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.)