in Sect. IV. When fibrous contractions succeed other fibrous contractions,
the connection is termed _association_; when fibrous contractions succeed
sensorial motions, the connection is termed _causation_; when fibrous and
sensorial motions reciprocally introduce each other in progressive trains
or tribes, it is termed _catenation_ of animal motions. All these
connections are said to be produced by _habit_; that is, by frequent
repetition.

12. It may be proper to observe, that by the unavoidable idiom of our
language the ideas of perception, of recollection, or of imagination, in
the plural number signify the ideas belonging to perception, to
recollection, or to imagination; whilst the idea of perception, of
recollection, or of imagination, in the singular number is used for what is
termed "a reflex idea of any of those operations of the sensorium."

13. By the word _stimulus_ is not only meant the application of external
bodies to our organs of sense and muscular fibres, which excites into
action the sensorial power termed irritation; but also pleasure or pain,
when they excite into action the sensorial power termed sensation; and
desire or aversion, when they excite into action the power of volition; and
lastly, the fibrous contractions which precede association; as is further
explained in Sect. XII. 2. 1.

* * * * *

SECT. III.

THE MOTIONS OF THE RETINA DEMONSTRATED BY EXPERIMENTS.

I. _Of animal motions and of ideas._ II. _The fibrous structure of the