If something cannot go on forever, it will stop

Unfamiliar Quotations by Herbert Stein. Herbert Stein was an economist and originator of Herbert Stein's Law, "If something cannot go on forever, it will stop," often restated as "Trends that can't continue, won't." Here, he has a great admonition, which he conveys through a quotation, not to waste time arguing with those who have chosen to forgo discourse for conviction.

If you meet a madman who says that he is a fish and that we are all fishes, do you take off your clothes to show him that you do not have fins?
--Milan Kundera, Risibles Amours, 1984

I love this in part because I am proud that I translated it from the French, which, in turn, was translated from the Czech. But I love it even more because it has saved me so much trouble. In the past when I encountered some outlandish inanity--often about taxes--I would sit down at my keyboard and write an answer. I am still tempted to do that, but since I encountered that quotation, I have resisted.

You may ask: How will I know if he is a madman? The answer is: Don't worry, you'll know. And if you are in doubt, assume he is mad and leave the refutation to others. You have plenty to do in the world without having to worry about debating people who may be mad.