The histories of patent law and medical practice in the United States have intersected in various ways over the past 150 years, beginning with the professional campaign against “patent medicines” in the late nineteenth century, and culminating, for now, in attempts to patent the diagnostic procedures discussed in this article. The patenting of diagnostic procedures provokes a set of fundamental questions about the episteme of patent law. These questions are not new. They emerged at the very origins of patent jurisprudence, centered on the question of what distinguished an invention from a law of nature, and this question of patentability has persistently reemerged over the past century in the contexts of plant breeding, biotechnology, and now diagnostic medicine. So far, the question has been addressed in terms that imagine the invention as a machine, understood in the figurative sense of a transformative organization of forces and elements. But diagnostic procedures, because they address the body informationally, as a system based on the recursive patterning of signals rather than a linear transformation of inputs into outputs, stretch the figure of the machine to the point at which it ceases to be effective. How then should one define and delimit invention?.