The purpose of time is to prevent everything from happening at once. Time is familiar to everyone, yet it's hard to define and understand. Physicists define time as the progression of events from the past to the present into the future. If a system is unchanging, it is timeless. Time can be considered to be the fourth dimension of reality, used to describe events in three-dimensional space. It is not something we can see, touch, or taste, but we can measure its passage. Time moves only in one direction. It's possible to move forward in time, but not backward.