The dissertation examines the temporal theories of Aristotle and the Muslim Aristotelian, Ibn Sînâ (Avicenna). After considering Aristotelian science and sketching Aristotle's theory of physics, the dissertation picks up a series of puzzles concerning the reality of time. The central puzzle is a dilemma, which seemingly shows that the now can neither change nor remain the same. The dilemma is important, since one's solution to it affects the way one envisions time. Aristotle's solution, I argue, is to show how the now remains the same. Thus he adopts a “static” theory of time, i.e., time is a magnitude marked off in motion by an unchanging now, and although time is a magnitude of motion, it itself does not flow or change. In contrast, Ibn Sînâ, who employs much of the same vocabulary as Aristotle, develops a “dynamic” theory of time, which envisions time as the “flow” of an ever changing now, and thus he attempts to explain how the now changes. The dissertation concludes with an application of Aristotle and Ibn Sînâ's accounts of time to McTaggart's argument for the unreality of time and Sydney Shoemaker's contention that conceptually time does not require change. In the end I argue that time must be static and cannot “flow.”