In perception, before we can
hope to understand an event in
the brain that underlies a
percept, we must possess an
idea of the process that leads to
the percept's formation.
Suppose we want to know why a
picture of a crater looks like a
mound when it is inverted. In
the absence of any
understanding of the process of
perception, we would have no
idea what to look for in the
brain. Suppose, however, that
through experimentation we
discover that, when people are
shown pictures of an enclosed
region with a shadow at the top,
they perceive it as a hole or
indentation, whereas, when
they are shown the same
picture with the shadow at the
bottom, they perceive a mound
or elevation. Now at least we
know a general principle about
the perception. We can try to
penetrate the problem further
by asking about the origin of
the principle. Since light in
our environment almost always
comes from above, a hole will
tend to be shadowed at its top.
Thus the principle might be one
that is learned. If we discover