Long before psychology 
became a science, philosophers 
debated this issue. As discussed 
in Chapter 1, Berkeley took the 
position that a two-
dimensional retina was 
logically incapable of directly 
yielding depth perception 
because it could receive only a 
two-dimensional image. We 
must learn to gauge the 
distance of things by a process 
of association. Accommodation 
and convergence are cues that 
ultimately become reliable 
signs of distance because we 
reach for or move toward things 
and this teaches us how to 
interpret these signs. Others 
emphasized inborn 
characteristics of mind such as 
the predisposition to locate 
things in three-dimensional 
space. If we did not by nature 
tend to organize the perceptual 
world spatially, so the 
argument goes, how could we 
ever learn about depth 
localization?

    In trying to resolve this issue 
by experimentation, 
psychologists have historically 
encountered the same problems 
they have in attempting to 
discover the origins of 
constancy. Except in response 
to the new "habituation" 
method (to be discussed in 
Chapter 5), newborn animals, 
human or otherwise, can only 
"tell" us what they perceive 
through performance on 
behavioral tests of learning, of 
which very immature 
organisms are usually not 
capable, while rearing animals 
without vision until they are 
mature enough to test leads to 
severe deterioration of the 
visual nervous system and 
prevention of normal 
maturation.