There was a man of Chu who sold shields and spears. Praising his shields, he said, 'My shields are so strong that nothing can pierce them.' Praising his spears, he said, 'My spears are so sharp that there is nothing they cannot pierce.' Someone asked, 'What if I take your spear and try to pierce your shield?' The man could make no reply. For a shield that cannot be pierced and a spear that pierces everything cannot both exist in the same world.