III. 1. Innumerable trains or tribes of other motions are associated with
these muscular motions which are excited by irritation; as by the stimulus
of the blood in the right chamber of the heart, the lungs are induced to
expand themselves; and the pectoral and intercostal muscles, and the
diaphragm, act at the same time by their associations with them. And when
the pharinx is irritated by agreeable food, the muscles of deglutition are
brought into action by association. Thus when a greater light falls on the
eye, the iris is brought into action without our attention; and the ciliary
process, when the focus is formed before or behind the retina, by their
associations with the increased irritative motions of the organ of vision.
Many common actions of life are produced in a similar manner. If a fly
settle on my forehead, whilst I am intent on my present occupation, I
dislodge it with my finger, without exciting my attention or breaking the
train of my ideas.

2. In like manner the irritative ideas suggest to us many other trains or
tribes of ideas that are associated with them. On this kind of connection,
language, letters, hieroglyphics, and every kind of symbol, depend. The
symbols themselves produce irritative ideas, or sensual motions, which we
do not attend to; and other ideas, that are succeeded by sensation, are
excited by their association with them. And as these irritative ideas make
up a part of the chain of our waking thoughts, introducing other ideas that
engage our attention, though themselves are unattended to, we find it very
difficult to investigate by what steps many of our hourly trains of ideas
gain their admittance.

It may appear paradoxical, that ideas can exist, and not be attended to;
but all our perceptions are ideas excited by irritation, and succeeded by
sensation. Now when these ideas excited by irritation give us neither