Another theory reverses the 
argument, in a manner of 
speaking. It assumes that the 
first step in processing is the 
creation of the illusory figure. 
The lightness effect occurs 
secondarily. The stimulus 
pattern, for example the one 
seen earlier in Chapter 5, leads 
initially to the perception of 
the colored fragments as figures 
on a white ground. That is what 
we should expect from bottom-
up processing based on 
principles of figure-ground 
organization. To perceive the 
white rectangular region, 
figure-ground reversal is 
necessary, and that does not 
always occur if the observers 
remain naive—–that is, if they 
do not know about the 
possibility. If reversal does 
occur, it is probable that it does 
so because it is cued by such 
factors as the incompleteness 
of the circular fragments, 
suggesting that something is 
occluding them, or by the 
perfect alignment of the edges 
of the rectangular gap across 
the dividing space. Whatever 
triggers the reversal, the fact is 
that it entails the perception of 
a white rectangle even though 
no contours are visible except 
at its corners. It is possible, 
therefore, that the perceptual 
system "invents" the extra 
whiteness so as to "explain" 
why one perceives contours 
where none exist. Following 
the suggested classification, 
this is a nonmechanistic 
theory, since the process 
entails perceptual 
reorganization based on the 
detection of cues. The outcome 
can be viewed as the preferred 
solution to the problem of what 
the stimulus pattern most 
likely represents. Once the 
illusory figure is perceived, it 
is difficult to perceive the 
pattern in any other way.