Time is one of the seven fundamental physical quantities in the International System of Units. Time is used to define other quantities â€” such as velocity â€” so defining time in terms of such quantities would result in circularity of definition. An operational definition
of time, wherein one says that observing a certain number of
repetitions of one or another standard cyclical event (such as the
passage of a free-swinging pendulum) constitutes one standard unit such
as the second,
is highly useful in the conduct of both advanced experiments and
everyday affairs of life.
The operational definition leaves aside the
question whether there is something called time, apart from the
counting activity just mentioned, that flows and that can be measured.
Investigations of a single continuum called spacetime bring questions
about space into questions about time, questions that have their roots
in the works of early students of natural philosophy.