ll the sides of the same cube coloured red; and then of it
coloured green; and then of it coloured blue; lastly, open your eyes as in
the former experiment, and after the first second of time allowed for the
contraction of the iris, you will not perceive any increase of the light of
the day, or dazzling; because now there is no accumulation of sensorial
power in the optic nerve; that having been expended by its action in
thinking over visible objects.

This experiment is not easy to be made at first, but by a few patient
trials the fact appears very certain; and shews clearly, that our ideas of
imagination are repetitions of the motions of the nerve, which were
originally occasioned by the stimulus of external bodies; because they
equally expend the sensorial power in the organ of sense. See Sect. III. 4.
which is analogous to our being as much fatigued by thinking as by labour.

6. Nor is it in our dreams alone, but even in our waking reveries, and in
great efforts of invention, so great is the vivacity of our ideas, that we
do not for a time distinguish them from the real presence of substantial
objects; though the external organs of sense are open, and surrounded with
their usual stimuli. Thus whilst I am thinking over the beautiful valley,
through which I yesterday travelled, I do not perceive the furniture of my
room: and there are some, whose waking imaginations are so apt to run into
perfect reverie, that in their common attention to a favourite idea they do
not hear the voice of the companion, who accosts them, unless it is
repeated with unusual energy.

This perpetual mistake in dreams and reveries, where our ideas of
imagination are attended with a belief of the presence of external objects,
evinces beyond a doubt, that all our ideas are repetitions of the motions
of the nerves of sense, by which they were acquired; and that this belief
is not, as some late philosophers contend, an instinct necessarily
connected only with our perceptions.

7. A curious question demands our attention in this place; as we do not
distinguish in our dreams and reveries between our perceptions of external
objects, and our ideas of them in their absence, how do we distinguish them
at any time? In a dream, if the sweetness of sugar occurs to my
imagination, the whiteness and hardness of it, which were ideas usually
connected with the sweetness, immediately follow in the train; and I
believe a material lump of sugar present before m