Unlike Plato’s Forms, which are eternal and unchanging, Aristotle’s substances are temporary and undergo alterations. Aristotle considered there to be four types of change: in respect of substance, quality, quantity, and place. These are generation or destruction; alteration; growth and diminution; and motion, respectively. Most of the Physics is devoted to change in its different forms. Change takes place in time and space, and the Physics offers intricate theories about the nature of time, of place, and of empty space. Since space and time are infinitely divisible, Aristotle analyses the notion of infinity. He also discusses a number of particular problems concerning the relation of motion to time.