Defeasible Reasoning
McCarthy (1986) gives an amusing description of how we might solve the "cannibals and missionaries" problem.
``Three missionaries and three cannibals come to a river. A rowboat that seats two is available. If the cannibals ever outnumber the missionaries on either bank of the river, the missionaries will be eaten. How shall they cross the river?'' Obviously the puzzler is expected to devise a strategy of rowing the boat back and forth that gets them all across and avoids the disaster.
Imagine giving someone the problem, and after he puzzles for a while, he suggests going upstream half a mile and crossing on a bridge. ``What bridge'', you say. ``No bridge is mentioned in the statement of the problem.'' And this dunce replies, ``Well, they don't say there isn't a bridge''. You look at the English and even at the translation of the English into first order logic, and you must admit that ``they don't say'' there is no bridge. So you modify the problem to exclude bridges and pose it again, and the dunce proposes a helicopter, and after you exclude that, he proposes a winged horse or that the others hang onto the outside of the boat while two row. You now see that while a dunce, he is an inventive dunce. Despairing of getting him to accept the problem in the proper puzzler's spirit, you tell him the solution. To your further annoyance, he attacks your solution on the grounds that the boat might have a leak or lack oars. After you rectify that omission from the statement of the problem, he suggests that a sea monster may swim up the river and may swallow the boat. Again you are frustrated, and you look for a mode of reasoning that will settle his hash once and for all.
McCarthy proposes "circumsciption" to solve this problem.
Hasse Diagram:
