Benjamin Rush’s twin 1786 letters on the different species of phobia and mania sit at an extended historical juncture at which an early modern quasi-medical troping of mental disorder in American social commentary sobered up to mental medicine. The letters’ satirical drive hinged on a perennial problem still occupying George Beard almost a century onward: which idiosyncratic trepidation or ill-grounded idea warranted the nomination of national and epochal ill? Rush’s mania letter exemplified an established genre identifying popular and especially political crazes; at the same time, it foreshadowed the early 19th-century rise and mid-century fall of monomania as forensic-nosological stopgap. The phobia text established the term’s dictionary (OED) sense of specific morbid fears, but did so in the form of a mobilisation of nosological jargon for social diagnostics purposes: an ambivalent prelude to Rush’s later formal engagement with unreasonable fears and follies. Both letters draw attention to a pervasive duality in early modern and Enlightenment conceptions of hydrophobia, aerophobia, syphilophobia and lyssophobia, between public-health and mental-hygienic follies.