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.
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