not to define, when we change two words of the same
signification one for another; which, when one is better understood than
the other, may serve to discover what idea the unknown stands for; but
is very far from a definition, unless we will say every English word in
the dictionary is the definition of the Latin word it answers, and that
motion is a definition of MOTUS. Nor will 'the successive application of
the parts of the superficies of one body to those of another,' which the
Cartesians give us, prove a much better definition of motion, when well
examined.

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 Des
Cartes's globules strike never so long on the retina of a man who was
blind by a gutta serena, he would ther