Furthermore, the 
phenomenon of figure-ground 
organization (as we shall see in 
the next chapter) suggests that 
there is a universal and 
probably unlearned tendency to 
perceive the region within 
certain contours as a solid two-
dimensional object with a 
particular shape on a 
background region. For 
example, in the illustration to 
the left, the circle appears to us 
not as a mere outline--which, 
physically speaking, is all it is
--but as a solid or "filled" disk. 
In fact, to produce an 
impression of a perimeter or 
outline object only, an artist 
would have to introduce 
additional lines or 
information--for example, by 
drawing a double outline or 
objects inside and outside the 
outline that are continuous 
with one another, as shown at 
left, below. If a region enclosed 
by a line is spontaneously 
perceived as a surface or a 
thing because that is how 
perceptual organization works, 
there is no need to invoke 
convention as an explanation 
of why we tend to perceive (and 
make) line drawings as 
representations of a world of 
objects.