All animals except humans live in a continual present, with no sense of the temporal distinctions of past, present and future. Our consciousness of time is therefore one of the most important distinguishing features of humankind, and one of the things that truly separates us from the lower animals. It comes as no surprise, then, that from time immemorial, philosophers, teachers and theologians have speculated on the true nature of time. Does time have a substance and, if so, what is it made of? How do we know that time really exists? Does time have a beginning and an end? Is it a straight line or a circle?
There is general agreement among philosophers that time is continuous (i.e. we do not experience it as stopping and starting, or darting about at random), and that it has an intrinsic direction or order (i.e. we all agree that events progress from past to present to future). There is also a more or less general agreement that is it objective, and not subjective or dependent on its being consciously experienced, which is borne out by the almost universal agreement on the time order of so many events, both psychological and physical, and the fact that so many different physical processes bear consistent time relations to each other (e.g. the rotation of the Earth, the frequency of oscillation of a pendulum, etc). However, even given that, many differing opinions and approaches to what time actually is have been put forward over the centuries.
The ancient philosophy of India and Greece was among the first to confront and question the real nature of many things that had been taken for granted (e.g. matter, space, nature, change, etc), and time was one of the many mysterious concepts they argued about at length. One major point of contention among the ancients was whether time is linear or cyclical, and whether it is endless or finite. (“Ancient philosophy”, for these purposes, is taken to include philosophical thought up to the late Middle Ages.)
During the Age of Enlightenment of the 17th and 18th Century, early modern philosophy began once again to consider questions of whether time is real and absolute or merely an abstract intellectual concept that humans use to sequence and compare events. In the 19th Century, philosophers began to question whether the present was really an instantaneous concept or a duration, and the conventionalists and phenomenologists all made their own contributions to the debate on time.
More recently, modern philosophy has continued to argue over whether time is real or “unreal”. But a whole host of other philosophical issues related to time have also surfaced, including whether time is tensed or tenseless, whether the present is instantaneous or a duration, whether the past and the future can be said to really exist, the manner in which objects persist though time, etc. Some of the novel ideas from modern physics have also generated new philosophical insights and hypotheses concerning the nature of time.