For this simple
idea, entering by the touch as well as sight, it is impossible to show
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 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 foreheads, 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 itself, as it is such a particular perception 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 itself, 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 Descartes's globules strike never so long on the retina of a
man who was blind by a gutta serena, he would thereby never have any
idea of light, or anything approaching it, though he understood
never so well what little globules were, and what striking on
another body was.
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