There ain't no such thing as a free lunch
"There ain't no such thing as a free lunch" (alternatively, "There's no such thing as a free lunch" or other variants) is a popular adage communicating the idea that it is impossible to get something for nothing. The initialisms TNSTAFL, TANSTAAFL, and TINSTAAFL are also used. Uses of the phrase dating back to the 1930s and 1940s have been found, but the phrase's first appearance is unknown. The "free lunch" in the saying refers to the nineteenth century practice in American bars of offering a "free lunch" as a way to entice drinking customers. The phrase and the acronym are central to Robert Heinlein's 1966 libertarian science fiction novel The Moon is a Harsh Mistress, which popularized it. The free-market economist Milton Friedman also popularized the phrase by using it as the title of a 1975 book, and it often appears in economics textbooks; Campbell McConnell writes that the idea is "at the core of economics".
The "free lunch" referred to in the acronym relates back to the once-common tradition of saloons in the United States providing a "free" lunch to patrons who had purchased at least one drink. All the foods on offer were high in salt (e.g. ham, cheese and salted crackers) so those who ate them ended up buying a lot of beer. Rudyard Kipling, writing in 1891, noted how he came upon a bar room 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."
TANSTAAFL, on the other hand, indicates an acknowledgment that in reality 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. For example, as Heinlein has one of his characters point out, a bar offering a free lunch will likely charge more for its drinks.
TANSTAAFL demonstrates opportunity cost. Greg Mankiw described the concept as: "To get one thing that we like, we usually have to give up another thing that we like. Making decisions requires trading off one goal against another." 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 not, a 'free lunch' can be had through a more efficient utilisation of resources. 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.
In the sciences, TANSTAAFL means that the universe as a whole is ultimately a closed system—there is no magic source of matter, energy, light, or indeed lunch, that does not draw resources from something else, and will not eventually be exhausted. Therefore the TANSTAAFL argument may also be applied to natural physical processes in a closed system (either the universe as a whole, or any system that does not receive energy or matter from outside). (See Second law of thermodynamics.) The bio-ecologist Barry Commoner used this concept as the last of his famous "Four Laws of Ecology".
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 in equilibrium.
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 speech not as in beer".
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