— 136 —
      §12. If then external Objects be not united to our Minds, when
they produce Ideas in it; and yet we perceive these original Qualities in
such of them as singly fall under our Senses,’tis evident, that some
motion must be thence continued by our Nerves, or animal Spirits,
by some parts of our Bodies, to the Brains or the seat of Sensation,
there to produce in our Minds the particular Ideas we have of them. And
since the Extension, Figure, Number, and Motion of Bodies of an
observable bigness, may be perceived at a distance by the sight, ’tis
evident some singly imperceptible Bodies must come from them to
the Eyes, and thereby convey to the Brain some Motion, which
produces these Ideas, which we have of them in us.
Locke Hum II, 8, §11, p. 136