But the sky? Doesn’t representation of the aerial element, like that of clouds, those “bodies without surfaces,” as Leonardo describe them, fall outside the skills necessary for linear perspective, which can only function, as a rule of construction, on the condition that everything escaping its jurisdiction be excluded from its field? How is one to represent, feature by feature, a body that has no contours? How is one to trace its “portrait”?

Hubert Damisch, A Theory of /Cloud/: Toward a History of Painting.