One can say that one event occurs after another event. Furthermore one can measure how much one event occurs after another. The answer to how much is the amount of time between the those two events. One way of defining the idea of 'after' is based on the assumption of causality. The work humanity has done to increasingly understand the nature and measurement of time, through the work of making and improving calendars and clocks, has been a major engine of scientific discovery.
Time in engineering and applied physics
In physics, time is defined as the distance between events along the fourth axis of the spacetime manifold. Special relativity showed that time cannot be understood except as part of spacetime, a combination of space and time. The distance between events now depends on the relative speed of the observers of the events. General relativity further changed the notion of time by introducing the idea of curved spacetime. An important unit of time in theoretical physics is the Planck time – see Planck units for more details.
Important questions in the philosophy of time include: Is time absolute or merely relational? Is time without change conceptually impossible or is there more to the idea? Does time "pass" or are the ideas of past, present and future entirely subjective, descriptions only of our deception by the senses?
Zeno's paradoxes fundamentally challenged the ancient conception of time, and thereby helped motivate the development of the calculus. A point of contention between Newton and Leibniz concerned the question of absolute time: the former believed time was, like space, a container for events, while the latter believed time was, like space, a conceptual apparatus describing the interrelations between events. McTaggart[?] believed, rather eccentrically and on the basis of a very shaky argument, that time and change are illusions. Parmenides (of whom Zeno was a follower) held a similar belief based on a similarly shaky, but rather more interesting argument[?].
