The lower of the conscious species possess only the faculty of sensation,
which is sufficient to direct their actions and provide for their needs. A
sensation is produced by the automatic reaction of a sense organ to a stimulus
from the outside world; it lasts for the duration of the immediate moment, as
long as the stimulus lasts and no longer. Sensations are an automatic response,
an automatic form of knowledge, which a consciousness can neither seek nor
evade. An organism that possesses only the faculty of sensation is guided by
the pleasure-pain mechanism of its body . . .
The higher organisms possess a much more potent form of consciousness: they
possess the faculty of retaining sensations, which is the faculty of
perception. A “perception” is a group of sensations automatically retained
and integrated by the brain of a living organism, which gives it the ability to
be aware, not of single stimuli, but of entities, of things. An animal is
guided, not merely by immediate sensations, but by percepts. Its actions are
not single, discrete responses to single, separate stimuli, but are directed by
an integrated awareness of the perceptual reality confronting it.