appear the way they do. In this,
illusions are as important as
correct percepts and have equal
status as facts. Once the
existence of moon craters is
known to the astronomer, for
example, the fact that this
discovery was made by a
perception, by looking through
a telescope, is of no further
consequence. What is of
interest is how the craters were
formed. To the student of
perception however, the
appearance of the crater is of
direct interest. How do we
perceive its depth? Why does a
crater look like a mound when
a picture of it is inverted?

Flipping this photograph of the
moon upside down transforms
the "mounds" into craters. The
perceptual question is, why do
we perceive the picture quite
differently depending on the
location of the shadows?