from 'The Republic,' by Plato

[Socrates and Glaucon are detained by a group of people who want them to stay in the city.]

"...you must either get the better of all these people, or else stay here."
"There is another possibility," I said. "We might persuade you that you should let us go."
"And do you really think you could persuade us," he said, "if we refused to listen?"
"Of course not," said Glaucon.
"In that case, make your decision on the assumption that we are not going to listen."