excited; though they can stimulate these parts with their own fingers
unmoved. Here the pleasureable idea of playfulness coincides with the
vellication; and there is no voluntary exertion used to diminish the
sensation, as there would be, if a child should endeavour to tickle
himself. See Sect. XXXIV. 1. 4.

6. And lastly, the motions excited by the junction of voluntary exertion
with irritation are performed with more energy, than those by irritation
singly; as when we listen to small noises, as to the ticking of a watch in
the night, we perceive the most weak sounds, that are at other times
unheeded. So when we attend to the irritative ideas of sound in our ears,
which are generally not attended to, we can hear them; and can see the
spectra of objects, which remain in the eye, whenever we please to exert
our voluntary power in aid of those weak actions of the retina, or of the
auditory nerve.

7. The temporary catenations of ideas, which are caused by the sensations
of pleasure or pain, are easily dissevered either by irritations, as when a
sudden noise disturbs a day-dream; or by the power of volition, as when we
awake from sleep. Hence in our waking hours, whenever an idea occurs, 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