It is inconceivable to doubt that light consists in the motion of some
sort of matter. For whether one considers its production, one sees
that here upon the Earth it is chiefly engendered by fire and flame
which contain without doubt bodies that are in rapid motion, since
they dissolve and melt many other bodies, even the most solid; or
whether one considers its effects, one sees that when light is
collected, as by concave mirrors, it has the property of burning as a
fire does, that is to say it disunites the particles of bodies. This
is assuredly the mark of motion, at least in the true Philosophy, in
which one conceives the causes of all natural effects in terms of
mechanical motions. This, in my opinion, we must necessarily do, or
else renounce all hopes of ever comprehending anything in Physics.

And as, according to this Philosophy, one holds as certain that the
sensation of sight is excited only by the impression of some movement
of a kind of matter which acts on the nerves at the back of our eyes,
there is here yet one reason more for believing that light consists in
a movement of the matter which exists between us and the luminous
body.

Further, when one considers the extreme speed with which light spreads
on every side, and how, when it comes from different regions, even
from those directly opposite, the rays traverse one another without
hindrance, one may well understand that when we see a luminous object,
it cannot be by any transport of matter coming to us from this object,
in the way in which a shot or an arrow traverses the air; for
assuredly that would too greatly impugn these two properties of light,
especially the second of them. It is then in some other way that light
spreads; and that which can lead us to comprehend it is the knowledge
which we have of the spreading of Sound in the air.