What separates science from pseudoscience? The distinction seems obvious, but attempts at a demarcation criterion - from Popper's 'falsifiability' to Langmuir's 'pathological science' - invariably fail, argues Michael D. Gordin.

A good place to start is a scholarly urban legend whose provenance is uncertain: “Nonsense is nonsense, but the history of nonsense is a very important science.” This statement is attributed to Saul Lieberman, a legendary Talmudic scholar, ostensibly when he was introducing the even more legendary Gershom Scholem. Maybe Lieberman said it; maybe he didn’t. Regardless, the content of the statement is far from being nonsense and opens up both a puzzle and a clue to its solution.

The puzzle is how we define what nonsense is in the first place. This is a central question of our moment, beset as we are with conspiracy theories and allegations of “fake news” and miracle cures. Indeed, it has been a central question ever since humanity began organizing its beliefs about knowledge: once you start that process, you need a way of dividing reliable claims from dubious ones. It’s not a solely academic problem. Everyone routinely sifts incoming information into (at least) two piles — we just don’t agree on how to do it. There are countless domains in which the puzzle is confronted.