The knowledge of lights and shades, which painting requires, is one of the most important and most essential branches of the art. We only see by means of light, and light draws and attracts the eye with more or less strength, as it strikes the objects of nature; for this reason the painter, who is the imitator of these objects ought to know and chuse the advantageous effects of light. . . .

This part of painting includes two things, the incidence of particular lights and shades, and the knowlege of general lights and shades, which we usually call the claro-obscuro. . . .

Claro implies not only anything exposed to a direct light, but also all such colours as are luminous in their natures; and obscuro, not only all the shadows directly caused by the incidence and privation of light, but likewise all the colours which are naturally brown, such as, even when they are exposed to light, mantain an obscurity, and are capable of grouping with the shades of other objects: Of this kind, for instance, are deep velvets, brown stuffs, a black horse, polished armour, and the like, which preserve their natural or apparent obscurity in any light whatever.

—Roger de Piles (1635–1709) from The Principles of Painting, as “translated by a Painter” and published in London in 1743.