Ingres was one of those artists to whom the outline was something sacred and magical, and the reason is that it was the means of reconciling the major conflict in his art, the conflict between abstraction and sensibility. The difference between what we see and a sheet of white paper with a few thin lines on it is very great. Yet this abstraction is one which we seem to have adopted almost instinctively at an early stage in our development not only in Neolithic graffiti but in early Egyptian drawings. And in spite of its abstract character, the outline is responsive to the least tremor of sensibility.

Ch. 4: Ingres I: The Years of Inspiration - The Romantic Rebellion (1973)