François Maspero said this quote

The faces you clutch at desperately slip away; it's when you're not thinking about them that their features flash past. It can happen on a street corner, at the turn of a staircase, because somebody said a word, because some image, an image has passed. Then the face is there for a split second, very fragile. One mustn't grasp at it, or it whisks away. One might as well try and catch a cloud. It was a cloud.