Paintings and drawings differ 
in other important respects 
from the scenes they represent, 
even aside from the different 
ways in which two artists 
might render the same scene. 
Consider, for instance, the line 
drawing to the left. We have no 
difficulty in perceiving that 
this drawing depicts a hand. Yet 
this object is represented only 
by lines. Furthermore, much 
detail of the figure is left out. 
Here, then, is a major 
difference between the real 
world and many pictures. In the 
world, the internal and outer 
contours of objects generally 
consist of edges. By an edge I 
mean the contour that 
separates one object surface of 
generally uniform solid color, 
lightness, or texture from 
another or from a background 
surface or region. In some 
pictures, however, such edges 
are often represented by lines—–
simply very narrow ribbons of 
uniform pigment or 
reflectance. A picture often 
does contain the same kind of 
information that edges do in 
the world, but, as is evident in 
the illustration under 
discussion, they need not. In 
the drawing, lines suffice to 
represent edges, and edges of 
different kinds at that. For 
example, a line can represent 
the visible edge or boundary of 
a finger occluding part of the 
palm behind it, or it can 
represent creases in the 
fingers, or it can represent the 
edge formed by the intersection 
of a finger and its nail. 
 
    The use of lines in pictures 
raises a larger issue about 
perception, pictures, and art in 
general that has generated 
much controversy among art 
historians and psychologists. Is 
our perception of pictures 
simply based on convention, or 
is it, as I have been arguing, 
rooted in the similarity of 
pictures to the "picture" the eye 
receives from the real scene?