Is past experience a factor 
here? In some cases, it 
undoubtedly is. Where 
incomplete fragments of 
familiar shape are present, 
they serve to suggest the 
complete object occluded by an 
interposed object. But this 
factor is not a necessary one. In 
any event, once the interposed 
region is perceived as figure 
with its illusory contours, it is 
irreversibly preferred.

    More generally, many line 
drawings would probably not 
look the way they do were it not 
for experience with the kind of 
object they represent. We saw 
an example of this in Chapter 3. 
Prior experience with real 
cubes or boxlike structures is 
quite probably necessary in 
order to perceive a simple line 
drawing of a cube that does not 
show converging sides as three-
dimensional since there are no 
known depth cues displayed.