Economist Herbert Stein (1916-1999) wrote in 1986: “But if it can’t go on forever it will stop. And if we never do anything that we can’t go on doing forever we will never do very much.”
 
Stein’s Law (“If something cannot go on forever, it will stop”) originally referred to a nation’s balance of payments deficits, but has been applied to other deifict situations as well.
   
 
Wikipedia: Herb Stein
Herbert Stein (August 27, 1916 – September 8, 1999) was a senior fellow at the American Enterprise Institute and was on the board of contributors of The Wall Street Journal. He was chairman of the Council of Economic Advisers under President Nixon and President Ford. In the 1970s, he was a professor of economics at the University of Virginia.
 
Stein was born in Detroit, Michigan, but his family moved to New York during the Great Depression. He enrolled in Williams College just before he turned sixteen. After graduating Phi Beta Kappa, he went to Washington, D.C. to work as an economist in various agencies. He received his Ph.D. in economics from the University of Chicago in 1958. He was well-known as a pragmatic conservative and was jokingly referred to as “a liberal’s conservative and a conservative’s liberal.” He was the author of The Fiscal Revolution in America.
 
In his article, “Adam Smith did not wear an Adam Smith necktie,” Stein wrote that the people who wear the Adam Smith tie do it “to make a statement of their devotion to the idea of free markets and limited government. What stands out in Wealth of Nations, however, is that their patron saint was not pure or doctrinaire about this idea. He viewed government intervention in the market with great skepticism. He regarded his exposition of the virtues of the free market as his main contribution to policy, and the purpose for which his economic analysis was developed. “Yet he was prepared to accept or propose qualifications to that policy in the specific cases where he judged that their net effect would be beneficial and would not undermine the basically free character of the system,” wrote Stein. “He did not wear the Adam Smith necktie.” In Stein’s reading, The Wealth of Nations could justify the Food and Drug Administration, the Consumer Product Safety Commission, mandatory employer health benefits, environmentalism, and “discriminatory taxation to deter improper or luxurious behavior.”
 
Stein was the formulator of “Herbert Stein’s Law,” which he expressed as “If something cannot go on forever, it will stop,” by which he meant that if a trend (balance of payments deficits in his example) cannot go on forever, there is no need for action or a program to make it stop, much less to make it stop immediately; it will stop of its own accord. It is often rephrased as: “Trends that can’t continue, won’t.”
 
He was married to Mildred Stein, who died in 1997 after 61 years of marriage, and he is the father of lawyer, author, and actor Ben Stein (Ferris Bueller’s Day Off, Win Ben Stein’s Money) and writer Rachel Stein.
   
Google Books
Washington Bedtime Stories:
The Politics of Money and Jobs
By Herbert Stein
New York, NY: Free Press
1986
Pg. 179:
But if it can’t go on forever it will stop. And if we never do anything that we can’t go on doing forever we will never do very much.
   
26 January 1986, Boston (MA) Globe, “Growing Debt Shadows U.S. Economy” by Thomas Oliphant:
If something cannot go on forever, it will stop,” said Herbert Stein, a leading conservative economist who helped make policy in the early 1970s.
   
Google Books
A symposium on the 40th anniversary of the Joint Economic Committee : hearings before the Joint Economic Committee, Congress of the United States, Ninety-ninth Congress, first session, January 16 and 17, 1986.
By United States. Congress. Joint Economic Committee
Washington, DC: U.S. Government Printing Office
1986
Pg. 262:
Well, I recently came to a remarkable conclusion which I commend to you and that is that if something cannot go on forever it will stop.
(Herbert Stein—ed.)
   
Google Books
Readings in Introductory Macroeconomics
By Peter D. McClelland
New York, NY: McGraw Hill
1988
Pg. 101:
As Herbert Stein, chairman of the Council of Economic Advisers under President Nixon, is fond of saying, “If something cannot go on forever, it will stop.
       
Slate Magazine
Herb Stein’s Unfamiliar Quotations
On money, madness, and making mistakes.
By Herbert Stein
Posted Friday, May 16, 1997, at 3:30 AM ET
(...)
If something cannot go on forever, it will stop.
—Stein’s Law, first pronounced in the 1980s

This proposition, arising first in a discussion of the balance-of-payments deficit, is a response to those who think that if something cannot go on forever, steps must be taken to stop it—even to stop it at once.
 
InfectiousGreed
If Something Cannot Go on Forever, It Will Stop
By Paul Kedrosky · Monday, May 18, 2009
In thinking about Barack Obama’s recent personal revelation about the evils of deficits (he worried aloud in a weekend speech about other countries “tiring” of lending the U.S. money), as well as the edge of budgetary apocalypse situation here in California, I got to thinking about one of my favorite Herb Stein quotes:
 
If something cannot go on forever, it will stop.
   
Paul Krugman Blog - NYTimes.com
January 14, 2010, 11:28 am
Stein’s Law, New Application
Stein’s Law: If something cannot go on forever, it will stop.
 
This is usually applied to things like trade deficits. But it also applies to matters non-economic.