There is a problem with this 
explanation, however. For 
animals that move their eyes as 
we do, displacement of 
contours over the retina is 
neither necessary nor 
sufficient for the perception of 
motion. It is not necessary 
because we often track a 
moving object by moving our 
eyes, thus holding its image 
more or less stationary on the 
retina. Nevertheless, we see 
the object moving. Moreover, in 
many illusions of motion--the 
moon in the clouds and the 
still images in the frames of a 
movie, for instance--the image 
of the object seen as moving is 
stationary. Neither is the 
displacement of contours over 
the retina sufficient, because 
frequently an object’s image 
displaces over the retina 
without creating an impression 
of motion. When we move our 
eyes across a room, for 
example, the location of chairs 
and tables appears unchanged, 
although the images of them on 
the retina move. Investigators 
refer to this phenomenon as 
position constancy.