Once a clever man was crossing a turbulent river. He wished to amuse himself by talking to the boatman. "Do you know mathematics?" he asked. "No sir." answered the boatman. "Then you have lost one quarter of your life," said the wise man. "Do you know history?" "No sir." answered the boatman. "Then you have lost three quarters of your life." Just then a sudden gust of wind overturned the boat. "Do you know how to swim?" asked the boatman. "No," answered the wise(="clever)" man. "Well then," replied the boatman, "you have lost your whole life." The clever man knew a lot of knowledge except   _  .
mathematics philosophy history swimming