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No two parts of Duration can be certainly known to be equal.       §21. But, perhaps, it will be said without a regular Motion, such
as of the Sun, or some other, how could it ever be known that such
Periods were equal? To which I answer, the Equality of any other
returning appearances might be known by the same way, that that
of Days was known, or presumed to be so at first, which was only
by judging of them by the train of Ideas had passed in Men’s Minds
in the Intervals, by which train of Ideas discovering inequality in the
natural days, but none in the artificial days, the artificial days, or
Νυχθήμερα were guessed to be equal, which was sufficient to make
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them serve for a measure: tho’, exacter search has since discovered
inequality in the diurnal Revolutions of the Sun, and we know not
whether the Annual also be not unequal: These yet by their pre-
sum’d and apparent Equality, serve as well to reckon time by,
(though not to measure the parts of Duration exactly) as if they
could be proved to be exactly equal. We must therefore carefully
distinguish betwixt Duration it self, and the measures we make use
of to judge of its length. Duration in it self is to be considered, as
going on in one constant, equal, uniform Course: but none of the
measures of it, which we make use of, can be known to do so; nor
can we be assured, that their assigned Parts or Periods are equal in
Duration one to another; for two successive lengths of Duration,
however measured, can never be demonstrated to be equal. The
Motion of the Sun, which the World used so long, and so confidently
for an exact measure of Duration, has, as I said, been found in its
several parts unequal: and though Men have of late made use of a
Pendulum, as a more steady and regular Motion, than that of the
Sun or (to speak more truly) of the Earth; yet if any one should be
asked how he certainly knows, that the two successive swings of a
Pendulum are equal, it would be very hard to satisfie himself, that
they are infallibly so: since we cannot be sure, that 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 perswade us of their
Equality.
Locke Hum II, 14, §21, pp. 189-190