— 423 —
Light.       §10. The Act of Perspicuous, as far forth as perspicuous, is another
Peripatetick definition of a simple Idea; which though not more
absurd than the former of Motion, yet betrays its Uselessness and
Insignificancy more plainly, because Experience will easily con-
vince any one, that it cannot make the meaning of the Word Light
(which it pretends to define) at all understood by a blind Man: but
the definition of Motion appears not at first sight so useless, because
it scapes this way of Trial. For this simple Idea, entring by the
Touch as well as Sight; ’tis impossible to shew an Example of any
one, who has no other way to get the Idea of Motion, but barely by
the definition of that Name. Those who tell us, that Light is a
great number of little Globules, striking briskly on the bottom of
the Eye, speak more intelligibly than the Schools: but yet these
— 424 —
Words never so well understood, would make the Idea, the Word
Light stands for, no more known to a Man that understands it not
before, than if one should tell him, that Light was nothing but a
Company of little Tennis-balls, which Fairies all day long struck
with Rackets against some Men’s Fore-heads, whilst they passed
by others. For granting this explication of the thing to be true; yet
the Idea of the cause of Light, if we had it never so exact, would no
more give us the Idea of Light it self, as it is such a particular per-
ception in us, than the Idea of the Figure and Motion of a sharp
piece of Steel, would give us the Idea of that Pain, which it is able to
cause in us. For the cause of any Sensation, and the Sensation it self,
in all the simple Ideas of one Sense, are two Ideas; and two Ideas so
different, and distant one from another, that no two can be more so.
And therefore should Des Cartes’s Globules strike never so long on
the retina of a Man, who was blind by a Guttâ Serenâ, he would
thereby never have any Idea of Light, or any thing approaching to it,
though he understood what little Globules were, and what striking
on another Body was, never so well. And therefore the Cartesians
very well distinguish between that Light which is the Cause of that
Sensation in us, and the Idea which is produced in us by it, and is
that which is properly Light.
Locke Hum III, 4, §10, pp. 423-424