For Aristotle, the natural world is the world of things subject to change. Accordingly, Aristotle’s natural philosophy essentially consists in a philosophical investigation of change. Aristotle deals with the most fundamental philosophical issues about change in the Physics. Here he determines the intrinsic constituents of a thing that make it possible for it to be subject to change (matter and form), he classifies the types of explanatory factor at work in the natural world (the distinction of the four causes), and in particular he argues for the claim that nature acts for an end (teleological explanation). He also gives a general definition of change, which relates the notion of change to the more basic notions of act and potency, he shows that every change is continuous, and he proves the existence of an eternal motion and an unmoved mover. In addition, he provides a philosophical treatment of the notions of time, place, the void, and the infinite, which are thought to be necessary parts of a complete discussion of change. Because of its extremely rich philosophical content, the Physics was intensely studied by medieval philosophers and became the focal text for the assimilation of Aristotle’s natural philosophy.