Or, what is nature? Is the peanut butter question the domain of chemists or philosophers?

The answer is somewhere in between, and the question of natural peanut butter is among the most widely relevant debates in the modern history of food. At the moment, in part because of peanut butter, the word “natural” may be the most influential claim on food labels. Americans spend $40 billion annually on food so labeled—more than on “organic,” a word that has a regulated, standardized definition. And when we buy products labelled natural, what is it we think we’re buying?

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Unlike most other foods, there is a legal definition of “peanut butter.” To use the name, a product must contain 90 percent peanuts. (Technically a person could sell a bag of peanuts and label it peanut butter, but, you know, why would you?) And while “natural” isn’t defined or regulated, peanut butter is one domain where the term means something predictable to most people: Natural peanut butter just contains peanuts (and usually salt).

This is the way peanut butter was understood in its earliest days—and it was actually defined as such by the Food and Drug Administration in 1940. At the time, foods weren’t required to bear labels that listed ingredients. So when peanut-butter manufacturers inquired with the agency about adding glycerin to keep the oil from separating to the top—a phenomenon that, even today, people describe to me as off-putting, unnatural—the agency responded that glycerin could be added, yes, but it would have to be duly announced on the peanut-butter container.

That was because, the FDA reasoned, people who buy peanut butter are not expecting glycerin: Peanut butter is “generally understood” to mean “a product consisting solely of ground roasted peanuts, with or without a small quantity of added salt.”

This definition, which seemed simple and obvious at the time, started a landslide of attempts at defining what foods are. What, exactly, can be added to or replaced in a food without changing its definition?

Defining parameters seemed like a reasonable task, and it would have been, had it not been for the torrent of postwar food technologies. What we weren’t making anew—Jell-O salads, SPAM, Tang, Twinkies, sprayable cheeses—we were making in better, brighter, everlasting forms.

And so came the embrace of ever cheaper, sweeter, gooier peanut butters.

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Ancient Incan people appear to have pulverized peanuts thousands of years before peanut butter was defined in an FDA office in 1940. By that time, modern peanut butter had been around for about 40 years. American farmers had taken up peanuts after the boll weevil devastated cotton cultivation at the turn of the century. In 1894, a snack-food maker in Saint Louis named George Bayle began producing and selling his new butter product. It took off after being featured in the 1904 World’s Fair in Bayle’s home city. By 1907, nationwide production was up to 34 million pounds.