaurant, or to my private hissing noise, the inner sounds certainly seem to be there, waiting for me to hear them; but though O’Shaughnessy explores the two hypotheses at length, and I am inclined to agree with his judgment, I think the question remains open.

O’Shaughnessy wants to “prise apart two closely intertwined psychological items: the visual field, and our awareness of it. The independent psychological reality of the visual field is the existence of visual sensations or visual sense-data”—sensations of which we are often not aware. What is the importance of this issue? It is that O’Shaughnessy is resisting the over-intellectualization of the mind. He wants to establish that the higher mental functions rest on a brute foundation that is meaningless, uninterpreted, and directly linked to the physical body and the direct impact on the body of the rest of the physical world. (In his earlier book on the will, he argued persuasively for a similar basis of action that was below the level of intention, which he calls subintentional action.) The first stage in perception is the direct physical causation of a wealth of sensations, imprinting the world in our mental flesh, so to speak. Only when the attention focuses on and makes sense of some of this material does experience arise—experience that can be the subject of introspection. And then, from experience, beliefs and knowledge can arise, along with the awareness of the world that makes intentional action possible.

This view goes against the widespread current tendency to see all psychological states as pervaded with thought, belief, concepts, and intentions—with meaning of some kind. According to that approach, the sensory qualities of a visual impression are simply identified with its representation of the external properties of objects we perceive—the roundness and orangeness of the image, for example, are identified by many philosophers with its seeming to me that there is an orange, or something very like an orange, in front of me. (This view is called representationism.) O’Shaughnessy objects, first, that I can only notice the orange by noticing the orange disk in my visual field; and, second, that the sensory orange disk may be there without my noticing it. If my eyes happen to be resting on a fruit bowl while a political catastrophe is announced on the radio, the orange disk will be included in my visual field, but I may not notice either it or the orange.

For O’Shaughnessy, then, the attention puts together the stream of consciousness from a selected portion of the abundant raw material of sensation, and it shapes that material into experiences and knowledge of the world, to be used in determining at every moment what to expect and what to do next. This is a compelling picture, and seems true to experience. On reflection, it is hard to deny that the contents of our minds are much larger than our fully conscious selves. The attentive self that is the subject of consciousness is in some sense the inhabitant and explorer of a vast mental territory; and uninterpreted data in our minds form the first boundary between the conscious self and the external world.

What is needed to complete this picture is an understanding of what attention itself is. It can’t be depicted as an internal perceiver of the contents of the mind without leading to a regress—since in that case the original sense-data would have to cause further sense-data of which the attention became aware, and the same question would arise about how it notices them. O’Shaughnessy’s account of attention is not easy to understand. He says that the awareness of an experience simply is the experience itself. But what happens when an unnoticed sensation becomes part of conscious experience by being noticed? O’Shaughnessy says it becomes immediately available for use in rational action and belief, having been picked out by the attention in its constant effort to make sense of the world. The attention, for him, is really a mental manifestation of the will, and consciousness is the product of the constant activity of the mental rational will in maintaining an intelligible and usable version of the world and our place in it.

While this may be a good account of the function of consciousness, it isn’t an account of the intrinsic difference between a noticed and an unnoticed sensation. But perhaps there is nothing more to be said about this, and we must be content with O’Shaughnessy’s aim of describing how the attention shapes the experiences of the present moment into an intelligible system.

Anyone who has experienced insomnia knows that the essence of consciousness is mental activity, and that we cannot lose consciousness unless we manage to give it up and become passive vehicles of sensation and fantasy, no longer imposing order or sense on them. O’Shaughnessy has some very interesting and astute discussions of dreams, drunkenness, and hallucinations—states in which the rational control of experience characteristic of full consciousness is severely diminished or absent, the exercise of will is weakened, and the connection to reality is broken.

He also presents a related theory of the imagination, arguing that imagination is very different from perception, in that it does not contain any “unnoticed but noticeable” parts. (This was also observed by Sartre.) If I visually imagine a bed of daffodils, I don’t have to imagine a specific number of daffodils; but when I actually see a bed of daffodils, then whether I notice them individually or not, there is a specific number of daffodils that I see and a specific number of daffodil images in my visual field. This difference reflects the absence from the imagination of the direct causal control of my senses by the external world that occurs in perception. The content of imagination, O’Shaughnessy argues, is fully determined by what is imagined—here, for example, a bed of daffodils, with no number specified. It is not merely a pale form of perceptual image.

I don’t know whether this interesting claim is true, and I doubt that its truth or falsity can be determined by philosophical reflection alone. I have a similar uncertainty about O’Shaughnessy’s sense-datum theory and his argument for the autonomy of the visual field. Certainly these are credible positions, and they are carefully developed. But I believe that physiological evidence would also be relevant to a final judgment about the interaction between attention, sensation, and sensory input.

O’Shaughnessy’s only excursion into physiological psychology is a long discussion of the pathological condition called blindsight, in which the brain-damaged subject has no visual experience but can still, in certain circumstances, make accurate guesses about what is before his eyes. Even though he may consider himself blind, the remaining parts of his visual system can still have direct influence on his beliefs. O’Shaughnessy is mainly concerned to argue that this response is not “seeing,” because it does not include visual experience. But there are also facts about the normal operation of vision that complicate the picture.

Consider for example the operation of a component of the neurological apparatus of vision called the dorsal system.1 The dorsal system allows visual input to play a part in the control of action without going through consciousness. When you run on a beach, for example, you avoid stepping on stones in your path, not because you perceive them but before you perceive them, in virtue of a direct contribution of visual information to the control of action. In other words your eyes can guide your feet without giving you a visual experience first.

This is known from the different effects of injuries to different pathways by which information from the retina reaches the brain. Because it involves the subtle control of intentional action it is different from the more familiar fact that one reflexively withdraws one’s hand from a hot surface before feeling the pain. These phenomena seem to show that the data of vision, insofar as they are directly caused by neural input, are more complex than can be captured in the idea of a single visual field whose contents are all sensations, whether noticed or not. Some of the inputs that are not noticed may be present in other forms than as sensations.

Another relevant example is provided by the evidence that the visual field cannot be described simply as an expanse with the color specified at each point, like the pixels on a computer screen. O’Shaughnessy argues that this bare minimum “atomistic” account is the one that describes the “visual given,” i.e., what is there before the attention goes to work on it. But much more processing seems to go on even at the earliest stages of the physical production of visual sensation, including, for example, detecting the boundaries of objects, identifying them as distinct things, and tracking their movements. Such processed visual data may already be part of the physically caused “raw material” on which the attention operates to generate conscious experience. Well-known studies of specialized detectors in the visual cortex point to such a conclusion.2 They may not prove the conclusion; rival theories have to be weighed against all the evidence. But if we take seriously the conception of the senses as paths by which the physical world directly affects the mind, physiological evidence for the antecedent receptive structure of the mind becomes important.

O’Shaughnessy is a remarkably gifted and solitary philosopher who pays almost no attention to anyone else, and it would be naive to expect him to behave like an ordinary member of the profession. Still, to read his book is a Stakhanovite task, even for someone who has spent a lifetime studying philosophy. It has seven hundred closely printed pages of dense argument, with hardly any references to the vast literature on these topics, and a Proustian exhaustiveness of detail that suggests, no doubt wrongly, that we are getting the entire contents of the author’s mind. My description gives only the bare bones of the theory; it would be impossible to convey the flavor of the book without quoting, for example, the detailed discussion of the forms of temporal knowledge someone can possess while in a coma, or the explanation of why the visual detectability of a particle passing through a cloud chamber doesn’t mean that the particle is visible. I fear that the sheer volume of O’Shaughnessy’s work will deter all but the most dedicated readers, and I get the uneasy feeling that O’Shaughnessy doesn’t care. This is too bad, because the book offers a great deal of insight about the most conspicuous and least understood thing in the universe—ourselves.

If we are to make progress over the long term in understanding how the mind fits into the world, it will take detailed phenomenological investigations like O’Shaughnessy’s as well as research on the brain and on behavior. There is a danger of assuming that we all know what we’re talking about when we set out to investigate visual perception and other forms of consciousness scientifically: it may seem that the subjective phenomena are familiar and we must search for their hidden physical basis, as we might investigate the structure of the eye. But that is too simple. The phenomena to be explained are in this case much more complex than our intuitive grasp of them reveals, and to deepen our understanding of them is in part a philosophical task, which must be carried out if the scientific questions are to be posed correctly. To seek inner understanding of the kind pursued by O’Shaughnessy, not by abstracting the perspective of consciousness from the physical world but by recognizing the primitive embeddedness of consciousness in that world, seems to me the right path to follow. This is so even if a great deal will remain hidden from a priori reflection, and equal attention will have to be paid to the physical evidence.

This Issue

April 11, 2002