OF SHADOWS ON BODIES.
When you represent the dark shadows in bodies in light and shade, always show the cause of the shadow, and the same with reflections; because the dark shadows are produced by dark objects and the reflections by objects only moderately lighted, that is with diminished light. And there is the same proportion between the highly lighted part of a body and the part lighted by a reflection as between the origin of the lights on the body and the origin of the reflections.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.