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TANSTAAFL is an acronym for the adage "There Ain't No Such Thing As A Free Lunch," popularized by science fiction writer Robert A. Heinlein in his 1966 novel The Moon Is a Harsh Mistress,^[1][2] which discusses the problems caused by not considering the eventual outcome of an unbalanced economy. This phrase and book are popular with libertarians and economics textbooks. In order to avoid a double negative, the acronym "TINSTAAFL" is sometimes used instead, meaning "There Is No Such Thing As A Free Lunch".

The phrase refers to the once-common tradition of saloons in the United States providing a "free" lunch to patrons, who were required to buy at least one drink. Rudyard Kipling, writing in 1891, noted how he

came upon a barroom full of bad Salon pictures in which men with hats on the backs of their heads were wolfing food from a counter. It was the institution of the "free lunch" I had struck. You paid for a drink and got as much as you wanted to eat. For something less than a rupee a day a man can feed himself sumptuously in San Francisco, even though he be a bankrupt. Remember this if ever you are stranded in these parts.^[3]

TANSTAAFL means that a person or a society cannot get something for nothing. Even if something appears to be free, there is always a cost to the person or to society as a whole even though that cost may be hidden or distributed. ^[4] For example, you may get complimentary food at a bar during "happy hour," but the bar owner bears the expense of your meal and will attempt to recover that expense somehow. Some goods may be nearly free, such as fruit picked in the wilderness, but usually some cost such as labor or the loss of food for local wildlife is incurred.

The idea that there is no free lunch at the societal level applies only when all resources are being used completely and appropriately, i.e., when economic efficiency prevails. If one individual or group gets something at no cost, somebody else ends up paying for it. If there appears to be no direct cost to any single individual, there is a social cost. Similarly, someone can benefit for "free" from an externality or from a public good, but someone has to pay the cost of producing these benefits.

To a scientist, TANSTAAFL means that the system is ultimately closed — there is no magic source of matter, energy, light, or indeed lunch, that cannot be eventually exhausted. Therefore the TANSTAAFL argument may also be applied to natural physical processes. (See Thermodynamics.)

In mathematical finance, the term is also used as an informal synonym for the principle of no-arbitrage. This principle states that a combination of securities that has the same cash flows as another security must have the same net price.

TANSTAAFL is sometimes used as a response to claims of the virtues of free software. Supporters of free software often counter that the use of the term "free" in this context is primarily a reference to a lack of constraint ("libre") rather than a lack of cost ("gratis"). Richard Stallman has described it as "free as in liberty not as in beer".

TANSTAAFL is the name of a snack bar in the Pierce dormitory of the University of Chicago. The name references the fact that the use of the term was popularized by Milton Friedman, the Nobel Prize–winning former University of Chicago professor.^[5]

• In 1950, a New York Times columnist ascribed the phrase to economist (and Army General) Leonard P. Ayres of the Cleveland Trust Company. "It seems that shortly before the General's death [in 1946]... a group of reporters approached the general with the request that perhaps he might give them one of several immutable economic truisms which he had gathered from his long years of economic study... 'It is an immutable economic fact,' said the general, 'that there is no such thing as a free lunch.'"^[6]
• The book TANSTAAFL, the economic strategy for environmental crisis, by Edwin G. Dolan (Holt, Rinehart and Winston, 1971, ISBN 0-03-086315-5) may be the first published use of the term in the economics literature.
• Spider Robinson's 2001 book 'The Free Lunch' draws its name from the TANSTAAFL concept.

• Free lunch
• No free lunch in search and optimization
• No-arbitrage bounds – mathematical relationships specifying simple limits on derivative prices
• Parable of the broken window – story by Frédéric Bastiat written to illuminate the notion of hidden costs
• Tanstagi – There Ain't No Such Thing As Government Interference in the Schrödinger's Cat trilogy
• Regiving – practice of giving away one's goods to others