Someone has told the story of meeting, on a train, a weird gentleman who took a bag down from the rack, opened it, extracted from his pocket a cabbage on the end of a string, and began to dangle it into the bag.  Consumed with curiosity, the traveler asked what was in the bag and was told that it was a mongoose.  'Well, why do you carry a mongoose?' 'Alas,' said the gentleman, 'I am an alcoholic and suffer from delirium tremens, so that I need this mongoose to keep away the snakes.'  'But surely you realize that those snakes are only imaginary?'  'Yes, indeed,' he answered, 'but so is the mongoose.

• illusions