10. Definitions of light.
"The act of perspicuous, as far forth as perspicuous," is another Peripatetic 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 convince 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 escapes this way of trial. 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. 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.
