In the modern day, we understand time to be a fundamental scientific variable. In Greco-Roman antiquity, however, timekeeping’s potential to facilitate scientific inquiry was only beginning to be explored. We have over 500 examples of sundials and water-clocks dating to as early as the 4th c. BCE, and literary and inscriptional evidence from the Imperial period suggests that, by this point, hourly timekeeping had become integrated into many aspects of society. Yet, few scholars have explored the impact that this new technology may have had on scientific investigation outside of the realm of technical astronomy. This dissertation aims to initiate such a conversation by analyzing how Galen of Pergamum, a renowned second-century physician, used sundials, water-clocks, and the unit of the hour in his medical and philosophical writings. It also contributes toward our larger understanding of horology’s roles in the Greco-Roman world by offering a rare case study of how one individual engaged with the technology around him., Two primary questions drive this dissertation. First, how, when, and why does Galen exploit clock technology in his disputes with fellow doctors and philosophers? Second, to what extent are Galen’s attitudes towards horology representative for his time? Chapters One and Two address these questions within the context of Galen’s On the Affections and Errors of the Human Soul, a treatise on epistemology and ethical psychology that, unusually, features an extended discussion of sundial and water-clock construction. Chapter One explores how Galen uses these processes to highlight the differences between his own method of scientific inquiry and the methods used by members of contemporary philosophical schools. I demonstrate that Galen associates clock-design with the positive concepts of verifiability, clarity, concord, utility, and long-term scientific progress, while associating sectarian philosophies with the opposite. Chapter Two contextualizes Galen’s attitude toward clocks by investigating the semiotic fields of sundials and water-clocks under the Roman Empire. I show that, across a range of media, sundials had become linked with the idea of the philosopher. I proceed to argue that, by incorporating clocks into his philosophical work, Galen adapts a common trope in order to promote his own scientific method as a way of life., Chapters Three and Four focus on how Galen integrates seasonal and equinoctial hours into two of his less-studied treatises, On Critical Days and Against Those Who Have Written on Types, which deal with intermittent fevers such as malaria. The third chapter examines how Galen works references to hourly timekeeping into his defense and refinement of Hippocratic “critical day” theory. I propose that, both in his fever case histories and in his astrological explanations for critical days, Galen uses hourly timekeeping to help himself defend two claims: (a) that he is familiar with astronomy, and (b) that his own theories align closely with Hippocratic teachings. The fourth chapter explores how Galen incorporates hours into his critique of complex fever classification systems and, in so doing, manages to highlight his own empiricism and rationality, the essential components of his scientific method. In the fifth and final chapter, I relate Galen’s interest in hourly timekeeping to his understanding of kairos, the “right moment” to engage in an action. Using On Hygiene as a case study, I demonstrate how Galen presents the relationship between kairoi and hours differently in different contexts, depending on whether the patient is sick, healthy, or simply old., Ultimately, my investigation reveals that, while Galen engages closely with contemporary scholarly and representational trends, the manner and degree to which he applies hourly timekeeping to medical and philosophical controversies is unique among our limited extant sources.