There is a more direct appeal to the imagination in line drawing than in possibly anything else in pictorial art.  The emotional stimulus given by fine design is due largely to line work.  The power a line possesses of instinctively directing the eye along its course is of the utmost value also, enabling the artist to concentrate the attention of the beholder where he wishes.  Then there is a harmonic sense in lines and their relationships, a music of line that is found at the basis of all good art.  But this subject will be treated later on when talking of line rhythm.

Most artists whose work makes a large appeal to the imagination are strong on the value of line.  Blake, whose visual knowledge was such a negligible quantity, but whose mental perceptions were so magnificent, was always insisting on its value.  And his designs are splendid examples of its powerful appeal to the imagination.

On this basis of line drawing the development of art proceeded.  The early Egyptian wall paintings were outlines tinted, and the earliest wall sculpture was an incised outline.  After these incised lines some man of genius thought of cutting away the surface of the wall between the outlines and modelling it in low relief.  The appearance of this may have suggested to the man painting his outline on the wall the idea of shading between his outlines.

At any rate the next development was the introduction of a little shading to relieve the flatness of the line-work and suggest modelling.  And this was as far as things had gone in the direction of the representation of form, until well on in the Italian Renaissance.  Botticelli used nothing else than an outline lightly shaded to indicate form.  Light and shade were not seriously perceived until Leonardo da Vinci.  And a wonderful discovery it was thought to be, and was, indeed, although it seems difficult to understand where men’s eyes had been for so long with the phenomena of light and shade before them all the time.  But this is only another proof of what cannot be too often insisted on, namely that the eye only sees what it is on the look-out for, and it may even be there are things just as wonderful yet to be discovered in vision.

But it was still the touch association of an object that was the dominant one; it was within the outline demanded by this sense that the light and shade were to be introduced as something as it were put on the object.  It was the “solids in space” idea that art was still appealing to.

“The first object of a painter is to make a simple flat surface appear like a relievo, and some of its parts detached from the ground; he who excels all others in that part of the art deserves the greatest praise,"[1] wrote Leonardo da Vinci, and the insistence on this “standing out” quality, with its appeal to the touch sense as something great in art, sounds very strange in these days.  But it must be remembered that the means of creating this illusion were new to all and greatly wondered at.