The most common number is 18. It probably represents the letter E. If he substitutes E into the message everywhere he sees an 18, then—Well, to be honest, then he’ll have to write out the whole message again, substituting Es for 18s, and it will take a long time, and it might be time wasted because he might have guessed wrong. On the other hand, if he just retrains his mind to construe 18s as Es—an operation that he thinks of as being loosely analogous to changing the presets on a pipe organ’s console—then what he sees in his mind’s eye when he looks at the blackboard is