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.