, which
is incongruous to our former experience, we instantly dissever the train of
imagination by the power of volition, and compare the incongruous idea with
our previous knowledge of nature, and reject it. This operation of the mind
has not yet acquired a specific name, though it is exerted every minute of
our waking hours; unless it may be termed INTUITIVE ANALOGY. It is an act
of reasoning of which we are unconscious except from its effects in
preserving the congruity of our ideas, and bears the same relation to the
sensorial power of volition, that irritative ideas, of which we are
inconscious except by their effects, do to the sensorial power of
irritation; as the former is produced by volition without our attention to
it, and the latter by irritation without our attention to them.

If on the other hand a train of imagination or of voluntary ideas are
excited with great energy, and passing on with great vivacity, and become
dissevered by some violent stimulus, as the discharge of a pistol near
one's ear, another circumstance takes place, which is termed SURPRISE;
which by exciting violent irritation, and violent sensation, employs for a
time the whole sensorial energy, and thus dissevers the passing trains of
ideas, before the power of volition has time to compare them with the usual
phenomena of nature. In this case fear is generally the companion of
surprise, and adds to our embarrassment, as every one experiences in some
degree when he hears a noise in the dark, which he cannot instantly account
for. This catenation of fear with surprise is owing to our perpetual
experience of injuries from external bodies in motion, unless we are upon
our guard against them. See Sect. XVIII. 17. XIX. 2.

Many other examples of the catenations of animal motions are explained in
Sect. XXXVI. on the Periods of Diseases.

* * * * *

SECT. XVIII.

OF SLEEP.

1. _Volition is suspended in sleep._ 2. _Sensation continues. Dreams
prevent delirium and inflammation._ 3. _Nightmare._ 4. _Ceaseless flow
of ideas in dreams._ 5. _We seem to receive them by the senses. Optic
nerve perfectly sensible in sleep. Eyes less dazzled after dreaming of
visible objects._ 6. _Reverie, belief._ 7. _How we distinguish ideas
from perceptions._ 8. _Variety of scenery in dreams, excellence of the
sense of vision._ 9. _Novelty of combination in dreams._ 10.
_Distinctness of imagery in drea