A line should be self-sufficient.
As early as the 10th century the masters of Chinese India ink painting attempted to draw a line that could be self-sufficient. They believed that they could secretly infuse the line with 気 (ch'i, ki). (Ch'i: ether, breath, gas, and life force.) Through the master's Ch'i a drawn line on paper could even free itself from the surface and fly through the universe. But when does the master know that the lines have Ch'i? He begins to contemplate his drawn lines, but by what criteria should he question them? I believe that was when aesthetic judgment at the level of perception began in the history of drawing and ink painting.
In the European tradition of drawing, the line did not intrinsically express itself for a long time. The lines were drawn on the surface in a similar manner and looked apparently identical. The difference in the expression of a drawing originated mostly by the use of single lines: for example, by the density of the line, one controls the brightness or expresses materiality in various ways. Even though the single stroke has no intrinsic meaning, if every stroke is positioned accurately, it begins to play an important role within the picture and serves to create the character of the whole. For the artist, it was always very important when drawing a line to be attentive to the correlation among the various lines. The single line of a drawing is often interchangeable and adaptable, thus requiring no aesthetic judgment. Koho Mori-Newton