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 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.


11. Simple Ideas, why undefinable, further explained.

Simple ideas, as has been shown, are only to be got by those impressions
objects themselves make on our minds, by the proper inlets appointed
to each sort. If they are not received this way, all the words in the
world, made use of to explain or define any of their names, will never
be able to produce in us the idea it stands for. For, words being
sounds, can produce in us no other simple ideas than of those very