5. Finally, though the figures of bodies are in truth resembled by the
figure of the part of the organ of touch, which is stimulated into motion;
and that organ resembles the solid body, which stimulates it, in its
property of solidity; and though the sense of hearing resembles the
vibrations of external bodies in its capability of being stimulated into
motion by those vibrations; and though our other organs of sense resemble
the bodies, that stimulate them, in their capability of being stimulated by
them; and we hence become acquainted with these properties of the external
world; yet as we can repeat all these motions of our organs of sense by the
efforts of volition, or in consequence of the sensation of pleasure or
pain, or by their association with other fibrous motions, as happens in our
reveries or in sleep, there would still appear to be some difficulty in
demonstrating the existence of any thing external to us.

In our dreams we cannot determine this circumstance, because our power of
volition is suspended, and the stimuli of external objects are excluded;
but in our waking hours we can compare our ideas belonging to one sense
with those belonging to another, and can thus distinguish the ideas
occasioned by irritation from those excited by sensation, volition, or
association. Thus if the idea of the sweetness of sugar should be excited
in our dreams, the whiteness and hardness of it occur at the same time by
association; and we believe a material lump of sugar present before us. But
if, in our waking hours, the idea of the sweetness of sugar occurs to us,
the stimuli of surrounding objects, as the edge of the table, on which we
press, or green colour of the grass, on which we tread, prevent the other
ideas of the hardness and whiteness of the sugar from being exerted by
association. Or if they should occur, we voluntarily compare them with the
irritative ideas of the table or grass above mentioned, and detect their
fallacy. We can thus distinguish the ideas caused by the stimuli of
external objects from those, which are introduced by association,
sensation, or volition; and during our waking hours can thus acquire a
knowledge of the external world. Which nevertheless we cannot do in our
dreams, because we have neither perceptions of external bodies, nor the
power of volition to enable us to compare them with the ideas of
imagination.

III. _Of Vision._

Our eyes observe a difference of colour, or of shade, in t