The two theories of 
constancy thus are perhaps not 
very different after all. Both 
involve quasi-cognitive 
operations. In the cases of 
spatial constancies, such as 
those of size and shape, the 
important retinal-image 
information is given absolutely 
(visual angle and shape of 
image), and this is assessed by 
the perceptual system in terms 
of other relevant information 
(distance and slant). While 
information concerning 
stimulus relations contributes 
to our perception of objects, it 
is not a sufficiently reliable 
indicator of size or shape 
because a given thing can be 
seen in any context and can 
change its context from 
moment to moment. Thus what 
matters most for arriving at a 
perception of an object’s size or 
shape is its distance or slant, 
and the visual nervous system 
has evolved adaptively to allow 
taking these into account.