Jason Holt set himself the goal to write a book on
consciousness and the phenomenon of blindsight. He intends to illuminate
philosophical questions concerning the nature of consciousness, the theory of
knowledge and perception, and the philosophy of mind.

The
introduction offers a short but comprehensive "crash course in the mind
body problem" (p.9ff) that can be highly recommended to the beginning student
of philosophy of mind and familiarizes the reader with the different positions.

Chapter 1 and 2
explain the phenomenon of blindsight and other disassociation cases. Holt
defines blindsight as "residual vision in a blind field without
concomitant awareness" (p.26). That means patients with localized damage
to the primary visual cortex (V1) experience cortical blindness in part of
their visual field. But even though they are not consciously aware of any
stimuli in this blind field they have access to some visual information about
these stimuli. When forced to guess about the shape, color, orientation of
objects in the blind field they perform well above chance. Holt distinguishes
blindsight from degraded forms of normal vision such as subliminal perception
and peripheral vision. The main difference is that blindsight, or rather the
loss of conscious visual experience, is caused by damage to V1. Degraded normal
vision occurs in healthy individuals. Even though conscious experience is
absent in blindsight the visual information that is accessible to the patient
is highly reliable. In this regard blindsight is similar to disassociation
cases of other sense perception such as "blind (numb) touch", "deaf
hearing", and of visual perception such as inability to see color, to see
motion, to see the orientation or shapes of objects. In all these cases the
patient has some localized brain damage and subsequently lacks conscious
awareness but has access to some information about stimuli. In Holt’s words: "The
system [brain] gets it but the subject does not" (p.36).

Holt urges the reader to resist the
temptation to accept radical explanations (such as the notion of unconscious
consciousness) to account for the phenomena or to trivialize them (maybe
consciousness does not play any role in sense perception). Instead he suggests
that a distinction between informational and experiential sensitivity can
account for the rather puzzling disassociation cases.

Chapter 3 presents a rebuttal of
eliminative materialism, which is proposed by Pat and Paul Churchland. Holt
explains that on the Churchlands’ view blindsight is just one more piece of
supporting evidence for the claim that consciousness has no role to play in the
explanation of human neurophysiology. They assert that folk psychology has lost
with color vision one of the main pillars of support for the need to invoke
consciousness in the explanation of natural phenomena. If it turns out that one
can have reliable access to information about colors without being consciously
aware of color, then what role is left for consciousness to play? In defense of
the causal efficacy of consciousness Holt points out that it is one thing to
inquire into the causal role of consciousness but quite another to deny that
the phenomenon exists. He is confident that both consciousness and folk
psychology can survive the eliminativist attack.

In chapter 4 we enter the world of
thought experiments Holt explains: "A thought experiment is an imagined
scenario invented to test theories, theoretical commitment, and intuitions for
conceptual integrity" (p. 57). He introduces us to Daniel Dennett, one of
the most prolific thought experimenters,. Dennett uses the real cases of blind
sight as a springboard into a world of super blindsighters (imaginary subjects
that learn to guess what is in their blind field without being prompted to
guess). This thought experiment is designed to support Dennett’s intuition that
qualia (the "what it is like" aspect of consciousness) do not make
any difference that makes a difference in explaining sense perception. A normal
perceiver (with qualia) and a super blindsighter (without qualia) are
functionally equivalent. Therefore, according to Dennett, qualia are
superfluous. Holt demonstrates that Dennett frequently equivocates and glosses
over important distinctions to keep his "intuition pumps" pumping.
Still it seems that he is not able to undermine Dennett’s contention that
qualia do not exist in normal subjects: Dennett’s thought experiment was not
designed to "destroy" qualia (rather he claimed they never existed)
but only to undermine our belief in qualia. This shows how difficult it is to
battle thought experiments with empirical data: we are operating in virtually
different worlds.

In chapter 5 Holt turns the tables
and uses blindsight to argue for conscious realism. He suggests that blindsight
helps to reveal the neural correlates for consciousness: because V1 damage
causes loss of conscious visual experience without loss of all visual functions
it is reasonable to assume that V1 plays a role in conscious experience. But
for Holt the "mechanics" of consciousness are less important than the
arguments that can be given before we have a full grasp of the neurological
basis of consciousness. He defends the introspective accounts of patients who
report that there is a difference in experience between blind-field "vision"
and normal vision. Further, Holt points out that blindsight poses a serious
problem for the behaviorist phenomenology because it is not clear that
blindsight patients lack a disposition to discriminate stimuli in their blind
field. Rather, the assumption that they lack qualia can account for their behavior
(for their correct guesses as well as for their denial to see anything). On
page 79 Holt provides a concise summary of the arguments for conscious realism.

Chapter 6 is an
attempt to use blindsight as supportive evidence in the case against
epiphenomenalism (the belief that even if qualia exist they do not play any
causal role). Holt introduces several well-known thought experiments (for
example phenomenal zombies, Mary the black and white color expert) and
demonstrates that clever arguments about logical possibility and/or
conceivability of worlds in which qualia play no causal role are no proof for
the claim that qualia play no causal role in the actual world. If we assume the
causal closure of the physical world, then it becomes impossible to maintain
that qualia are mere correlations to brain states.

Chapter 7 supports Holts own
metaphysical view: identity theory, a form of reductive materialism. Here Holt
demonstrates that neither the dualist suspicion nor a non-reductive token
identity form of materialism can survive detailed analysis. Following an
argument by Kripke dualists rely on the apparent disanalogy between "acceptable"
physical identities (between temperature and mean kinetic energy) and "suspicious"
identities (between brain-states and pain). Holt shows that the disanalogy is
caused by our intuitions rather than by a metaphysical difference: we are
willing to trust the evidence accumulated by science that temperature is
mean kinetic energy but we are hesitant to accept the still incomplete
psychological story about the identity of brain states and pain. Similarly,
non-reductive materialism relies on our intuitions and on a rather coarse
grained analysis of psychological phenomena. Holt offers convincing examples to
undermine these intuitions and to keep the door open for type-identity between
brain states and mental states.

Chapter 8 contains Holt’s response
to the "hard problem" of consciousness and suggestions for how to
close the "explanatory gap". These arguments have been introduced by
David Chalmers who holds that the "hard problem" is the problem of
explaining why and how the neural correlates of consciousness give rise to
consciousness. Holt rejects Chalmers’ suggested solution, which is a "radical
metaphysical rejigging" (p.106) that requires us to accept a form of
panpsychism. According to this view information is the only fundamental substance
(mind and matter are just different aspects of this substance). Holt points out
that it is not evident how this rather counterintuitive view can solve the hard
problem. He admits that there is a conceptual gap between consciousness and its
neural correlates but he expresses confidence that the gap can be closed as
neurosciences progresses. His explication of the Nagel/Akins debate about the
importance of the "what is it like" aspect of consciousness
illuminates that focus on either first or third person perspective is the
difference that makes a difference.

Chapters 9 and 10 are dedicated to
the philosophical applications of blindsight to the theory of knowledge and the
conceptual foundations of vision science respectively. Holt attempts to
persuade the reader that blindsight patients do have perceptual beliefs, not
just subdoxastic states. Holt’s subtle points about justification, warrant and
reliability not withstanding, I remain unconvinced that blindsight patients do
in fact have perceptual beliefs and knowledge about the stimuli in their blind
field. A simple method to decide whether we are justified in attributing "blindsight-beliefs"
to patients would be to test whether they are able to defend their
guesses when challenged. We should remember here that the patient depends on
either the experimenter or his intact visual field to confirm that his guess
was in fact correct. How would he react if the experimenter lies to him about
the stimulus in the blind field? If he were to trust the experimenter’s claim
more than his own guess then he would not have knowledge.

The concluding chapter illuminates
some issues about theories of perception that are mainly of interest to
philosophers. Holt suggests that blindsight can support inferential theories of
perception. Furthermore, it might be possible to use the dorsal/ventral stream
model for processing of visual information suggested by Millner and Goodale to
support the view that at least some visual perception is direct.

Overall Holt’s book is an excellent
contribution to contemporary debates in the philosophy of mind. He offers solid
empirical evidence in support of his arguments and — as an added bonus — his
writing is a pleasure to read.

© 2005 Christina Behme

Christina
Behme is in the process of obtaining a MA in Philosophy at Dalhousie University.

Categories: Psychology, Philosophical