The Supreme Court’s recent cases on patent-eligible subject matter have struggled to draw the line between unpatentable fundamental principles, such as laws of nature and abstract ideas, and patentable inventions. In Mayo v. Prometheus, the Court suggested that only “inventive applications” of fundamental principles fall within the domain of the patent system. Both Mayo and its intellectual forebear, Parker v. Flook, anchored this doctrine in Neilson v. Harford, the famous “hot blast” case decided by the Court of Exchequer in 1841. But the Supreme Court has founded the inventive application doctrine on a basic misapprehension. Neilson’s patent on the hot blast was sustained not because his application was inventive, but because it was entirely conventional and obvious. In both England and the United States, the hot-blast cases taught that inventors could patent any practical application of a new discovery, regardless of the application’s novelty or inventiveness. And for over one hundred years, American authority consistently maintained that practical application distinguished unpatentable discovery from patentable invention. The inventive application test in fact originated in 1948, in Funk Brothers v. Kalo Inoculant, which departed radically from the established standard of patent eligibility. In the wake of Funk Brothers, the lower courts struck down a series of patents unquestionably within the technological arts—arguably the precise innovations the patent system sought to promote. This history is largely forgotten today, but it should serve as a cautionary tale of the patents that could be invalidated if the Court maintains inventive application as the test for patent eligibility.