by habit, that is, by frequent repetition. These laws of animal causation
will be evinced by numerous facts, which occur in our daily exertions; and
will afterwards be employed to explain the more recondite phænomena of the
production, growth, diseases, and decay of the animal system.

* * * * *

SECT. V.

OF THE FOUR FACULTIES OR MOTIONS OF THE SENSORIUM.

1. _Four sensorial powers._ 2. _Irritation, sensation, volition,
association defined._ 3. _Sensorial motions distinguished from fibrous
motions._

1. The spirit of animation has four different modes of action, or in other
words the animal sensorium possesses four different faculties, which are
occasionally exerted, and cause all the contractions of the fibrous parts
of the body. These are the faculty of causing fibrous contractions in
consequence of the irritations excited by external bodies, in consequence
of the sensations of pleasure or pain, in consequence of volition, and in
consequence of the associations of fibrous contractions with other fibrous
contractions, which precede or accompany them.

These four faculties of the sensorium during their inactive state are
termed irritability, sensibility, voluntarity, and associability; in their
active state they are termed as above, irritation, sensation, volition,
association.

2. IRRITATION is an exertion or change of some extreme part of the