Despite great variation in  
the images on the eye that  
represent a given object in the  
world, the object looks much  
the same to us. It appears to be  
about the same size despite the  
changes in the size of its  
images (size constancy), to have  
much the same shape despite  
changes in the shape of its  
images (shape constancy), and  
to have much the same  
orientation (tilted, upright,  
upside down) despite changes  
in how its images are oriented  
on the retina (orientation  
constancy). It also appears to be  
located in much the same  
direction in relation to  
ourselves and other objects  
despite where on the retina its  
image is located (constancy of  
direction or position  
constancy), and much the same  
lightness, or shade of gray,  
despite changes in the  
intensity of light reaching the  
eye from its surfaces (lightness  
constancy). 
 
    How does the brain  
construct from these varying  
images a visual world that is  
characterized by constancy?  
Currently, there are two,  
apparently conflicting,  
answers to this question  
entertained by students of  
perception, the stimulus- 
relation theory and the taking- 
into-account theory. The  
former derives from the  
psychophysical tradition and  
the latter primarily from the  
work of Helmholtz and other  
Inference theorists. Let us first  
consider the explanations for  
constancy of size.