Woland to Matthew Levi:

 “But would you be so kind as to ponder this question: what would your good do if evil didn't exist, and what would the earth look like if all the shadows disappeared? After all, shadows are cast by things and people. Here is the shadow of my sword. But shadows also come from trees and living beings. Do you want to strip the entire globe of all trees and living things just because of your fantasy of enjoying naked light? You are stupid."

(M. Bulgakov, The Master and Margarita, ch. 29)