search diligently for some time. The woman finally asks the man where he
thinks he lost them, to which he replies, ‘‘Down the street in the middle of the
block.’’ When she then asks why he is looking here at the corner, he replies,
‘‘Because this is where the light is.’’ The problem is that consciousness does not
seem to be where behavioral science can shed much light on it.
Physiological Criteria Modern science has another card to play, however, and
that is the biological substrate of consciousness. Even if behavioral methods
cannot penetrate the subjectivity barrier of consciousness, perhaps physiologi-
cal methods can. In truth, few important facts are yet known about the bio-
logical substrates of consciousness. There are not even very many hypotheses,
although several speculations have recently been proposed (e.g., Baars, 1988;
Crick, 1994; Crick & Koch, 1990, 1995, 1998; Edelman, 1989). Even so, it is pos-
sible to speculate about the promise such an enterprise might hold as a way of
defining and theorizing about consciousness. It is important to remember that
in doing so, we are whistling in the dark, however.
Let us suppose, just for the sake of argument, that neuroscientists discover
some crucial feature of the neural activity that underlies consciousness. Perhaps
all neural activity that gives rise to consciousness occurs in some particular
layer of cerebral cortex, or in neural circuits that are mediated by some partic-
ular neurotransmitter, or in neurons that fire at a temporal spiking frequency of
about 40 times per second. If something like one of these assertions were true—
and, remember, we are just making up stories here—could we then define
consciousness objectively in terms of that form of neural activity? If we could,
would this definition then replace the subjective definition in terms of ex-
perience? And would such a biological definition then constitute a theory of
consciousness?
The first important observation about such an enterprise is that biology can-
not really give us an objective definition of consciousness independent of its
subjective definition. The reason is that we need the subjective definition to
determine what physiological events correspond to consciousness in the first
place. Suppose we knew all of the relevant biological events that occur in hu-
man brains. We still could not provide a biological account of consciousness
becausewewouldhavenowaytotellwhichbraineventswereconsciousand
which ones were not. Without that crucial information, a biological definition
of consciousness simply could not get off the ground. To determine the bio-
logicalcorrelatesofconsciousness,onemustbeabletodesignatetheevents
to which they are being correlated (i.e., conscious ones), and this requires a
subjective definition.
For this reason, any biological definition of consciousness would always be
derived from the subjective definition. To see this in a slightly different way,
consider what would constitute evidence that a given biological definition was
incorrect. If brain activity of type C were thought to define consciousness, it
could be rejected for either of two reasons: if type C brain activity were found
to result in nonconscious processing of some sort or if consciousness were
foundtooccurintheabsenceoftypeCbrainactivity.Thecrucialobservation
for present purposes is that neither of these possibilities could be evaluated
without an independent subjective definition of consciousness.
18 Stephen E. Palmer