Lightness and Brightness and Other Confusions

creasingly difficult to think of the mind as something distinct from the physical
world as more and more of the processes and features of our mental lives
are not only mapped onto brain processes, but are shown to depend upon
them. Just as striking is the realization that, contrary to Descartes, the greater
part of our cognition and volition takes place at an unconscious level. That
said, many reflective people are uncomfortable with the claim that conscious
events are identical with brain events. Is my pleasure at the deployment of
green in a Degas painting nothing more than a pattern of neural activity?
Complexity aside, does the difference between us and robots just come down
to the difference between hardware and wetware?
There is a second bifurcation that is exacerbated by the first: the split between
my consciousness and yours. In Aristotle’s view, qualities were part and parcel
of the physical world. Perception involved a process whereby a property of
the world, which he called a “form,” impressed itself on the mind of the percipient
(hence our word “information”). Red is out there in the world, and it
imprints itself on your mind just as it does on mine, so the question of
whether you and I have the same colour experience doesn’t arise. But after
Galileo and Descartes, the fact that a spectral pattern strikes your retina in
much the same way as it does mine fails to guarantee that the colour that is
generated in your mind is like the one that is generated in mine. It should
come as no surprise that the question as to whether what I experience as red
you experience as green is first raised in the seventeenth century in the writing
of Locke. The “inverted spectrum” problem has plagued us ever since.
The progress in neuroscience that seems to have narrowed the gap between
experiences and brain events can give us some measure of comfort concerning
our perceptual similarities and differences. What we currently understand
about colour processing in the eye-brain points to very substantial similarities
between what goes on in your visual pathways and mine. However, most of
the neural underpinnings of our colour vision remain mysterious. For instance,
although we can find a neural analogue of the hue circle, we have not
yet been able to find the brain mechanisms that underlie the difference between
unitary and binary hues.
We do know that there are differences between the colour vision of one person
and that of another. Everyone knows about colour-deficient people. Much
less appreciated are the significant differences that exist between “colour
normals,” i.e., people who pass the standard tests for colour deficiencies. A
“normal” or “average” observer is assumed for all colour-order systems, but of
course this is a statistical concept. It is interesting to see how wide the distribution
is. Sixty-two years ago, Ralph Evans remarked,
A rough estimate indicates that a perfect match by a perfect “average”
observer would probably be unsatisfactory for something like 90 percent
of all observers because variation between observers is very much
11