 Physics I is a well-constructed logos. It aims at knowledge of the principles of physical change: it establishes that there are such principles and determines what they are and how many. Even if the results are general, its theories are fundamental for the entire Aristotelian study of the natural world. This introduction reconstructs the general structure of Physics I and its connection to Physics II. It distinguishes four stages in Physics I: an introduction, a doxographical section in which Aristotle is in dialogue with previous philosophers, a section in which he solves the main puzzles about change, and an endoxical confirmation of the theory established in the preceding section. Other points discussed are: to what kind of public Physics I is addressed, the difference between Aristotle's theory and ancient cosmologies, the examples he uses, how to identify a single event, and his conception of essences.