Louis Kahn: 

I once gave myself an assignment: to draw a picture that demonstrates light. Now if you give yourself such an assignment the first thing you do is escape somewhere, because it is impossible to do.  You say that a white piece of paper is the illustration; what else to do? But when I put a stroke of ink on the paper, I realized that the black and white was where the light was not, and then I could really make a drawing, because I could be discerning where I put the black.