And it is certain that this
sensation of pain is not less different from the motion that causes
it, or from that of the part of our body which the sword cuts, than
are the sensations we have of colour, sound, odour, or taste. On
this ground we may conclude that our mind is of such a nature that
the motions alone of certain bodies can also easily excite in it all
the other sensations, as the motion of a sword excites in it the
sensation of pain.
CXCVIII. That by our senses we know nothing of external objects
beyond their figure [or situation], magnitude, and motion.
Besides, we observe no such difference between the nerves as to lead
us to judge that one set of them convey to the brain from the organs
of the external senses anything different from another, or that
anything at all reaches the brain besides the local motion of the
nerves themselves. And we see that local motion alone causes in us
not only the sensation of titillation and of pain, but also of light
and sounds. For if we receive a blow on the eye of sufficient force
to cause the vibration of the stroke to reach the retina, we see
numerous sparks of fire, which, nevertheless, are not out of our
eye; and when we stop our ear with our finger, we hear a humming
sound, the cause of which can only proceed from the agitation of the
air that is shut up within it.
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