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.
Although, chronologically, man’s consciousness develops in three stages: the stage of sensations, the perceptual, the conceptual — epistemologically, the base of all of man’s knowledge is the perceptual stage.
Sensations, as such, are not retained in man’s memory, nor is man able to experience a pure isolated sensation. As far as can be ascertained, an infant’s sensory experience is an undifferentiated chaos. Discriminated awareness begins on the level of percepts . . .
Percepts, not sensations, are the given, the self-evident. The knowledge of sensations as components of percepts is not direct, it is acquired by man much later: it is a scientific, conceptual discovery . . . .
(It may be supposed that the concept “existent” is implicit even on the level of sensations — if and to the extent that a consciousness is able to discriminate on that level. A sensation is a sensation of something, as distinguished from the nothing of the preceding and succeeding moments. A sensation does not tell man what exists, but only that it exists.)
Sensations are the primary material of consciousness and, therefore, cannot be communicated by means of the material which is derived from them. The existential causes of sensations can be described and defined in conceptual terms (e.g., the wavelengths of light and the structure of the human eye, which produce the sensations of color), but one cannot communicate what color is like, to a person who is born blind. To define the meaning of the concept “blue,” for instance, one must point to some blue objects to signify, in effect: “I mean this.” Such an identification of a concept is known as an “ostensive definition.”