Holmes: "You see, but you do not observe. The distinction is clear. For example, you have frequently see the steps which lead up from the hall to this room."
Holmes: "How often?"
Watson: Well, some hundreds of times."
Holmes: "Then how many are there?"
Watson: "How many? I don't know."
Holmes: "Quite so! You have not observed. And yet you have seen. That is just my point. Now, I know that there are seventeen steps, because I have both seen and observed."
From: "A Scandal in Bohemia" in The Complete Sherlock Holmes (New York: Doubleday, 1927)