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 the prominences
and depressions of objects, and that those shades uniformly vary, when the
sense of touch observes any variation. Hence when the retina becomes
stimulated by colours or shades of light in a certain form, as in a
circular spot; we know by experience, that this is a sign, that a tangible
body is before us; and that its figure is resembled by the miniature figure
of the part of the organ of vision, that is thus stimulated.

Here whilst the stimulated part of the retina resembles exactly the visible
figure of the whole in miniature, the various kinds of stimuli from
different colours mark the visible figures of the minuter parts; and by
habit we instantly recall the tangible figures.

Thus when a tree is the object of sight, a part of the retina resembling a
flat branching figure is stimulated by various shades of colours; but it is
by suggestion, that the gibbosity of the tree, and the moss, that fringes
its trunk, appear before us. These are ideas of suggestion, which we feel