Time is a measured or measurable period, a continuum that lacks spatial dimensions. "What then, is time? If no one asks me, I know what it is. If I wish to explain it to him who asks me, I do not know." In this remark St. Augustine in the 5th century AD drew attention to the fact that while time is the most familiar of concepts used in the organization of thought and action, it is also the most elusive. It cannot be given any simple, illuminating definition. In the face of this problem philosophers have sought an understanding of time by focussing on two broad questions: What is the relation between time and the physical world? And what is the relation between time and consciousness?

According to those, such as Sir Isaac Newton, who adopt an absolutist theory of time, the answer to the former question is that time is like a container within which the universe exists and change takes place. Time's existence and properties are independent of the physical universe. Time would have existed even if the universe had not. Time is held to be nonending, nonbeginning, linear, and continuous. That time has these properties is established philosophically, without reference to scientific investigation. According to the rival relationist theory, time can be reduced to change. Time is nothing over and above change in the physical universe; all hypotheses about time can be translated into hypotheses about the physical universe. Consequently the question "Has time a beginning?" becomes "Was there a first event in the history of the universe?" Also, investigating the properties of time is to be done scientifically. Relationists explore the possibility that physics could show time to have structure: it might consist of discrete particles (chronons), for instance, or it might be cyclical.

It has been realized in the 20th century that time cannot be treated in isolation from space. Consequently philosophers now tend to focus attention on space-time, conceived, after Einstein, as a continuum. While the temporal aspects of space-time remain importantly different from its spatial aspects, there is an interdependence that is shown in the case of measurement: the measure of an interval of time assigned by a clock depends on the path and speed with which it is moved. The fundamental controversy between the absolutist and the relationist remains; some philosophers argue that Einstein's theories of relativity vindicate relationist theories, others that they vindicate the absolutist theory.

Excerpt from the Encyclopedia Britannica without permission.