I thought of a dream related by Leibniz, in which he found himself at a great height overlooking the world–with provinces, towns, lakes, fields, villages, hamlets, all spread beneath him. If he wished to see a single person–a peasant tilling, an old woman washing clothes–he had only to direct and concentrate his gaze: “I needed no telescope except my attention.”

Oliver Sacks, “Bull on a Mountain,” from A Leg to Stand On