It happened that in the Kingdom of Riddle, the grain cellars were always empty despite record harvests year after year. The King knew that either the farmers were not delivering the grain as they were ought, or a thief was stealing it. He summoned his noblemen, who asked, "What is the problem you wish us to solve?" "Our grain cellar is empty, despite bountiful crops," said the King. "When I ask the sentries who guard the cellars to explain, I get no straight answers. You must fix this problem so that the grain delivered is accurately reported and then protected from thieves." The noblemen had faced similar challenges before, and hurried off to find out from the sentries if the farmers were lying or if thieves were stealing the grain. Despite their best efforts, the noblemen could not get straight answers to their questions. Whatever one sentry said, the other disputed. Under pressure to save face with the King, the noblemen devised a system that took advantage of the two sentries' dislike for each other. If the sentries would go so far as to contradict everything that the other said, surely they would never conspire together to steal grain. The noblemen set up one sentry to count the bags of grain a farmer delivered. The other sentry would make sure the farmer loaded the grain down the cellar shoot and not through the garbage shoot, where thieves could come at night and steal it. To make sure neither sentry was able to hide what he was doing, the noblemen had them alternate between the two tasks. The new system failed miserably. The counting house records of how much grain was delivered never matched with the amount of grain in the cellar. The King summoned his wisemen who asked, "What is the problem you wish us to solve?" "Our grain cellar is empty, despite bountiful crops," said the King. "When I ask the sentries who guard the cellars to explain, I get no straight answers. The noblemen devised a system, but still we have problems. You must fix this system so that the grain delivered is accurately reported and then protected from thieves. The wisemen listened to what the King said and asked, "What do you mean you get no straight answers from the sentries?" "The sentries are cursed," replied the King. "When asked a question, one will tell only lies and the other the truth, but I know not which is which. When I ask them question, I am driven in circles. When the noblemen ask them questions, they are driven in circles." The wisemen considered what the King said and made their way to the sentries who guard the grain. They found that when a farmer delivers grain, one of two things happens. Either one sentry lies about how much grain is delivered and the other directs the farmer to the correct shoot, or the correct amount of grain is recorded and the farmer is sent to the wrong shoot. The wisemen realized any system that relied on one sentry counting sack of grain and the other telling farmers where to deliver it would not work To fix the new system, the wisemen recommended that farmers report the amount of grain delivered directly to the King's counting house. When they deliver grain to the cellar, they are to ask either sentry the following question, "If I were to ask the other sentry which shoot I should use, what would he say?" The farmer should then load the grain down the other shoot. This is because one of two things happens. They lying sentry says the truthful sentry would direct the farmer to the wrong shoot, or the truthful sentry says the lying sentry would direct the farmer to the wrong shoot. The King was quite pleased. Since the kingdom's farmers were generally honest folk, the counting house records were reasonably accurate and the King always knew how much grain was in the cellar. Less grain was lost to thieves because both sentries were making sure it was delivered to the correct shoot, despite that one tells only lies and other only the truth.
Moral: The moral of the story is two-fold: (1) you need to ask the right questions of the right people in order to understand what is going on, and (2) a system you design will not work if you don't consider the abilities and limitations of the people who use it. The noblemen neglected to ask the King enough about the problem and missed valuable information about the sentries. The wisemen were able to take into account the limitations of the sentries and design a more appropriate solution that worked.
These stories are adapted examples written in my class, IS 6840 (formerly MSIS 488).
© Vicki L. Sauter. All rights Reserved.