'A Thousand Futures': Galen on Prediction and Prognōsis

Medical prediction was a central concept of Galenic medicine. Scholars usually examine it through Galen’s On Prognosis, which narrates a series of case studies from his time in Rome, culminating in a cure for the emperor Marcus Aurelius. In this article, I show that we can know significantly more about Galen’s theory of medical prediction if we read On Prognosis in parallel with his commentaries, particularly Commentary on Hippocrates’ Prognostic and Commentary on Hippocrates’ Prorrhetic. Doing so allows us to see how Galen expanded Hippocratic notions of prediction. Unlike the Hippocratics, Galen referred to medical prediction consistently as prognōsis, a technical term that designated it as a science (technē). Although prognōsis could be compared to other forms of future-prediction including divination, Galen believed that it stood out in that crowded field because its empirical footing and logical-deductive method securely grounded its epistemic claims. Galenic prognōsis also claimed a new level of precision: predictions were often made down to the hour, which had become technically feasible since the Hippocratic period due to the spread of water-clocks and sundials. Correspondingly, an increased popular attention to hourly timekeeping meant that predictions at this level of precision were desirable and persuasive to patients. Despite Galen’s claims that his concepts of medical prediction had their roots in Hippocratic medicine, his prognōsis represents a significant departure.