9. All the parts of extension are extension, and all the parts of duration are duration.
There is one thing more
wherein space and duration have a great conformity, and that is, though they are justly reckoned amongst our
simple ideas, yet none of the distinct ideas we have of either is without all manner of composition: it is the very
nature of both of them to consist of parts: but their parts being all of the same kind, and without the mixture of any
other idea, hinder them not from having a place amongst simple ideas. Could the mind, as in number, come to so
small a part of extension or duration as excluded divisibility, that would be, as it were, the indivisible unit or idea;
by repetition of which, it would make its more enlarged ideas of extension and duration. But, since the mind is not
able to frame an idea of any space without parts, instead thereof it makes use of the common measures, which, by
familiar use in each country, have imprinted themselves on the memory (as inches and feet; or cubits and
parasangs; and so seconds, minutes, hours, days, and years in duration);--the mind makes use, I say, of such ideas
as these, as simple ones: and these are the component parts of larger ideas, which the mind upon occasion makes
by the addition of such known lengths which it is acquainted with. On the other side, the ordinary smallest
measure we have of either is looked on as an unit in number, when the mind by division would reduce them into
less fractions. Though on both sides, both in addition and division, either of space or duration, when the idea
under consideration becomes very big or very small its precise bulk becomes very obscure and confused; and it is
the number of its repeated additions or divisions that alone remains clear and distinct; as will easily appear to any
one who will let his thoughts loose in the vast expansion of space, or divisibility of matter. Every part of duration
is duration too; and every part of extension is extension, both of them capable of addition or division in infinitum.
But the least portions of either of them, whereof we have clear and distinct ideas, may perhaps be fittest to be
considered by us, as the simple ideas of that kind out of which our complex modes of space, extension, and
duration are made up, and into which they can again be distinctly resolved. Such a small part in duration may be
called a moment, and is the time of one idea in our minds, in the train of their ordinary succession there. The
other, wanting a proper name, I know not whether I may be allowed to call a sensible point, meaning thereby the
least particle of matter or space we can discern, which is ordinarily about a minute, and to the sharpest eyes
seldom less than thirty seconds of a circle, whereof the eye is the centre.
