Where light falls on an object and is reflected back to the eye, we see a highlight; where it strikes at an angle and is reflected back other than directly to the eye we see halftones; where no direct light falls on the object we have shadows: and these highlights, halftones and shadows are modified by light reflected into them by other objects and by other parts of the same object.

p.55 - Pictorial Composition in Photography (1920) - Chapter II