These color transformation arguments are telling criticisms against the com-
pleteness of any definition of conscious experience based purely on behavior.
Their force lies in the fact that there could be identical behavior in response to
identical environmental stimulation without there being corresponding identi-
cal experiences underlying them, even if we grant that the other person has
experiences to begin with.
Phenomenological Criteria Let us return to the issue of criteria for conscious-
ness: How are we to tell whether a given creature is conscious or not? Clearly,
phenomenological experience is key. In fact, it is the defining characteristic, the
necessary and sufficient condition, for attributing consciousness to something.
I know that I am conscious precisely because I have such experiences. This
is often calledfirst-person knowledgeorsubjective knowledgebecause it is avail-
able only to the self (i.e., the first-person or subject). In his classic essay
‘‘What Is It Like to Be a Bat?’’ philosopher Thomas Nagel (1974) identifies the
Figure 1.
A biological basis for red-green-reversed trichromats. Normal trichromats have three different pig-
ments in the retinal cones (A), whereas red-green color blind individuals have the same pigment in
their L and M cones (B and C). People with the genes for both forms of red-green color blindness,
however, would be red-green-reversed trichromats (D).
Visual Awareness 15