This general principle is 
closely related to the "generic 
viewpoint assumption": The 
visual system assumes that it 
does not have a privileged or 
special viewpoint of the scene. 
In other words, the possibility 
that there is coincidental 
arrangement between the 
viewing direction and the 
internal geometry of the scene 
is usually not considered. For 
example, if the scene in 
Chapter 3 of the interposed 
rectangle and "circle" were 
perceived veridically, then the 
smooth continuation of the 
contours of the rectangle with 
the notch of the circle would 
simply be the result of a 
coincidental or accidental 
alignment. This alignment 
could only occur from a single 
viewing direction.

    With the emergence of 
shaped, segregated units in 
perception, stored 
representations of prior visual 
experience enter into the 
story. Most typically, this step 
is merely a continuation of the 
sequence of bottom-up 
processing, consisting simply 
of an enrichment of 
perception, in the sense that 
the object, if familiar, can be 
recognized and identified. In 
some instances, however, the 
representation from memory 
can influence the very shape 
and organization achieved. We 
have seen several examples of 
such top-down processing in 
previous chapters, one of 
which was the reorganization 
of fragmented figures that 
occurs prior to, or at the 
moment of, their recognition. 
Even in these instances, 
however, the effect of past 
experience depends upon the 
bottom-up achievement of some 
degree of organized shape 
perception. Only then is access 
to the appropriate memories 
possible.