In the first century B.C.E., a group of Greek physicians called the Methodists denied that medicine could be based on such "hidden causes" as humors, atoms, or elements. They argued that the inner workings of the body were ultimately unknowable, existing beyond the limits of human knowledge and inference. Yet they insisted that medical certainty was still possible, claiming that every disease shared one of three directly apprehensible "manifest commonalities"--stricture, laxity, or some mixture of the two. Medicine could therefore be a science; it was simply noncausal in structure. This essay examines these medical theories in light of Herbert Simon's concept of "bounded rationality," suggesting that the Methodists were proposing a type of medical "heuristic" in response to the limitations of human knowledge and processing power. At the same time, the essay suggests that such an epistemology had its consequences, setting up an ontological crunch whereby the demands formerly placed on diseases and their causes transferred to "affections" and the commonalities, with successive generations of Methodists disagreeing about the status of symptoms, signs, and diseased states. Borrowing vocabulary from the Methodists themselves, the essay calls the consequent ontological slippage between causes and effects "metalepsis".