(A joke told by Aleister Crowley, which, if it is understood, will explain all magic.)
There were these two men sharing a railway carriage. They didn't know each other. They just happened to be traveling together. One of the men had, resting in his lap, a cardboard box, with holes punched in the top.
After some time spent contemplating what might be inside his traveling companion's box, the other man at last could not contain his curiosity. He said, "Excuse me, but I couldn't help noticing your box. Does it by chance contain some animal varietal?"
The other man, though obviously surprised by this impertinent intrusion from a stranger, smiled politely as he answered.
He said, "You're absolutely right. There is indeed a creature kept inside this box. And furthermore, I may reveal, the animal in question is a mongoose."
The first man, who had initiated the inquiry, was astonished by this revelation. Sputtering with surprise, he sought some further explanation of this certain provocative disclosure made by his strange fellow-traveler. "A mongoose? Sir, I must confess I had expected it to be perhaps a cat, or rabbit, not a creature so exotic and outlandish. The animal you mention so excites my curiosity that I must beg you, sir, to tell me more. Where are you bound with such a specimen, if I may be so bold?"
The other man, who sat with the perforate box on his lap, shrugged wearily as he replied. "Well," he said, "it's something of a personal matter, as it concerns a family tragedy. However, since I am confident I may rely on your discretion, I suppose I don't mind sharing my unfortunate account with you."
"You see," the man went on, "this sorry tale concerns my elder brother. He's always been what I suppose you might refer to as the black sheep of the family. He has for many years indulged himself in a predictable and commonplace array of vices, of which the worst is his fondness for strong spirits. His drinking has progressed until he is now int he final melancholy stages of delirium tremens. Me brother now sees serpents everywhere, which is the reason I am taking him this mongoose, that he may be ride of them."
"Excuse me," the other man interjected, looking puzzled, "but these snakes your brother sees...aren't they imaginary?"
"Indeed," his fellow traveler replied. "But this," he said, as he gestured meaningfully to the perforated box set on his lap, "is an imaginary mongoose."
(Taken from Alan Moore's Promethea)