what is actually going on in his mouth when a tooth is being bored by a
modern rotating drill. It may be found that the same principle helps to
account for the exaggerated importance which we attach to the
impressions of our dreams.

It is evident that all indistinct impressions are liable to be wrongly
classed. Sensations answering to a given colour or form, are, when
faint, easily confused with other sensations, and so an opening occurs
for illusion. Thus, the impressions received from distant objects are
frequently misinterpreted, and, as we shall see by-and-by, it is in this
region of hazy impression that imagination is wont to play its most
startling pranks.

It is to be observed that the illusions arising from wrong
classification will be more frequent in the case of those senses where
discrimination is low. Thus, it is much easier in a general way to
confuse two sensations of smell than two sensations of colour. Hence the
great source of such errors is to be found in that mass of obscure
sensation which is connected with the organic processes, as digestion,
respiration, etc., together with those varying tactual and motor
feelings, which result from what is called the subjective stimulation of
the tactual nerves, and from changes in the position and condition of
the muscles. Lying commonly in what is known as the sub-conscious region
of mind, undiscriminated, vague, and ill-defined, these sensations, when
they come to be specially attended to, readily get misapprehended, and
so lead to illusion, both in waking life and in sleep. I shall have
occasion to illustrate this later on.

With these sensations, the result of stimulations coming from remote
parts of the organism, may be classed the ocular impressions which we