M01_LEV1475_01_SE_C01.QXD 10/15/10 5:34 PM Page 1 CHAPTER ONE VISUAL AWARENESS STEPHEN E. PALMER 1.1 PHILOSOPHICAL FOUNDATIONS The first work on virtually all scientific problems was done by philosophers, and the nature of human consciousness is no exception. The issues they raised have framed the discussion for modern theories of awareness. Philosophical treatments of consciousness have primar- ily concerned two issues that we will discuss before considering empirical facts and theo- retical proposals: The mind-body problem concerns the relation between mental events and physical events in the brain, and the problem of other minds concerns how people come to believe that other people (or animals) are also conscious. 1.1.1 The Mind-Body Problem Although there is a long history to how philosophers have viewed the nature of the mind (sometimes equated with the soul), the single most important issue concerns what has come to be called the mind-body problem: What is the relation between mental events (e.g., per- ceptions, pains, hopes, desires, beliefs) and physical events (e.g., brain activity)? The idea that there is a mind-body problem to begin with presupposes one of the most important philosophical positions about the nature of mind. It is known as dualism because it pro- poses that mind and body are two different kinds of entities. After all, if there were no fun- damental differences between mental and physical events, there would be no problem in saying how they relate to each other. Dualism. The historical roots of dualism are closely associated with the writings of the great French philosopher, mathematician, and scientist René Descartes. Indeed, the classical version of dualism, substance dualism, in which mind and body are conceived as two different substances, is often called Cartesian dualism. Because most philosophers find the notion of physical substances unproblematic, the central issue in philosophical debates over substance dualism is whether mental substances exist and, if so, what their nature might be. Vivid sensory experiences, such as the appearance of redness or the feeling of pain, are From chapter 13 in Palmer, Stephen E., Vision Science: Photons to Phenomenology, pp. 618–630, © 1999 Massachusetts Institute of Technology, by permission of The MIT Press. 1 M01_LEV1475_01_SE_C01.QXD 10/15/10 5:34 PM Page 2 2 SECTION I PHILOSOPHICAL FOUNDATIONS among the clearest examples, but substance dualists also include more abstract mental states and events such as hopes, desires, and beliefs. The hypothesized mental substances are proposed to differ from physical ones in their fundamental properties. For example, all ordinary physical matter has a well-defined posi- tion, occupies a particular volume, has a definite shape, and has a specific mass. Conscious experiences, such as perceptions, remembrances, beliefs, hopes, and desires, do not appear to have readily identifiable positions, volumes, shapes, and masses. In the case of vision, how- ever, one might object that visual experiences do have physical locations and extensions. There is an important sense in which my perception of a red ball on the table is located on the table where the ball is and is extended over the spherical volume occupied by the ball. What could be more obvious? But a substance dualist would counter that these are proper- ties of the physical object that I perceive rather than properties of my perceptual experience itself. The experience is in my mind rather than out there in the physical environment, and the location, extension, and mass of these mental entities are difficult to define — unless one makes the problematic move of simply identifying them with the location, extension, and mass of my brain. Substance dualists reject this possibility, believing instead that men- tal states, such as perceptions, beliefs, and desires, are simply undefined with respect to position, extension, and mass. In this case, it makes sense to distinguish mental substances from physical ones on the grounds that they have fundamentally different properties. We can also look at the issue of fundamental properties the other way around: Do experiences have any properties that ordinary physical matter does not? Two possibilities merit consideration. One is that experiences are subjective phenomena in the sense that they cannot be observed by anyone but the person having them. Ordinary matter and events, in contrast, are objective phenomena because they can be observed by anyone, at least in prin- ciple. The other is that experiences have what philosophers call intentionality: They inher- ently refer to things other than themselves.1 Your experience of a book in front of you right now is about the book in the external world even though it arises from activity in your brain. This directedness of visual experiences is the source of the confusion we mentioned in the previous paragraph about whether your perceptions have location, extension, and so forth. The physical objects to which such perceptual experiences refer have these physical prop- erties, but the experiences themselves do not. Intentionality does not seem to be a property that is shared by ordinary matter, and if this is true, it provides further evidence that con- scious experience is fundamentally different. It is possible to maintain a dualistic position and yet deny the existence of any sepa- rate mental substances, however. One can instead postulate that the brain has certain unique properties that constitute its mental phenomena. These properties are just the sorts of expe- riences we have as we go about our everyday lives, including perceptions, pains, desires, and thoughts. This philosophical position on the mind-body problems is called property dual- ism. It is a form of dualism because these properties are taken to be nonphysical in the sense of not being reducible to any standard physical properties. It is as though the physical brain contains some strange nonphysical features or dimensions that are qualitatively distinct from all physical features or dimensions. These mental features or dimensions are usually claimed to be emergent properties: attributes that simply do not arise in ordinary matter unless it reaches a certain level or type of complexity. This complexity is certainly achieved in the human brain and may also be achieved in the brains of certain other animals. The situation is perhaps best understood by M01_LEV1475_01_SE_C01.QXD 10/15/10 5:34 PM Page 3 CHAPTER ONE VISUAL AWARENESS 3 analogy to the emergent property of being alive. Ordinary matter manifests this property only when it is organized in such a way that it is able to replicate itself and carry on the required biological processes. The difference, of course, is that being alive is a property that we can now explain in terms of purely physical processes. Property dualists believe that this will never be the case for mental properties. Even if one accepts a dualistic position that the mental and physical are somehow qualitatively distinct, there are several different relations they might have to one another. These differences form the basis for several varieties of dualism. One critical issue is the direction of causation: Does it run from mind to brain, from brain to mind, or both? Descartes’s position was that both sorts of causation are in effect: events in the brain can affect mental events, and mental events can also affect events in the brain. This position is often called interactionism because it claims that the mental and physical worlds can interact causally with each other in both directions. It seems sensible enough at an intuitive level. No self-respecting dualist doubts the overwhelming evidence that physical events in the brain cause the mental events of conscious experience. The pain that you feel in your toe, for example, is actually caused by the firing of neurons in your brain. Convincing evidence of this is provided by so-called phantom limb pain, in which amputees feel pain — sometimes excruciating pain — in their missing limbs (Chronholm, 1951; Ramachandran, 1996). In the other direction, the evidence that mental events can cause physical ones is decid- edly more impressionistic but intuitively satisfying to most inter-actionists. They point to the fact that certain mental events, such as my having the intention of raising my arm, appear to cause corresponding physical events, such as the raising of my arm — provided I am not paralyzed and my arm is not restrained in any way. The nature of this causation is scientif- ically problematic, however, because all currently known forms of causation concern phys- ical events causing other physical events. Even so, other forms of causation that have not yet been identified may nevertheless exist. Not all dualists are interactionists, however. An important alternative version of dualism, called epiphenomenalism, recognizes mental entities as being different in kind from physical ones yet denies that mental states play any causal role in the unfolding of physical events. An epiphenomenalist would argue that mental states, such as perceptions, intentions, beliefs, hopes, and desires, are merely ineffectual side effects of the underlying causal neural events that take place in our brains. To get a clearer idea of what this might mean, consider the following anal- ogy: Imagine that neurons glow slightly as they fire in a brain and that this glowing is some- how akin to conscious experiences. The pattern of glowing in and around the brain (i.e., the conscious experience) is clearly caused by the firing of neurons in the brain. Nobody would ques- tion that. But the neural glow would be causally ineffectual in the sense that it would not cause neurons to fire any differently than they would if they did not glow. Therefore, causation runs in only one direction, from physical to mental, in an epiphenomenalist account of the mind-body problem.