then of a square; and after having excited these ideas repeatedly, you
excite the idea of their difference, which is that of another right-angled
triangle inverted over the former; you are said to reason upon this
subject, or to compare your ideas.

These ideas of recollection, like the muscular motions above mentioned,
were originally excited by the irritation of external bodies, and were
termed ideas of perception: afterwards the pleasure or pain, that
accompanied these motions, induced a repetition of them in the absence of
the external body, by which they were first excited; and then they were
termed ideas of imagination. At length they become voluntarily practised in
succession or in combination for the common purposes of life; as when we
make ourselves masters of the history of mankind, or of the sciences they
have investigated; and are then called ideas of recollection; and are
performed with strength and velocity in proportion to the energy of the
volition that excites them, and the quantity of sensorial power.

II. 1. The muscular motions above described, that are most frequently
obedient to the will are nevertheless occasionally causable by painful or
pleasurable sensation, as in the starting from fear, and the contraction of
the calf of the leg in the cramp.

2. In like manner the sensual motions, or ideas, that are most frequently
connected with volition, are nevertheless occasionally causable by painful
or pleasurable sensation. As the histories of men, or the description of
places, which we have voluntarily taken pains to remember, sometimes occur
to us in our dreams.

III. 1. The muscular motions that are generally subservient to volition,
are also occasionally causable by irritation, as in stretching the limbs