Leonardo da Vinci Quote
A shadow may be infinitely dark, and also of infinite degrees of absence of darkness. The beginnings and ends of shadow lie between the light and darkness and may be infinitely diminished and infinitely increased. Shadow is the means by which bodies display their form. The forms of bodies could not be understood in detail but for shadow.
The Notebooks of Leonardo da Vinci (Richter, 1888) - III Six books on Light and Shade