Past, Present and Future
Another way of looking at time is as the totality of three separate elements: the past, the present and the future.
The past may be defined as those events which occurred before a given point in time, events which are usually considered to be fixed and immutable. It can be accessed through memory or, since the advent of written language, recorded history. The study of the past, in particular as it relates to humans, is called history.
The present may be defined as the time associated with the events perceived directly and for the first time, i.e. not as a recollection of the past or as a speculation of the future. It is equivalent to the word “now”, and is the period of time located between the past and the future. Just how long a period of time the present incorporates, however, depends on the context, and can vary from an infinitesimal or durationless moment to a day to a whole era, depending on how it is being used.
The future is the indefinite time period after the present moment. It is the portion of the projected time line that is anticipated to occur, and may be considered as potentially infinite in its extent, or as circumscribed and finite, depending on the context. While some people may see the future as fixed and predetermined, most see it as essentially unknown (and perhaps unknowable), and open to many different possibilities and permutations. The study of postulating possible, probable and preferable futures and worldviews is called futurology.