'And to us it is enough that gravity does really exist, and act
according to the laws which we have explained, and abundantly serves
to account for all the motions of the celestial bodies, and of our sea.
'And now we might add something concerning a certain most subtle Spirit
Spirit which pervades and lies hid in all gross bodies; by the force and
action of which Spirit the particles of bodies mutually attract one
another at near distances, and cohere, if contiguous; and electric
bodies operate to greater distances, as well repelling as attracting the
neighbouring corpuscles; and light is emitted, reflected, refracted,
inflected, and heats bodies; and all sensation is exited, and the
members of animal bodies move at the command of the will, namely
by the vibrations of this Spirit, mutually propagated along the solid
filaments of the nerves, from the outward organs of sense to the
brain, and from the brain into the muscles. But these are things that
cannot be explained in a few words, nor are we furnished with that
sufficiency of experiments which is required to an accurate determina-
tion and demonstration of the laws by which this elastic and electric
spirit operates.'

NEWTON