Smoking Out the Tenebrists: What Are Shadow and Light?

Shadow is the absence of light and only arises from the oppositions of dense bodies, when opposed to luminous rays. Shadow is of the nature of darkness and illumination is of the nature of light. The one conceals and the other reveals. They are always joined together in company on bodies, and shadow is of greater power than light, because it banishes and completely deprives bodies of light, and light can never wholly chase away the shadows of bodies, that is to say of dense bodies.

Shadow derives from two things that are dissimilar to each other, because one is corporeal and the other immaterial. The corporeal one is the opaque body and the immaterial is the light. Thus light and body are the causes of shadow.

Shadow is a mixture of darkness with light and it will be of greater or lesser depth according to whether the light that is mixed with it is of greater or lesser strength.

--Leonardo da Vinci (Leonardo on Painting, Ed. Martin Kemp)