A Treatise on Painting/Chapter 176
Chap. CLXXVI.—Which of the two is the most useful Knowledge, the Outlines of Figures, or that of Light and Shadow.
The knowledge of the outline is of most consequence, and yet may be acquired to great certainty by dint of study; as the outlines of the different parts of the human figure, particularly those which do not bend, are invariably the same. But the knowledge of the situation, quality, and quantity of shadows, being infinite, requires the most extensive study.