The general replied, "I will explain it to you with an example. A filthy man is standing outside a bath house. Will he go in?"
"Of course," replied the adjutant.
"No, you're wrong," said the general. "A filthy man is filthy by his nature, and will not go in to the bath house. Only clean men, knowing the virtues of cleanliness, will bathe."
"I understand, comrade general."
"Now, let me give you another example. A filthy man is standing outside a bath house. Will he go in?"
"Absolutely not," replied the adjutant immediately.
"You're wrong again," said the general. "Why should a filthy man not enter a bath house? He is dirty, the bath house is there to enable him to become clean, and he will use it."
"I think I understand, comrade."
"Now, one last example. A filthy man is standing outside a bath house. Will he go in?"
"How the hell should I know?"
"Now, comrade, you truly understand the meaning of Marxist dialectic."
(A very similar joke from Leo Rosten's The Joys Of Yiddish (If you haven't got a copy of this book, then drop everything and go out and buy a copy--it's full of jokes, folk sayings and other assorted witticisms.))