hat
the cause of that motion, which is unknown to us, shall always operate
equally; and we are sure that the medium in which the pendulum moves is
not constantly the same: either of which varying, may alter the equality
of such periods, and thereby destroy the certainty and exactness of the
measure by motion, as well as any other periods of other appearances;
the notion of duration still remaining clear, though our measures of
it cannot (any of them) be demonstrated to be exact. Since then no two
portions of succession can be brought together, it is impossible ever
certainly to know their equality. All that we can do for a measure
of time is, to take such as have continual successive appearances at
seemingly equidistant periods; of which seeming equality we have no
other measure, but such as the train of our own ideas have lodged in our
memories, with the concurrence of other PROBABLE reasons, to persuade us
of their equality.

22. Time not the Measure of Motion

One thing seems strange to me,--that whilst all men manifestly measured
time by the motion of the great and visible bodies of the world, time
yet should be defined to be the 'measure of motion': whereas it is
obvious to every one who reflects ever so little on it, that to measure
motion, space is as necessary to be considered as time; and those
who look a little farther will find also the bulk of the thing moved
necessary to be taken into the computation, by any one who will estimate
or measure motion so as to judge right of it. Nor indeed does motion any
otherwise conduce to the measuring of duration, than as it constantly
brings about the return of certain sensible ideas, in seeming
equidistant periods. For if the motion of the sun were as unequal as
of a ship driven by unsteady winds, sometimes very slow, and at others
irregularly very swift; or if, being constantly equally swift, it yet
was not circular, and produced not the same appearances,--it would not
at all help us to measure time, any more than the seeming unequal motion
of a comet does.

23. Minutes, hours, days, and years are, then, no more Minutes, Hours,
Days, and Years not necessary Measures of Duration. necessary to time or
duration, than inches, feet, yards, and miles, marked out in any matter,
are to extension. For, though we in this part of the universe, by the
constant use of them, as of periods set out by the revolutions of the
sun, or as known parts of such periods, have fixed the