experiment, nor the alternate presence and evanescence of them, as in the
fifth experiment, nor the perpetual change of colours of them, as in the
last experiment, could exist.

IV. The subsequent articles shew, that these animal motions or
configurations of our organs of sense constitute our ideas.

1. If any one in the dark presses the ball of his eye, by applying his
finger to the external corner of it, a luminous appearance is observed; and
by a smart stroke on the eye great slashes of fire are perceived. (Newton's
Optics.) So that when the arteries, that are near the auditory nerve, make
stronger pulsations than usual, as in some fevers, an undulating sound is
excited in the ears. Hence it is not the presence of the light and sound,
but the motions of the organ, that are immediately necessary to constitute
the perception or idea of light and sound.

2. During the time of sleep, or in delirium, the ideas of imagination are
mistaken for the perceptions of external objects; whence it appears, that
these ideas of imagination, are no other than a reiteration of those
motions of the organs of sense, which were originally excited by the
stimulus of external objects: and in our waking hours the simple ideas,
that we call up by recollection or by imagination, as the colour of red, or
the smell of a rose, are exact resemblances of the same simple ideas from
perception; and in consequence must be a repetition of those very motions.

3. The disagreeable sensation called the tooth-edge is originally excited
by the painful jarring of the teeth in biting the edge of the glass, or
porcelain cup, in which our food was given us in our infancy, as is further
explained in the Section XVI. 10, on Instinct.--This disagreeable sensation
is afterwards excitable not only by a repetition of the sound, that was