There are two distinct views on the meaning of time. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence, and time itself is something that can be measured. This is the realist's view, to which Sir Isaac Newton subscribed.[1]
A contrasting view is that time is part of the fundamental intellectual structure (together with space and number) within which we sequence events, quantify the duration of events and the intervals between them, and compare the motions of objects. In this view, time does not refer to any kind of entity that "flows", that objects "move through", or that is a "container" for events. This view is in the tradition of Gottfried Leibniz[2] and Immanuel Kant,[3][4] in which time, rather than being an objective thing to be measured, is part of the mental measuring system. The question, perhaps overly simplified and allowing for no middle ground, is thus: is time a "real thing" that is "all around us", or is it nothing more than a way of speaking about and measuring events?
In science, time (and space) are considered fundamental, (=not definable via other quantities because other quantities are defined via time and space) thus they can only be defined via measurement - using operational definitions that specify the units of measurement that quantify time. Regularly recurring events and objects with apparent periodic motion have long served as standards for units of time. Examples are the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and, currently, oscillations of Cesium atoms.
Time has long been a major subject of science, philosophy and art. The measurement of time has occupied scientists and technologists, and was a prime motivation in astronomy. Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human lifespans.